Doc G`s Info Site

Transcription

Doc G`s Info Site

Doc G's Info Site
Home Page:
The Doc G Observatory
With Photos of Construction
Doc's Biography
Bibliography:
History and Biography
History of Astronomy
Biographies of Famous Astronomers
Applications
Telescopes and Equipment
Information and Pictures
Books with Photographs and Images
About the Objects in the Univers
Attachments:
Tubes and Adapters
Telescope Backplate Apertures
Vignetting by Adapters
Adapters for Camera Lenses
Focusing Film Cameras
Piggyback Mounts for Cameras
JMI Focuser with DRO
Parfocal Attachments for Imagers
Design of Guider Mount
Optec Filter Slider
Lumicon Off Axis Guider
Weighted Eyepiece Adapter
Discussion of 2" Diagonals
Video Camera and Attachments
Field Rotation and De-Rotators
Optical Equipment
Adapters for Cameras and Imagers
Focal Reducers and Extenders
Star Field Rotation and De-Rotators
Rotation in Polar Mounted Telescopes
Design of Guider Mounts
ETX as a Guider Telescope
Parfocal Attachments for the ST-7
Focussing Accessories for Film
Cameras
Eyepieces and Field Stops Discussion
with Photographs
Filters for Imaging and Observing
Design of a Projection Attachment
Video and Digital Cameras:
Video and Digital Cameras
Sensitive Video Camera
Video Attachments
Digital Camera - Olympus
Viewing, Perception and Filters:
The Eye and Perception
Use of Filters for Viewing
Filters for Three Color Imaging
Solar Light
The Use of Solar Filters
Description of Optec Filter Slider
Astronomical Software:
LX200 Astro Programs
Using T-Point with the LX200
Using T-Point with modified LX200
Electrical Equipment:
Motors and Controls
Principles of DC motor Operation
Position Control Systems
Observatory Design:
Observatory Design
Thermal Problems
Control Rooms
Winch Design
Many Other Issues
NCRAL 2000 Paper:
The Role of CCD Cameras
in Amateur Astronomy
SPECIAL SECTION:
M. Hart on Astrophotography
Preface and Dedication
Choosing Color Filters
Thermal Effects in Guiding and
Focusing SCTs
Effective Use of PEC and Polar
Alignment
Imaging:
Film Photography
List of Interesting Objects
Guider Design for Piggyback
Photography
3 Color Filters for Film & CCD Imagers
Tubes and Adapters for Cameras
Focal Reducers and Magnifiers
Focussing Film Cameras
Film and CCD Resolution
CCD Imagers and Accessories
3 Color Filters for Film & CCD
CCD Resolution Compared to Film-(Pixels Shmixels)
Tubes and Adapters for CCD Imagers
ST7 Long Cable
ST7 as a Guider
Cooler Box for Digital Camera
LX200 Information:
LX200 Mechanical Analysis
Flexibility of the Mount
Mount Vibrations
Mount Oscillations
Repair of the Declination Drive
Rebuilding the Declination Drive
Declination Drive Adjustment
Electrical Aspects of the Dec Drive
Focusing Mirror and Knob
PEC Operation and Training
LX200 Electrical Analysis
Main Computer Board Analysis
Keypad Operation
Keypad Code List
Keypad - Hot Plugging
Control Panel Ports
Notes on Plugs and Cables
Details about the Declination Drive
LX200 Mount Information
LX200 Saddle Mount
Doc G, shown above with the observatory is Emeritus Professor of Electrical and Computer Engineering at the
University of Wisconsin - Madison.
This roll-off building with a 12" LX200 telescope and accessories was donated to the Madison Astronomical
Society by Doc G in June of 1996. When rolled back, a windbreak remains which is supplied with various shelves for
computers and accessories as well as a comfortable place to sit to operate the telescope locally from the computer.
The telescope is also remotely controllable, by computer, from the nearby clubhouse.
In addition to the Meade 12" LX200 telescope in the DocG observatory, there is a second domed building shown in its
most recent incarnation below. The recently rebuilt ten foot building sports a new Pro Dome installed in June 2001.
The photo shows the installation of the dome almost completed. The Pro Dome with full automation using Digital
Dome Works and a second Meade 12" LX200 telescope will be operational in September 2001. The Pro Dome, the
LX200 telecope and numerous accessories including an SBIG ST-4 imager/guider were donated to the MAS by Dr.
Greiner. It is owned and operated by the Madison Astronomical Society.
The MAS now has two computer controlled telescopes which can be operated from the nearby club house.
Additionally there is a 17" Dobsonian in a roll-off building and a 16" CAT. The 16" CAT is a long focal length, 7900
mm, f19 designed mainly for planetary observation. The club house is a large all season building used for meetings
and additional equipment. It is heated and air conditioned. The dark site is the Yanna Research Station located
near Brooklyn, Wisconsin about 30 miles South of Madison, Wisconsin. It is owned and operated by the Madison
Astronomical Society.
More Pictures of the Observatory
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Brief Biography of R. A. "Doc G" Greiner
For those who might be interested, this biography contains the highlights of Doc G's education, employment, hobbies and other
interests.
R. A. Greiner is currently retired (12 years now) from his profession of Professor of Electrical Engineering at the University of
Wisconsin in Madison Wisconsin. He was Professor in the Department of Electrical and Computer Engineering for 36 years. (from
age 26 through 62).
"Doc" or "Prof" were the names used for him by members of an Electrical Engineering Fraternity (Kappa Eta Kappa) for which he
was advisor (for 25 years) starting in the early 1960's. (that was to his face of course) "R A" was the common "name" used by
friends and colleagues in the department. (this use of the first two initials among some of the faculty was a gesture of friendship)
Many of R. A. Greiner's lifelong interests were formulated in the 1940s as expressed in high school interests. These included a
significant interest in photography, a love of mathematics and a great interest in chemistry and physics. The high school
programs, in a suburb of Milwaukee, in these areas, were excellent and extensive. (most students went on to the university. RA
did much of the photography for the high school yearbook and was a photographer for the local weekly newspaper as well. Large
format photography, color photography and three color separation photography were done in the days of Kodacolor with a speed
of ASA 8 and printing with the dye transfer method. RA went on to the university to study chemistry, physics, mathematics and
astronomy. RA's interest in music and sound reproduction resulted in the design and construction of a three way tri-amplified
loudspeaker system while he was still a junior in high school. This interest carries over to today with music and sound reproduction
among his major hobbies.
R. A. Greiner received his bachelors degree in Physics in 1954 with minors in mathematics and astronomy. (with honors) The
masters degree, received in 1955, was in Physics with graduate work concentrating on molecular spectroscopy, nuclear
spectroscopy and solid state physics. Electronic instrumentation was a major part of all of these studies. Optical instrument design
including interferometer and precision mechanical design was a part of this research.
He switched to studies in the Department of Electrical Engineering which resulted in a PhD in 1957. The doctoral work as in
photoconductivity and electrical conductivity in solids which were related directly to the behavior of semiconductor devices. Dr.
Greiner was appointed Assistant Professor of Electrical Engineering in 1957 (age 26). He taught graduate courses in electronics,
including vacuum tube design, design of transistors and application of solid state devices in the Department. He started a solid
state device fabrication laboratory in 1960 and wrote a book on solid state devices and applications. (McGraw Hill 1961) The book
was about the physics of discrete transistor operation and applications of a variety of solid state devices to analog and digital
circuits. A lifelong interest in high fidelity sound reproduction included numerous articles about power amplifiers and loudspeakers
and led to his election as a Fellow of the Audio Engineering Society in 1984.
In 1961 Dr. Greiner was appointed full Professor in the Electrical Engineering Department (age 30). His research continued in solid
state devices and applications with a graduate program that produced 46 advanced degrees in the next 10 years. In 1972 he
joined the central administration of the University of Wisconsin System where he served as a Senior Academic Advisor to the
President's Office in the area of the Physical Sciences. By 1979 the "same old administrative problems" came around for the third
time and he decided to return to teaching in the Department of Electrical and Computer Engineering. Professing turns out to be
one of the finest of occupations. (in his opinion)
It was time to pursue new interests and a fresh research program. This turned out to be acoustics, electro-acoustics,
instrumentation, digital signal processing and control. A new graduate program was started and produced another 47 advanced
graduate degrees in the period 1979 through 1992. An extensive laboratory was established for research in acoustics and signal
processing which formed the core of work supported by industrial grants. Work in signal processing and adaptive digital control
generated many degree thesis projects and resulted in the formation of a new company in the area. Some of the signal processing
research led to publication in areas of statistics of musical signals and digital processing of audio signals in the early years of
development of such work (1982). Many graduate students went into digital signal processing, computer control, adaptive control
and similar areas.
Over all these years Dr. Greiner consulted in noise and vibration control in industrial settings. This included advanced design of
balancing machines for a major manufacturer of equipment for the automotive industry. Since retirement, Dr. Greiner has
continued to consult in acoustics, noise control and vibration control for industrial applications. Of course, the usual array of
papers in technical journals, patents and the like resulted from all the above activity. High speed flash photography, lens testing,
macro and micro photography are ancillary interests still engaged in from time to time.
Dr. Greiner is a member of Eta Kappa Nu, Sigma Xi, Phi Kappa Phi, Tau Beta Pi, Kappa Eta Kappa, The Institute of Electrical and
Electronics Engineering and the Audio Engineering Society. He is a Registered Professional Engineer in the State of Wisconsin.
In retirement "Doc G" remains active in consulting (just a little), gardening (a small but nice perennial garden), photography
(mainly 35 mm film but with a new interest in digital), fiddling with computers and of course astronomy (LX200s and CCDs). Doc's
astronomy interests are mainly in refinement of amateur equipment, viewing of extended objects and imaging of deep space
objects. Astronomy has turned out to be a very challenging hobby. The pointing accuracy required is totally unreasonable, the
stability of the platform required astonishing and the imaging of faint, zero contrast objects incredibly difficult. This is a nice new
challenge. "Doc G" is currently very active in the Madison Astronomical Society and edits the bi-monthly newsletter "Capitol
Skies."
Go to Home Index for Doc G's Info Site
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Books Which Concentrate on the:
History of Astronomy Including some biographical material
Building of Telescopes Including some biographical material
Biographies of Astronomers
Including discovery of planets and some historical material
All reviews by Doc G of books in his personal library
A book that fits all categories:
The Astronomy and Astrophysics Encyclopedia, Stephen P. Maran, Van Nostrand Reinhold 1992.
A big, big book. It is indeed encyclopedic in nature. There are a lot of really good contributors to this tome. All in all
a good book for quick reference to almost everything astronomical.
Three books about cosmology (simplified):
A Brief History of Time, Stephen Hawking, Illustrated Version, Bantam 1996. When I read the first version of this
book I didn't understand what Hawking was talking about. With this new version, I still don't. Even though I have a
minor in astronomy and a masters degree in physics and took courses in quantum mechanics and the like, I have no
real understanding of what Hawking is talking about. I don't think this stuff can be popularized. Those who claim to
understand Hawking are, I think, faking it. :-)
Before the Beginning: Our Universe and Others, Martin Rees, Addison Wesley 1997. If you liked Hawking's A Brief
History of Time, you will love this book. It contains, in only 257 pages a brief description of every cosmological
thought stated by every cosmologist who has existed over all of time. As best as I can tell nothing has been
omitted. Unfortunately, nothing is explained either. The speculations of cosmology are stated without significant
discussion and remain just that speculation. The conclusion I see is that man has been and still is speculating about
the nature of the universe and that there is little hope of ever figuring it out. Never-the-less, I am rather satisfied
that I managed to get through this book. I repeat, if you liked Hawking's A Brief History of Time, you will love this
book.
Einstein's Greatest Blunder? Donald Goldsmith Harvard University Press 1997. This a catchy title caught me into
purchasing the book. It is a lightweight review of cosmology from the beginning to the end. Again one of those
books that tries to give you the impression that it is teaching you something about the fundamentals of astronomy.
When you are all through with it, you have learned little or nothing, in my opinion.
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Books Which Concentrate on Astronomical Applications
Photography References - Film Photography
Imaging References - CCD Imaging
Just Viewing
Optical and Mechanical Considerations
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Books About the Structures of The Universe:
The Solar System
The Galaxy
The Universe
Books About the History of Astronomy
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Index of Information on Tubes and Adapters
Telescope Backplate Aperture Sizes
Vignetting by Tubing Near the Focal Plane
Adapters for Imagers and Camera Lenses
Focusing Devices for Film CamerasWith Removable Pentaprisms
Using the ETX as a Guider Telescope
Information on the Lumicon Giant Off AxisGuider
Use of the Optec Filter Slider with the JMI-NGFS
Photographs and Design of a Weighted2" to 1 1/4" Eyepiece Adapter
Discussion of 2" Diagonals: Meadeand TeleVue Compared
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Index of Optical & Mechanical Attachments of Various Types
ETX Used as a Guider Telescope
Use of The Lumicon Off Axis Guider (OAG)
Design of a Projection Attachment for a Video Camera
Discussion of the Design of Projection Attachments
Focusing Device for Cameras With Removable Pentaprisms
Photos and Information About Eyepieces
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Information on Video & Digital Cameras
Video Attachments for Video Camera
For astronomical use
A fine point and shoot camera
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The Eye and How It Works When Viewing
Faint Astronomical Objects
The eye has an amazing ability to see detail and perceive contrast in both very bright and very dim objects. In order to appreciate
the brightness range that the eye can utilize, it is appropriate to look at the range of brightness that the eye may normally
encounter. First it is essential to establish a unit of measure for light. Units like watts or watts per square meter would make
sense. But traditionally, light was measured in terms of a standard candle. This leads to a set of measures that are special to light.
The basic measure of illuminance is called a candela. This is the total visible light emitted in all directions by a standard
candle. The standard candle is an actual candle made of a certain mixture of waxes and of a certain size. Another measure of
illuminance is the lumen. One candela is equal to one lumen per steradian. Since there are 4*pi steradians in a complete sphere,
a lumen is a candela divided by 4*pi. (about 12.57). Note that these two units are measures of the power of the source and the
power through a steradian around the source.
Extended objects need another measure of their brightness. This is the power per unit area and it is called luminance. The units
are candelas per square meter or lumens per square meter. Thus these two words are thus both measures of power (brightness)
but one is the power of the source, illuminance. The other the power of or through a surface of a given area, luminance. (most
unfortunate that these words are so similar)
To get some idea about the luminance caused by a stellar object inspect the following table. The stellar magnitude is the usual
brightness rating given to objects by astronomers. Each change in magnitude of 1 unit is a factor of the 1/5 root of 100. Thus 5
magnitudes is 100 times and 10 magnitudes is 10 E4 times. The table gives the luminance or the power through a surface in
terms of lumens per square meter. This number is important since it tell how much light energy is intercepted by a telescope of a
given aperture.
Object
Sunlight
Moonlight (full moon)
Total of All Starlight
Venus
0th Magnitude Star
6th Magnitude Star
lumens/square meter
130 E3
0.267
1.0 E-3
1.4 E-4
2.65 E-6
1.0 E-8
Stellar Magnitude
-27
-12.5
-6
-4
0
6
Dark Adaptation
The physiology of dark adaptation dark is complex. The phenomenon is highly dependent upon the individual viewer. So, as with
all biological effects, only average behavior can be specified. In its simplest form, it is a fact that a viewers ability to perceive light
changes and gets better if the eye is allowed to remain in the dark for some time. This is a chemical effect in the retinal of the
eye. (too complex to describe here) Never-the-less everyone experiences this effect. Typically the change in sensitivity is from 2
to 6 magnitudes after 20 to 30 minutes of darkness. It may vary greatly from person to person for reasons related to physical
condition of the eye. Typical variations for persons with otherwise normal sight are about 2 magnitudes.
These numbers mean that the sensitivity of the eye may increase after 30 minutes by as much as 250 times (6 magnitudes). That
is a large improvement. Brief exposure to bright lights wipe out this improvement almost immediately. Thus viewers should shield
their eyes from any light while viewing and especially from very actinic light. Dim red light is the least damaging but even that
causes some decrease in acuity. Adaptation also depends on the size of a spot of light shown on the retina in a complex way. The
best advice is to severely limit exposure of the retina to any light to retain maximum brightness acuity.
These numbers and variations from person to person show why some viewers claim to see Mag 8 stars regularly while other have
trouble seeing Mag 4 stars under similar conditions. People's brightness acuity simply varies by a great deal and may depend
significantly on the use of tobacco, alcohol and other chemicals. On a broad average, most persons can see Mag 6 stars on a clear
dark night.
The Structure of The Retina of The Eye
The structure of eye is complex, here are outlined only a few factors that directly affect astronomical viewing. The very center of
viewing, that is, the point in space that attracts our direct attention is focused on a region of the eye called the fovea centralis.
This portion of the eye, only a few degrees in size, is crammed with visual cones. These cones have the ability to see color but are
not highly sensitive to brightness. Immediately surrounding the fovea centralis is a large ring of receptors called rods. The rods
have little sensitivity to color but are quite sensitive to brightness. They see in black and white. There are of course some cones
mixed in with the rods so color is perceived everywhere but only when the excitation is sufficiently bright. The rods are about 4
magnitudes more sensitive to light than the cones.
There is a spot about 15 to 18 degrees to the nasal side of the retina where the optic nerve enters the eye and is attached to the
retina. This spot is blind and may be a couple of degrees in diameter. Notice that since the spot is to the nasal side, the blind
region on the surface being observed is in the temporal direction because the lens of the eye turns the image upside down and left
to right. But it is important to recognize that when viewing objects they should not be viewed is such a way as to place them on
the blind spot.
On the other hand, to the temporal side of the retina, especially at 15 to 20 degrees, there are an abundance of cones. This
makes the region 15 to 20 degrees to the temporal side of the retina very sensitive to brightness. Thus astronomers use what is
called averted vision. By forcing the eye to concentrate attention just a bit in the temporal direction, the object is moved onto the
region of the eye with the greatest brightness sensitivity. As one eye moves the object into the region of greater sensitivity the
other eye moves the object into the blind spot. But viewing is generally do with one eye and whatever eye is used, moving the
center of attention toward the temporal side does the desired function.
It is also necessary, when using averted vision to hold the object on the sensitive spot for some time to get the full effect of
averted vision. A period of 4 to 7 seconds is usually optimal.
Thus, is required concentration and practice to use averted vision techniques successfully. However, it is worth while to practice
this technique since the increase in brightness sensitivity is considerable.
Sensitivity and Contrast
In order to see very faint stars and some detail in those enticing gray smudges in the sky, it is necessary not only to have the best
possible brightness sensitivity but also an image size that optimizes contrast within the faint objects. This topic is even more
obscure and strange if you will than the above facts. There are really two things that must be realized that are physiological facts
and will have to go without proof. One is that the eye can discern contrast better when the scene is bright and much more poorly
when the scene is dim. The other is that the ability of the eye to discern contrast differences increases with the size of the objects
placed in conjunction to each other. These two factors work together in a complex way when trying to see contrast within dim
objects.
The first effect is quite obvious. As the skies get darker, we see more and more, fainter and fainter stars. With the telescope we
see even more stars since the object is greatly magnified thus spreading out the background light and still gathering the star,
which is a point source, into a relatively good point image. When looking at a diffuse object with a telescope, both the background
and the object are spread out in a similar way so the actual contrast between them does not change. Still, most astronomers
claim to see, and in fact do see, increasing contrast as the magnification is increased within a certain range. This fact is caused by
the second property of the eye. Namely, the eye can discern contrast better if the objects viewed are larger.
These facts and psychophysical phenomena combine to hint that there might be an optimum magnification factor for viewing
extended objects through a given telescope. All even slightly experienced viewers will say "this is certainly true." It is partially the
reason for having so many eyepieces for various magnifications and fields of view. (the other is the innate desire for having more
toys of course) So it is in fact not only good but necessary to have this array of viewing tools.
What is happening is that there is a struggle between the eyes ability to see contrast, which gets poorer with decreasing
brightness and the eyes improving ability to see contrast as the image gets larger. There should be and in fact is an optimum
field of view and magnification for every object and for each persons eyes. Now it is possible to take much data available in the
literature on the eye and make some estimates of the optimum magnification for an object with a given surface brightness. The
most brilliant of these analyses is given in a fine book by Roger N Clark "Visual Astronomy of the Deep Sky," Sky Publishing
Corporation and Cambridge University Press, 1990. It is however rather difficult to follow all of the details.
One could go through a lengthily analysis of limiting magnitudes for various telescopes, various sky conditions, various dark
adaptation and even the effect of filters on the ability to see objects and detail in them. Elaborate formulas are however not the
object of this discussion. Anything that brightens the image and increases the contrast helps to see detail and separate the object
from the background. This includes, larger diameter telescopes which gather more light, more perfect optics and various filters
which darken light pollution and still allow the spectral lines of the objects being observed through.
Thus here we will only make a few observations on the most promising practice to follow for most amateurs. The contrast
detectable by the eye improves by about a factor of ten when the size of the object at the eye goes from 1 to 10 arc minutes.
After that, the improved contrast detectability seems to stabilize. This depends to some extent on the brightness but that is a
secondary factor. Thus, a useful viewing tactic is this. Start with low magnification. The object is probably too small for the eye to
detect detail easily even though it is bright. Move to a higher magnification. Then detail becomes easier to detect and the general
view of the object improves. Continue to increase magnification. Finally as the magnification becomes too great, the brightness
gets too low and the eye is no longer able to detect detail as well.
At this point, I refer you to the book mentioned above. It gives a great deal of information about the issues mentioned above with
many charts and calculations. In addition there is an extended discussion of viewing of all the Messier objects one by one with
both images and drawings. The discussion give a realistic evaluation of what might be seen with an 8 inch telescope and at what
magnification the object is best viewed. Details about the objects size and surface brightness are also given so one can estimate
its visibility with a larger telescope. There is also a list of over 600 objects and the optimum magnification to use for a variety of
telescope sizes.
Unfortunately this book, printed first in 1990 is now out of print and hard to find. It is very well worth hunting for.
Return to Beginning
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Index of Information on Filters
Filters Used for Three Color Imaging
Solar Light and the Use of Solar Filters
Solar Filter Notes Using the Calcium Line
Description of the Optec Filter Slider
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Astronomical Programs Used with the LX200
Using T-Point with a modified LX200
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Index for Information on the Application of
Motors and Control Systems
Properties of Small DC Motors
Discussion of Control Systems
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Observatory Design Considerations
Pictures of Construction of the Doc G Observatory
Pictures of the Computer Control Room for the
Doc G Observatory
General Considerations for Selection of Observatory Type
Expanded Discussion of Design Considerations
Thermal Effects in Observatories
Control Room Equipment and Environment
Construction Methods
Winch Design for Observatory Roofs
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The Role of CCD Cameras in Amateur Astronomy
Presented at NCRAL 2000 May, 6 2000 by R. A. Greiner
Abstract: This presentation will include specific discussion of middle and advanced level CCD equipment commonly used by
amateur astronomers for digital imaging. Both the camera equipment and the control software will be discussed. The specifications
of the equipment required to accomplish specific guiding and imaging goals will be emphasized. Current available hardware and
software and how they interrelate are of concern and will be detailed. The presentation will include discussion of complete setups,
from camera to telescope to software, which need to be coordinated to successfully accomplish specific imaging goals. Speculation
on the state of the current art and the future of digital imaging will be presented.
0. Amateur and Professional Cameras Compared
One needs to be careful when using the terms amateur and professional. In this paper, the distinction is made without any
denigration of the amateur in any way. It is made only to narrow the field of concerns and applications regarding CCD cameras.
Generally the discussion here will be for cameras that are applied to smaller telescopes, say under 16 inches and in a price range
that is within the reach of a serious amateur.
While amateur and professional cameras have mainly similar goals. The amateur camera is most often used to make pretty
pictures while the professional camera is used for scientific imaging, quantitative measurements, spectral studies and the like.
That is not to say that many amateur astronomers do not do science of a high order. Indeed, this is especially possible with some
recent cameras and accessories. There is no doubt that good science can be done with what is generally considered amateur
equipment.
The main distinctions between professional cameras and amateur cameras are usually the size of the CCD chip, its freedom from
defects and the amount of cooling provided to the chip. The distinction of size is not accidental at least partly because the field of
view of the imager is determined by the size of the chip and the focal length of the telescope. The cost of the chip is directly
related to its size and its perfection and is often a significant part of the total cost of the camera.
Thus we have modest chips, say 4 by 5 mm, in modestly cooled cameras that cost $ 2500. We have 12 by 12 mm chips in better
cooled cameras for $ 7,000 or so. Both of these cameras have been popular with amateur imagers. These cameras are suitable for
use with 10 inch Schmidt Cassegrains at $ 3000, 5 inch Apochromatic refractors at $15,000 or 15 inch Ritchey-Chretien
Cassegrains at $ 25,000. Such telescopes are also popular with many amateur astronomers. Attention will be directed to cameras
in the under $10,000 category. Their characteristics, features and design problems apply, generally, to all CCD cameras.
Typical amateur cameras considered here would be those with the Kodak 400 series chips, like the SBIG ST-7 and Meade 416XT,
at the low end and those with the Kodak 1600 series chips, like the SBIG ST-8 and Meade 1616 in the middle. Higher end cameras
using chips like the Kodak 1300 or various SITe chips are available from Apogee Instruments and Finger Lakes Instruments. They
will be discussed briefly as well.
In addition to chip size and quality, a major factor in camera quality and usefulness is the depth and stability of the cooling
provided for the chip. The cooling factor in camera design is both important and complex and so will be discussed at some length.
It will be seen that a temperature differential of 20 to 30 degrees C. below ambient is relatively easy to attain. Steady
temperatures of -40 degrees C. are much more difficult to attain. These require sophisticated cooling systems and elegant camera
body design.
The electronics associated with the downloading of the information on the chip is also an important design factor, but as will
become apparent is of secondary difficulty compared to other physical factors. That is to say, the design of the electronics is
relatively easy and the quality of the electronics can be made high enough to be a minor problem in the overall camera design
picture. Note in passing that ordinary photographic cameras are available for under $1000 with pixel arrays of 1500 by 1200. The
large differential cost between these cameras and astro-imaging cameras is quite striking and seems to be increasing. This can be
explained by the very different requirements of these cameras. The arguments justifying the cost difference must be differed to
another time and place.
1. A. CCD Cameras for Imaging - Overview
Chip Size
Let us look specifically at some of the chips sizes available which are commonly used in CCD astro-imaging cameras. The size of
the CCD chips is one of the most obvious factors which differentiate CCD chips.
Chip Designation
Chip Size mm
Pixel Array Size
Pixel Size microns
KAF 0400
4 X 6.9
512 X 768
9X9
KAF 1600
9.2 X 13.8
1024 X 1536
9X9
KAF 1300
16.4 X 20.4
1024 X 1280
16 X 16
SITe TK 1024
24.6 X 24.6
1024 X 1024
24 X 24
35 mm film
26 X 36
2600 X 3600
Approx. 10
This table is shown in graphical form below where the impact of chip size is even more striking.
Chip size determines the imaging field of view of the telescope and thus the actual angular size of the object which can be imaged
with a single image. There are several other factors that are just as important. Among these are the size of the pixels, the quality
of the chip and the spectral sensitivity.
Chip Quality
Chips also come in a variety of qualities ranging from 0 to 4. The rating depends on the number of point, cluster and column
defects. In general, the 0 quality chips have no defects and are the most expensive. Astro-cameras usually will have 1st or 2nd
quality chips but often the purchaser has a choice. The general feeling among amateur imagers seems to be that quality 2 chips
are satisfactory for pretty pictures because the images can be repaired, if necessary, with digital image processing programs.
Quite a few more perfectionist imagers go for the quality 1 chips. Few use 0 defect chips because of the very high cost.
Defect frequency grading is listed for chips of various qualities below.
KAF 0400 or 1600
Grade 0
Grade 1
Grade 2
Point Defects
0
5
10
Cluster Defects
0
0
4
Column Defects
0
0
2
In general the quality rating of the chips used in amateur imaging cameras is not widely advertised. However, a few companies,
especially Apogee Instruments and Finger Lakes Instruments, list quality as a feature and have a price list which reflects the
various quality levels available. The chips are steeply graded in price as quality improves.
As is clear, the chips vary widely in both size of the chip and the size of the pixels. The array of chips available from several
manufacturers is way too long to discuss in detail here. The approximate characteristics of film are given simply so a comparison
of size and resolution to typical color film can be made. As is clear, that most CCD chips in amateur cameras are relatively small
compared to the 35 mm frame. Those with smaller pixel sizes provide resolution similar to film.
Field of View
The purpose of the telescope optics and the CCD camera together is of course to image a celestial object. With any camera, to
photograph an object, the object must fit on the film. In the case of the CCD imager the objects must fit on the chip. For this
discussion we are baring doing a mosaic because it is another level of complexity. Thus the combination of the size of the chip and
the focal length of the telescope must be selected to allow the angular extent of the object to fit the chip dimensions. Since the
angular size of desirable objects varies from a few arc-seconds to several degrees, it is not possible for one chip size and one focal
length to do a good job of resolving every object. Some images will fill the chip, some will be very tiny and some will overrun the
chip. Ideally, the image of the object would be large enough to more or less fill the chip and thus use the pixel resolution available
to its best advantage.
Here are three examples of what one might get with three different telescope focal lengths but the same CCD chip size. The first
image is rather pleasantly framed since it shows M51 in relation to the surrounding stars. The second image shows M51 filling the
CCD chip frame fairly tightly. This framing is good as well and will give the best resolution since the image utilizes the pixels
effectively. The final image will give even better resolution of the object, but it is clearly not esthetically satisfying.
The point of this discussion being that the individual imager must try to choose the right setup for the particular objects to be
imaged.
For a given telescope focal length and camera, the angular object size that can be encompassed by the chip is usually adjusted by
the use of a focal reducer or focal extender. Such reducers can be made in strengths of 0.50 or more commonly 0.68 without
serious optical compromise. Focal expanders (Barlow lenses) are also available in similar expansion strengths. Focal extenders are
required for planetary imaging, where the objects are smaller than 40 arc seconds. It is up to the user to decide by the choice of
focal length of the primary instrument and auxiliary lenses which classes (sizes) of objects will most effectively fill the chip area..
With this information in hand, it can be determined what focal length and chip size will encompass the objects of interest. Since
interesting deep space objects range from a few arc minutes to a degree or so, it is clear that with a given chip it is nearly a
requirement to have telescopes of several focal lengths. If planetary imaging is included where the objects are only tens of arcseconds It is even more clear that image magnification is required. Fortunately there are a variety of ways to get high
magnification while maintaining high optical quality. Similarly, shorter focal lengths than a typical telescope has can easily be
obtained with standard camera telephoto lenses. Some specific camera/telescope combinations will be discussed in the following
material.
Resolution Within the Image
The resolution attainable is controlled by the size of the pixels. More specifically by the angular field of view of each individual
pixel. With pixels of size 9 microns or less, the resolution is similar to that of photographic color film. The angular resolution
required of each pixel should be compared to the quality of the seeing. It is not sensible to try to have each pixel image a fraction
of an arc-second of the sky since seeing is rarely that good. On the other hand, if a pixel images too many arc-seconds, the
angular resolution of the image will be poor compared to the seeing. For example, it is often possible to resolve the four stars in
the double-double in Epsilon Lyra. When this is possible, the seeing is about 1 arc-second. Then to image these stars, a pixel
resolution of better than 1 arc-second is required.
On the other hand, even when the double-double is quite clearly separated to the eye, it might not be perfectly steady. Since the
exposure for the imaging is bound to be time integrated over at least several tens of seconds or even more, the apparently good
seeing may well translate into time integrated resolution of several arc-seconds. Experience has shown that the best resolution of
a star image will almost never be better than 2 to 2.5 arc- seconds. Allowing about 4 or 5 pixels to generate a nicely resolved star
image suggests that one might set 0.5 to 1 pixels per arc-second as a suitable resolution target. Most experienced imagers would
find this resolution to be somewhat too great because the price paid in the sensitivity of the total imaging system becomes too
high.
The literature is replete with discussions, heated at that, of the number of pixels required to image a star. Theoretically, the star
image is a point the size of the diffraction limit of the telescope optics. This is a finite size which must be captured by the pixels.
Some argue, wrongly I think, that only two pixels in orthogonal directions are required to image a star by using some mysterious
application of the Nyquist criterion. In fact such stinginess with pixels will result in square looking stars. It has been more sensibly
argued by the best astrophotographers that 3 to 5 pixels across the diameter of the star image will give reasonably good looking
(round) stars. When two few pixels make up the star image, it is said to be undersampled and will appear to be made up of
individual squares. With more pixels than are necessary for a good image are used, the stars are oversampled. Oversampled
images tend to have a nice photographic look, but precious pixels, field of view and total system sensitivity may be wasted as
described later..
The following two images show the problems of under-sampling on the left and slight over-sampling on the right. The star image is
approximately 3 arc-seconds in diameter. On the left the selection of 1 Pixel per arc-second gives a star image made up of about 3
pixels over its diameter. On the right the selection of 4 pixels per arc-second gives a much smoother image. The images are
greatly enlarged to show the effect. Only two or three pixels per star makes the stars blocky and unpleasant looking. The slightly
oversampled star on the other hand will look quite round. But there is a high price to be paid for this high resolution. The better
resolved image requires 16 times the exposure.
When the seeing is less than very good, taking less resolution is usually a better choice. Many amateurs find that time integrated
viewing is almost never better than 4 to 6 arc seconds. In that case a pixel resolution of 2 arc seconds might be a good
compromize. An informed choice must be made by the person doing the imaging which balances field of view, resolution and
overall sensitivity of the imaging system. This decision depends on a variety of factors, many of which are outlined in this paper.
An image of Jupiter emphasizes the point. On the left the resolution is about 0.3 arc second per pixel, in the center it is 1 arc
second per pixel and on the right about 2 arc seconds per pixel. Jupiter was about 30 arc-seconds at the time. It is clear that for
planetary imaging the optical gain of the telescope has to be increased greatly over that which works well for deep space objects.
Fortunately this can be done since the planets that can be imaged are very bright and thus the exposure times short. It is good
practice to sample generously so that the digital images have a more photographic quality. Again, it is not wise to significantly
over-sample but to take whatever resolution the seeing at the moment will allow.
Cooling of the Chip
Cooling of any chip used for astronomical imaging is essential. The reason is that the brightness of the image is exceedingly low.
As we all know, with our first look at a deep sky object, even through a large amateur telescope, most of the objects, so
gorgeously pictured in astronomy books, appear to be not much more that gray smears. This is a terrible disappointment for most
amateurs when they take their first look at a "faint fuzzy." But then we realize that the eye has difficulty seeing color and contrast
at very low light levels. One of the major reason for using CCD chips is because the light level is so low that photographic film
exhibits severe reciprocity failure. In the same way, at normal ambient temperatures, the electronic noise level in the pixels is so
severe as to wipe out the image of faint deep sky objects. No amount of additional exposure time will overcome this problem.
Fortunately, it is relatively easy to cool the chips so that the electronic noise becomes almost negligible. The noise decreases by
half for each 6 degrees C. of cooling. This is shown in the graph. below.
Thus if the chip can be cooled to -20 or better to - 40 degrees C., the electronic noise becomes an almost negligible factor for
amateur imaging applications. The more modestly cooled cameras, those with a single stage of TEC cooling, will attain a
differential between the ambient temperature and the chip of 25 to 30 degrees C. But note that the ambient temperature is not
the temperature of the air, but rather the temperature of the heat sink. The heat sink at maximum cooling effort will be easily 10
degrees C. above ambient air temperature. Thus a cooling effort of 30 degrees C. will give a chip temperature of only 0 degrees C.
Also, this temperature will depend on the ambient temperature if the cooler is already exerting maximum effort. This amount of
cooling is considered adequate by some imagers, but quite marginal by many imagers. In more sophisticated cameras, sufficient
cooling is provided to hold a given low temperature, regardless of the ambient temperature. (within reason) Such cameras will
generally cool to a fixed temperature of -40 degrees C.
These results are dramatically shown in the graphic below. The patches show pixel noise for a dark exposure of a fixed time. From
the left to right are shown cooling of the chip to -10 C., -20 C., -30 C., and - 40 C.
The cost of top line cameras that cool to the lowest temperatures in the face of summertime ambient temperatures is quite high.
This is caused by several factors including the cooler equipment and the more elaborate camera design that must be effected.
Even with increased cost, cooling should be considered carefully when choosing a CCD camera.
Spectral Sensitivity of the Chip
A factor of great importance is the spectral sensitivity of the chip, more accurately, the quantum efficiency of the chip, with
respect to wavelength. This property varies tremendously among chips as can be seen in the following graph.
The standard front illuminated chips all have sensitivities similar to the red line shown in the following graph. The new KAF 0400E
chip and similarly the KAF 1600E have the response shown by the green curve. They have greatly increased sensitivity in the blue
region of the spectrum and are thus are good choices for CCD cameras Almost all newer cameras use the type "E" chips. They
have sensitivity of about 60% from 550 to 750 microns. Also shown is the sensitivity of the back illuminated chips made by SITe.
These are much more sensitive than any of the front illuminated chips at all wavelengths. They are also significantly more costly
because of the great difficulty in their manufacture.
The issue of the sensitivity of CCD chips cannot be over emphasized because both overall sensitivity increase and more uniform
response over the spectrum greatly shortens the exposure times required. Exposure time is a vital issue because it greatly speeds
up the entire imaging process and reduces the demands on guiding. This can be seen by briefly considering the procedures for
CCD imaging in color.
Deep sky objects have a brightness per square arc-second of about 21st to 22nd magnitude. Thus, depending on the telescope
speed, a typical monochrome CCD exposure time might be 10 to 30 minutes. In order to do color imaging, it is necessary to take
three exposures through three color filters and possibly a luminance exposure as well. Additionally, the blue exposure with the
original chips had to be 3 to 5 times as long as the red and green exposures. It could take a nominal time of at least 2 hours to
get a complete set of images for a color image.
With the newer "blue" chips and especially the back illuminated chips, this time can be reduced to an hour orso. Many of these
factors will be discussed in more detail in Section 2 of this paper.
1. B. CCD Cameras for Guiding
One of the very important uses of CCD cameras has been for automated guiding of telescopes. Anyone who has guided
photographic exposures of 30 minutes or longer using a guide scope or an off axis guider (OAG) knows the true pain of
astronomical photography. Stories abound of astronomers getting their eyelashes frozen to the eyepiece. Manual guiding was a
tedious task. Hubble tells the story that he knew who was working the 100 incher just by the rhythm of the control relays
clattering away, and in the dead of Winter feeling sorry for the person doing it. The electronic autoguider puts an end to this task
and the CCD chip is paramount to making accurate automatic guiding possible.
This can now be done on any telescope at a reasonable price. The most popular of these guider cameras was and is the ST-4 from
SBIG. It uses a tiny chip, the TC 211, which is only 2.6 by 2.6 mm and has a pixel array of 192 X 164. In the ST-4 the chip is
cooled but not regulated and it has a high dark current by any imaging chip standards. Never the less, it is a good choice for
guiding because it can be read out rapidly and thus provides fast correction information for on the fly guiding. To do the guiding, a
guide star is centered on the chip through a separate guide scope or through an off axis guider, OAG. The CCD is read out several
times per second and any movement of the star off of the original pixel set is detected immediately by the digital electronics and
the necessary guidance commands are sent to the main telescope. Guiding to sub-pixel accuracy is possible.
Both Meade and SBIG make cameras suitable for guiding. These are priced in the range of $400 to $900. These guiders, with their
very tiny chips, are generally not used by serious amateurs for imaging. Some of the cameras do have imaging capability. The
SBIG ST-4 system is shown on the left and the Meade 216XT is shown on the right. The ST-4 has a separate box used to control
the operation of the guider. It can also be computer controlled. The 216XT can be used without a computer through a rather
arcane series of menus and button presses in the back. It can be better controlled with a computer via a serial connection.
Adequate software is provided in both cases.
An exciting new guider camera has been announced by SBIG, the STV. They call it a Digital Integrating Video Camera and
Autoguider. It has particularly interesting characteristics. It provides a real time video signal that can be viewed on any TV set.
Additionally, it can integrate the image for up to 600 seconds. Thus it acts like a small chip digital camera. It uses a TC237 chip
which has 656 X 480 pixels of 7.4 micron size. This is a small chip, but has TEC cooling and good low dark current. It would not be
considered suitable for high resolution imaging as are the other cameras under discussion here. But for a TV display, it is adequate
to show images in real time. As an auto-guider it has excellent characteristics. It is significantly more sensitive than earlier
guiders, has a full time and very fast guiding capability of up to 30 corrections per second and the control electronics are optimally
tuned to do guiding. The unit can be remotely controlled from a computer through a serial connection so that complete guiding
functions can be effected remotely. The chip image can be simultaneously viewed on any standard TV screen. While the STV is self
contained, including an LCD display, it has control software that allows full function of the unit from a remote computer. This
should make finding suitable guide stars and remotly controlling the guiding process much easier than ever before. The STV
system is shown below.
The stand alone guiders described above can be used in several ways. For film or CCD imaging they can be used on a separate
guide tube to guide the main OTA or they can be used on the main OAT to guide the telescope with the imaging camera on a piggy
back arrangement. For greater precision and longer focal length imaging, the auto guider can be used on off axis guide equipment.
(in place of the human eye and hand)
Obtaining rigidity between the main OTA and a separate guider scope is not a trivial matter but can be done. Similarly, setting up
an OAG is not trivial. The choice of operating method is up to the personal taste of the operator and will depend on the final
imaging goals and the operators expertise and experience. Guiding to a few arc-seconds is in any case not a trivial pursuit.
Directly below, is shown a typical separate guide scope on the left and the components of a typical OAG on the right. The guide
scope is chown with a flip mirror for easy finding of a guide star and centering it on the guiding camera. The camera in this case is
a Meade 216XT. The OAG is shown with some accessories that might be used including a large focal reducer lens. This OAG is
large enough to be used for medium format film photography as well as with a CCD camera.
One cannot terminate this topic without mentioning the self guiding CCD cameras. SBIG has provided, for some years, high quality
imaging cameras with built in guiding. The most notable cameras are the ST-7 and ST-8. They differ only in the size of the
imaging chip. Each contains a second chip of small size which is used as a guider chip. This chip, since it is a part of the camera
structure does not suffer from mechanical problems of rigidity or flexure or even mirror flop. This camera design has greatly
increased the usability of CCD imaging for the amateur and has been a strong driving force in popularizing CCD imaging among
amateurs. This camera should be seriously considered as a first CCD camera for anyone interested in CCD imaging because it
solves so many of the fundamental and persistent problems of imaging and guiding. The ST-8 will be used as the camera of choice
for the more specific example in Section 2.
Guiding with a CCD camera is a great advance over manual guiding. Such guiding is relatively easy to set up, guides to sub-pixel
accuracy and best of all never gets tired or distracted. Automated guiding is now almost a necessary technique for any serious
imager.
2. CCD Cameras for Digital Imaging - Specifics
Specific Example of SBIG ST-8 Camera and LX200 Telescope
At this point several cameras, their specifications and application will be considered so as to firm up the general overviews outlined
above. Among the most popular CCD cameras used by amateurs are those made by the Santa Barbara Instrument Group, SBIG.
They make a wide array of cameras for all aspects of amateur CCD imaging. Their most sensational cameras of the past several
years have been the ST-7 and ST-8. In reality these two cameras are based on a single basic design with two standard chip sizes.
Several recent modifications and very useful and innovative accessories have extended the life and range of application of these
cameras. Since both the glories of use and the problems with CCD imaging are manifest in these cameras, they will be used as
examples.
A unique and significant feature of the ST-7/8 cameras is that they are self guiding. (as described above) That is, they contain a
second small CCD chip which is near to and par-focal with the main imaging chip. This chip is used for guiding and since it is firmly
connected to the imaging chip, it gives guiding accuracy that is as perfect as is possible. Any flexure or motion of the optical tube
which would move the image, is immediately corrected by the guide chip.
This is an advantage over other guiding techniques, especially for the amateur who may not be conversant with the many
mechanical problems of establishing a solid, flexure free guiding system as is required with either an OAG or especially a separate
guide telescope. That is not to say that the latter techniques are not effective but that they require special care and experience. A
discussion of these mechanical factors would take another paper or two and so will not be pursued here.
The ST-7/8 cameras come with either the Kodak KAF 0400 or KAF 1600 chip. These two chips have recently been upgraded to the
E series chips which have better blue sensitivity as described previously. The chip sizes, pixel array and pixel sizes are: 6.9 X 4.6
mm 765 X 510 pixels 9 micron square, and 13.8 X 9.2 mm 1530 X 1020 pixels 9 micron square.
These are not large chips, but are of a size common in amateur cameras and are priced within the reach of most serious
amateurs, at about $ 2500 to $7500.
The cameras come with either anti-blooming gate arrays (ABG) or standard, non-ABG arrays, of pixels. Which chip type to use has
been discussed over and over among users. The trend seems to be toward the NABG chips since the ABG type has its sensitivity
reduced by about 30%. The NABG chips have the disadvantage that some blooming takes place on bright stars. But with modern
image processing techniques readily available, these artifacts can be removed effectively. Improved sensitivity is traded for a bit
more digital image processing.
The Implications of Cooling the Camera
Both cameras come with a standard single stage thermal electric cooler, TEC. This design is capable of cooling the chip to a
differential temperature of about 25 to 30 degrees C. below the ambient temperature of the heat sink. Of course the heat sink has
to be well above ambient in order to dissipate the total heat load of the camera. This might easily be 10 degrees C. When working
near the maximum differential temperature, the chip temperature becomes dependent on the heat sink temperature. This
situation for cooling has to be considered modest and in some cases inadequate. It might be fine in the Arctic but not too good in
the Mojhavi. After some extended discussion of the cooling problem last year, SBIG made available a retrofit cooling modification
which includes a second TEC stage and a fluid cooled heat exchanger. It is now possible to cool the chip to temperatures which are
limited only by other factors in the design of the camera such as the chip chamber thermal characteristics, the design of the body
of the camera and the design of the electronics.
Without going into excessive detail, the ultimate chip temperature is limited by frosting of the chip, dewing of the camera window,
dewing of the internal structure and electronic circuit boards and ultimately dewing of the entire camera body to the point where it
becomes dripping wet. All of these elements depend on the atmospheric conditions, especially the ambient dew point temperature.
The ST-8, with the added cooling option, is shown on the back of a Meade 10" LX200 telescope below. The optics have a focal
length of 1600 mm and a focal ratio of f 6.3. The liquid cooling tubes are apparent. The camera is coupled to a JMI focuser. Two
inch tubes are used to insure minimum flexibility of the mounting. The focuser might be considered excess baggage, but it is
required since the LX mirror is locked down to prevent mirror flop. It seems to be common practice to use a good linear focuser
instead of depending on focusing with the mirror on SCT type telescopes.
Cooling directly affects the dark current which is an important factor in determining the signal to noise ratio, S/N, in the image.
For the 0400 and 1600 series chips, which have 9 micron square pixels, used in this example, the dark current is about 1 e per
second per pixel at a temperature of - 10 degrees C. (e is the charge on an electron) There is some variation of these
specifications in the literature. The values used here are conservative. If a S/N ratio of 30 is desired with a half full pixel well, the
dark current accumulation should be less than about 1000 e. This allows for an exposure of 1000 seconds. (16 minutes) This is an
adequate exposure time to capture a monochrome image of many deep space objects. But when color filters are used, only a slice
of the spectrum is allowed to reach the chip. Additionally, the infra-red part of the spectrum is eliminated entirely. This greatly
lengthens the required exposure. Exposures of 30 to 60 minutes are not unusual. Then the dark current produces an accumulated
charge of 1800 e. When compared to a half filled pixel well of 30,000 e this dark current is not negligible. Thus, dark current noise
is an issue until the chip temperature gets well below -10 degrees C.
Chip Size and Field of View
At prime focus, a 1600 mm focal length and ST-8 (1600 size) chip gives a field of view of 19.7 X 29.5 arc-minutes and a resolution
of 1.15 arc-seconds. With a focal reducer or a focal extender these values can be changed significantly. The following table shows
a variety of possibilities using a basic 10" f 6.3 telescope. The last column is for a typical star image of 2.5 arc-seconds time
integrated size, which is typical under very good seeing conditions. A time integrated star size of 4 to 5 arc-seconds might be
more typical of average to good seeing.
Focal Length
Focal
Field View
Ratio
arc minutes
Resolution
arc seconds
Pix/Star
Basic scope
1600 mm
f 6.3
20 X 30
1.2
2.2
0.63 reducer
1000 mm
f 4.0
31 X 47
1.8
1.4
0.5 reducer
800 mm
f 3.2
40 X 60
2.4
1.1
2X extender
3200 mm
f 12.6
10 X 15
0.6
4.4
3X extender
4800 mm
f 19
7 X 10
0.45
6.6
400 mm lens
400 mm
f 2.8
80 X 120
4.8
0.6
200 mm lens
200 mm
f 2.8
160 X 240
9.6
0.3
The fields of view can be directly related to the classes of deep sky objects that can be squeezed on the chip by considering the
objects listed in the "List of Nice Objects" which is summarized here.
Object Size
Number_of
Objects
Arc Minutes
Number_of
Optical
Attachments
Alternate
Choice
M Objects
Smaller than 5
70
16
X2
X3
5 to 10
66
25
Prime
X2
10 to 20
70
16
Prime
20 to 40
25
10
0.63
0.5
40 to 80
15
6
400 mm lens
300 mm
Larger than 80
7
4
200 mm lens
135 mm
Planets
4
X3
X5
Clearly no single focal length is good for all objects since they range so greatly in size. But with a wise choice of the prime focal
length and with good basic optical quality, the focal length can be adjusted optically to cover a broad range of objects with good
framing and good resolution.
For planetary imaging a 3X extender could be used to give a satisfactory size image for the larger planets. For example with a 3X
extender: Jupiter would have an image diameter of about 1 mm or 100 pixels. Saturn and Mars are half that. Venus varies greatly
in size but on the average is in the same range. Thus it is apparent that a camera and telescope selected for deep sky imaging is
not a good choice for planetary imaging.
Focal lengths over 10,000 mm would be a better choice. Since not too many amateurs have several telescopes, though some do, it
is more likely that most persons will use optical attachments to change the effective focal length of their instruments. Optical
elements like the focal reducer/field flattener will generally allow for shortening the focal length by a factor of 0.63 or even 0.5.
These devices can be effective with smaller CCD chips, but vignetting and optical distortion are apparent with larger chips. Focal
extenders (Barlow lenses), on the other hand, work well up to magnifications as high as 5 times. The extender lens expands the
circle of illumination and so there is no problem covering the larger chips. But, this means that the light is also spread out more
and the sensitivity of the overall imaging system is greatly reduced.
Taking these matters into account, it is wise to choose a telescope with a basic focal length that is shorter rather than longer. That
is, a focal length of between 1000 and 2000 mm is about right. Many 4 to 6 inch refractors fall into this class as do the faster SCTs
in the 8 to 12 inch size. With a telescope on the shorter end of this range but with a fairly low focal ratio, the basic optics can be
shortened by a factor of 0.5 and extended by a factor of 5 to give coverage of a large selection of deep space objects as well as
bringing planetary imaging within reach. The 10 inch f 6.3 is a good choice in many ways.
If one wants to do wider field imaging, a very short telescope is necessary. At some point, it is more effective to use a good
telephoto lens such as one of the 300, 400 or 800 mm f 2.8 lenses commonly available for 35 mm cameras. These lenses cover a
large field with high resolution and can be easily mounted on a larger telescope in a piggy back arrangement as show. Shown
below is a Canon 500 mm f 2.8 lens piggy backed on a 10" f 6.3 LX200 telescope. The telescope does all the work of guiding with
a 216XT camera mounted at prime focus. The lens is shown with a film camera attached but a CCD camera could be used just as
well.
Next are photographs of two typical CCD imaging setups. It starts to look like a lot of "stuff" on the back of the telescope and it
really is. On the right is shown an SBIG ST-8 with cooling tubes mounted on a JMI focuser. In this case an EXT is used for guiding
with a Meade 216XT CCD camera attached. The ST-8 could also be used in its self guiding mode. At the right is shown the Lumicon
giant OAG with the Meade 216XT CCD camera used as a guider. The imaging in this case is being done with a Canon 35 mm
camera. The camera has a high powered magnifier attached for precision focusing.
The SBIG cameras are excellent cameras and would probably satisfy many if not most amateurs as their only cameras. They work
well, have adequate software and are reliable. They have excellent upgradability. Additional cooling is now available and the chips
can be upgraded. There are a number of very fine accessories such as a color filter wheel, an adaptive optics (OA) unit and now
even a spectrograph.
Other Top Line Cameras
A number of top line cameras are made by both Apogee Instruments and Finger Lakes Instruments. They have in some cases
better thermal design than the SBIG cameras. They are available with a wide variety of larger and more sensitive CCD chips,
including Kodak, Thompson and SITe chips. These companies also make cameras comparable to the SBIG cameras, at competitive
prices. They have in the past concentrated on making cameras with larger chips which would qualify as high end amateur cameras
or low end professional cameras. These cameras are typically in the $7000 to $12,000 range depending on the chip quality
selected.
The most attractive Apogee Instruments camera for the amateur is probably the AP-7p. It has an array of 512 X 512 pixels which
are 24 microns square. The chip is a SITe back illuminated chip and thus provides the high sensitivity described above. It has a
complete array of attachments including, a color filter wheel and a liquid cooling accessory. The price at this moment is somewhat
less than the ST-8E camera. Another of their cameras carries the KAF 1300 chip which has been mentioned above. It is a larger
chip, 16.4 by 20.4 mm with 16 micron pixels in an array that is 1024 by 1280 pixels. It is mounted in a very high quality camera.
Many CCD users consider Apogee Instruments cameras to be the gold standard for both amateur and professional applications.
Apogee Instruments has a very detailed web site which includes specifications about their cameras but also a large series of
articles and papers which describe characteristics of CCD chips in general. It covers most of the topics discussed in this article and
a few more specific to CCD chip characteristics. Reading their site provides a whole course in CCD camera properties. It is at www.
apogee-ccd.com.
Just a few weeks ago, Apogee Instruments made an striking announcement about a new series of lower priced cameras designed
specifically to meet the needs of the amateur imager. It is called the Lisaa system (Low cost Imaging System for Amateur
Astronomy) Because of the newness of this system, it has not yet been evaluated by the imaging community. It certainly deserves
investigation and consideration.
Finger Lakes Instruments also makes cameras of high quality which are used by many professionals. They make a complete line of
cameras using both the Kodak and the SITe chips. The camera is well designed, has excellent cooling and of course all of the
accessories one would expect. They too have recently announced a very attractive camera, the IMG1024S. This has a SITe back
illuminated chip with 1024 by 1024 pixels. The size of the pixels is 24 by 24 microns. This is a very large chip, 24.6 by 24.6 mm
which gives it an area of about 80% of a 35 mm frame. It is priced at only $ 7500 and for good reason is called the Dream
Machine. Finger Lakes Instruments also has an excellent web site with extensive information about CCD imaging in general. It is at
www.fli-cam.com.
A photo of one sample of each of their cameras is shown below. The Apogee camera sports with a regular camera lens and the FLI
camera sits naked with its cover off. It is interesting and promising that several top line camera manufacturers are going after the
amateur market.
Exposure Times and Procedures
One of the raves about CCD cameras is that they are so wonderfully sensitive that they can take images in just a few minutes
where film takes an hour. This is true for simple monochrome images. For monochrome images the entire spectrum of the star is
captured including the infra-red portion of the spectrum. The CCD is especially sensitive to infra-red light and has no reciprocity
failure. Thus many deep space objects can be captured in 5 to 15 minutes. But, and it is a big but when one wants to do color
imaging, it is another story entirely. While one can do many objects in 30 to 60 minutes with film, it takes as long or longer to do
these same objects with three or four color CCD imaging.
A principal reason is that three separate exposures have to be taken through filters that each pass only a small fraction of the
spectrum. Additionally for realistic color the infra-red is thrown out completely. So exposures that give good signal to noise ratios
are often 30 to 40 minutes or longer for each filter. Additionally, dark frames and flat fields have to be taken. All of these images
and auxiliary frames are then combined in a digital image processing program. This latter step may take another hour or more.
Galaxies and nebula are typically 21st to 22nd magnitude per square arc-second. For a given scope using a focal reducer increases
the light flux on each pixel and thus reduces the required exposure time. Binning the pixels also increases the sensitivity of the
chip but reduces the resolution. For this reason, some imagers bin the color images and obtain the resolution in a monochrome
image. The monochrome image is then combined with the three color images to create the final image. This is known as the LRGB
color imaging technique. Some imagers consider it less satisfactory that the normal RGB technique
Most of the well known imagers use scopes that are, or are reduced to an f 6.3 focal ratio. While some of these telescopes are
Schmidt Cassgrain type, many are also refractors. But because it is important to get as much light flux as possible to the chip, an
amateur might well consider a 16 inch f 4.0 Newtonian with a good coma corrector. Such an instrument would give shortened
exposures and good results for larger deep sky objects.
Most deep sky objects are only a few percent brighter that the sky. This means that it is necessary to distinguish very low contrast
objects. The CCD camera is very good at this since the image can be selectively stretched about the average value of the image.
This differentiates small brightness differences and creates quite good images where almost nothing existed. Ultimately, the sky
brightness limits the exposure and the ability to image very low contrast objects.
Color and Color Filters
Significant additional equipment is required for doing color imaging. It all starts with color filters and color filter wheels. To help
automate the process as much as possible, camera manufacturers provide accessory color filter wheels which hold the required
filters. They are computer controlled so that the filters can be shifted and a succession of exposures taken. This whole process is
generally automated under control of a computer program. The basic filter wheels provided by camera manufacturers look similar.
An example is show below on the left. Another form of filter attachment is the Optec filter slider. It is shown on the right. It has
some advantages in that it uses larger filters and they are further away from the chip. This allows them to cover larger chips and
prevents any dust from casting a sharp shadow on the chip.
Filter sets come in many, many varieties. There is constant argument over the best filter set to use for color imaging. Each imager
has a favorite set of filters. Only a sample of the filter spectra are given here to show their general nature. A long study of filtering
techniques must be undertaken to become expert in this area. The filters shown are for normal RGB imaging at the top left. This
set of filters has infra-red rejection built into it. Filters without this characteristic come back up in the infra-red region of the
spectrum and would need to have a rejection filter added. If no rejection filter were used each color image would be contaminated
by the strong infra-red light from the object.
As can be seen in the other views, there are filters made for capturing the image in the H alpha emission lines and filter sets to
match certain professional standards. Many imagers use sets of the standard Wratten color filters. There are at least a half dozen
sets described on my web site. It is necessary to study the color imaging literature at length so as to make and informed decision
about filter selection.
There seems to be no absolutely correct filter set. The choice depends on whether the imager wants to make pretty pictures, do
scientific imaging or do photometry. Generally manipulation of the digital image afterward using any number of image processing
programs alters the content of the image significantly in any case.
4. Processing Digital Images
Digital processing of the images is a forgone conclusion. The raw digital images are usually full of problems ranging from bloomed
stars to streaks and non-uniform illumination to dark noise. These artifacts can and must be removed prior to full scale image
processing.
Often multiple images will be taken and added to improve overall signal to noise ratios. This can amount to combining two or three
exposures to combining dozens of short exposures. The latter technique is often applied when the tracking ability of the telescope
is deficient. It is then called track and accumulate imaging. Sometimes, shorter exposures are taken when there is a unusually
bright star in the field. This tactic prevents the bright star from overloading a set of pixels and causing blooming on the chip.
Dark frame subtraction is usually done to remove bright pixels and the basic noise associated with the dark current in the chip. If
the dark current is kept small by deep cooling of the chip. Sometimes a bias count is removed from all pixel readings to do away
with any residual noise in the chip and its readout electronics. This usually amounts to only 100 counts or so.
A much more persistent problem arises from one of the great advantages of the CCD and the processing of the digital image. The
digital image can be stretched over a relatively small count range so as to exaggerate or distinguish very low contrast objects from
sky background. This makes it possible to "see" very faint and low contrast objects with the CCD camera. In fact, this is a principle
advantage of the CCD camera. However, this also has a consequence which arises this way. If there is any non-uniformity of the
illumination of the chip caused by the optical system of the telescope, including things like dust, fingerprints and smears that
affect the image focused on the chip, they too will show clearly. Sensitivity variations of the individual chips also contribute to a
non-uniform image. To control these problems, a so called "flat field" exposure is taken of a uniformly illuminated "gray" field. Any
optical non-uniformity across the chip area will be detedted. This "flat" is than multiplied into the image and a corrected image
results which has the non-uniformity removed.
This is a good idea in principle, but the flat field must be taken with exactly the same optical setup as the image. That includes the
same vignetting and dust and at the same temperature of the chip as the desired image. It turns out to not at all trivial to take
these flat fields. Ideally, different flat fields are taken to match each exposure and each filter. This greatly lengthens the time
required to get a set of exposures that can be used for processing the final image. Absolutely clean optical systems, freedom from
all dust and absence of vignetting are important so as to reduce the task of flat fielding.
So, one of the great advantages of CCD cameras, great sensitivity and enhancement of contrast is also one of the headaches of
the process.
Software for Capturing and Processing Digital Images
There are as many types and qualities of software as cameras and then some. Only a very few will be mentioned here. These fall
into three basic categories. There is considerable overlap among them. The three basic types are planetarium programs, telescope
control programs and camera control programs. For some time these programs were rather exclusive. But in the past few years
the camera manufacturers have seen that releasing their camera control codes was an advantage since then others would write
programs that included their cameras.
Among planetarium programs, The Sky from Software Bisque and Starry Night from Sienna Software are well known. These
programs contain the data bases and the command codes required to point the telescope. The control program code for the Meade
LX series of telescopes has become an almost de-facto standard. Many telescope manufacturers make mounts that use this code
structure.
The most useful planetarium programs also contain the telescope control code and thus overlap the second category of programs,
telescope control programs. There are however programs that are designed to control the telescope and use other planetarium
programs as the source of their pointing information. One which is becoming well known is Astronomer's Control Panel from DC-3
Dreams. This type of program is more versatile than others since it can import a great variety of data bases for different purposes.
This program, ACP, is especially interesting since it is part of and compatible with the new standards which are being established,
by the ASCOM Initiative (Astronomy Common Object Model Initiative) for general control of everything related to automated
astronomy including telescopes, cameras and domes. The purpose is to bring scientific digital control to the amateur community.
This group now encompasses a considerable number of the major suppliers of amateur software.
Each camera manufacturer will have programs which control the actual taking of images. These usually include automated taking
of dark frames and flat fields. Some also include programs for automated guiding, centering of objects, focusing the telescope and
even doing mosaics of larger objects. SBIG, Meade, Apogee and others have camera control programs. Providers external to the
camera manufacturers provide such programs as ACP, MaxIm and Sky to mention just a few which overlap still more of the
process. That is they include telescope control. camera control and varying levels of image processing.
In the image processing area the best known programs are MaxIm and Mira. Though other programs are used. Very few
experienced digital imagers use only one set of programs. While astronomical images have special properties since they are bright
spots on a black background or very low contrast images, some persons use photoshop-like programs to manipulate their images.
In general the specialized astronomy programs are more suitable for digital astro-images since they are designed to handle the
appropriate file types and do specialize digital processing functions on the images.
Of considerable interest recently has been an attempt to integrate all categories of programs into a unified system which enables
the compatible programs to talk to one another easily. This effort, carried out under the auspices of ASCOM, results in the ability
to script large control scenarios for total automation of what are otherwise tedious tasks. A discussion of these new directions can
be found at: www.dc3.com.
6. Speculation On the Future of Digital Imaging
This is a great time to be interested in astronomical digital imaging. A number of advances are on the near horizon.
Digital photography, ordinary photography that is, is just coming into its own. More and more digital cameras in both still and
video formats are appearing, almost daily. This means the chip manufacturers are revving up their ability to make better and
better chips. This has already affected the prices of astro-cameras, with larger and better chips dropping in price by factors of two
in the last year.
Astro-imaging is also coming into wider use. This has resulted in several digital camera companies recognizing a profit center and
getting into the amateur camera business. This has and will continue to result in better cameras with more features and at a lower
price.
Several entirely new concepts mentioned above will continue to pur the pressure on camera companies to meet the needs of
amateurs with a broader spectrum of cameras. Camera prices will drop significantly and/or twice the camera will become available
for the same price. Rumors abound about a $1000 camera coming out soon with capabilities similar to one that last year might
have cost $ 3000.
The really big move will be to full color astro-cameras. These will come about with the improvement of chips used in video
cameras to the point that they are suitable for astro- imaging. Indeed, it is a great time to be interested in astronomy and digital
astro-imaging.
References:
Most of the information in this paper can be found on the web at a variety of commercial sites. These sites are listed in addition to
a series of books because they are really the best way to get the most up to date information in this rapidly changing field.
CCD Astronomy, Christian Buil, William-Bell 1991
This is about the best of the early books about CCD cameras and image processing. It covers everything from building a camera
(out of date) to discussing the principles of imaging and image processing.
The Art and Science of CCD Astronomy, David Ratledge (ed.) Springer 1997
This book is a collection of CCD imaging experiences related by a dozen amateur imagers. It is rather inspirational to see what
some amateurs have accomplished. The book is quite over priced in my opinion. $40 for 150 paperbound pages. I found most of
the articles of some value but a bit disjointed because the several authors have not been edited together carefully. There are
several interesting appendices. Probably a book to get, for the tidbits scattered about in it, but then again somewhat of a
disappointment.
A Practical Guide to CCD Astronomy, Patrick Martinez and Alain Klotz Cambridge University Press 1998. This is a recent book
with detailed discussion of the design of CCD imagers. The imagers discussed seem to me to be somewhat arcane. There is also a
long discussion of image processing. There is a certain sense of authority in the writing. The images used as examples are quite
terrible. The book gives the overall feeling of and old book in a modern binding. I found it useful but not exciting to read. Lessons
given on the SBIGUSER group are much more up to date and applicable to use of the SBIG ST imagers.
Astronomical Image Processing, Richard Berry, William-Bell 1991. A nice pamphlet on the basics of image processing. This
material is most readable and has some nice examples. Additionally it has some software for the PC. Unfortunately, it is well out of
date.
Electronic Imaging in Astronomy, Ian S. McLean, Wiley 1997. This is an astonishing collection of about everything you could
want to know about electronics as applied to astronomical imaging, photometry and measuring techniques. Written for the
professional. The book is essential to those who want to know much about the practice and theory that underlies electronic
astronomy. Still after a thorough reading I found that it did not help me much with understanding how to use my current
equipment to do imaging. I felt that is was useful to know all of this stuff but I still am looking for a book that helps with the day
today problems of imaging with a modern CCD imager.
Web Sites of Interest:
Santa Barbara Instruments Group: www.sbig.com
Meade Instruments: www.meade.com
Finger Lakes Instruments: www.fli-cam,com
Apogee Instruments: www.apogee-ccd.com
Astronomers Control Panel: www.dc3.com
MaxIm control and signal processing software: www.cyanogen.com
Mira image processing software: www.axres.com
Optec optical attachments: www.optecinc.com
The Sky planetarium and control: www.bisque.com
In addition to these references, a great deal can be learned from the manuals that come with CCD cameras and the software
discussed in this article. Many of these are downloadable from the manufacturers web sites.
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Astrophotography Techniques - A Practical Approach by M. Hart
This series of articles is written entirely by M Hart and posted here on Doc G's Info Site as a service.
00.
Introduction
01.
Tricolor Filters for Imaging
02.
03.
Thermal Effects in Guiding and Focusing SCTs
04.
Effective Use of PEC Training and Polar Alignment
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Index of Information Relating to Film Imaging
Three Color Filters for Film and CCDs
A Brief Discussion of Reciprocity Problems with Film
A Nice List of Interesting Objects
Focusing Devices for Film Cameras with Removable Pentaprisms
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Index of Information Relating to CCD Imaging
Three Color Filters For Film and CCD
A List of Nice Objects by Size for CCD Imaging
Long Cables for the ST7
Using the ST7 as a Guider
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Cooling the Canon 10D Digital Camera
Or Other Camera D10
For the past few weeks I have been working on a method to cool a digital camera. My camera is the Canon 10D. As
we all know, the dark noise of the camera chip is reduced by a factor of about 2 for every 6 degrees Celsius reduction
in the camera temperature. This equipment is able to reduce the temperature of the camera by 24 degrees below
ambient. Thus at room temperature, 22 Celsius, the camera can be cooled to 0 Celsius. This gives a reduction in the
dark noise generated by the camera of about 16 times.
If one has selected a camera with excellent low noise in the first place, this amount of reduction in the noise reduces
it to a point where is almost not a problem at all. Obviously this is a significant advantage with hot summer nights.
Of course, on colder nights, the camera temperature can still be lowered by 24 Celsius and thus temperatures well
below freezing can be accomplished. The 10D will operate to temperatures well below freezing.
Lowering the temperature is attained with a liquid cooled by a Peltier cooler package and pumped from the Peltier
unit to the heat exchanger integral to the cooler box. The liquid used is ethanol alcohol and water in proportion to
prevent freezing of the liquid. The cooler is a totally enclosed aluminum box which holds the camera and which has a
built in heat exchanger. The box runs close to the temperature of the coolant.
Since the box is totally closed and very well insulated, there is no condensation within the box and little heat flow
from ambient to the box. The camera is inside the box with power and control cables run out of it to a computer and
exposure controller.
The cooler box is designed to go on the back of our 12" LX200 telescope. The connecting element is designed to take
a simple filter or a focal reducer. It is made of black, opaque delrin to reduce heat flow from the back of the
telescope to the cooler box.
A photo of this cooler attached to the Doc G telescope is shown below.
Here's a link to images being taken with this set-up: <http://btlguce.digitalastro.net/latest.htm>
For another site with some nice 10D astro images, visit: <http://www.ricksastro.com/Gallery/htm/10D.htm>
Check out also the digital_astro Yahoo group. They have a home page here with many images:
<http://velatron.com/dca/>
The cooler system is shown in closed and open situations in the two final photographs below. In the closed condition,
only the fan which drives out the heated air from the Peltier unit protrudes above the lid. The hot air from the Peltier
cooler and its power supply exit from the front and rear sides of the cooler system box. A dual thermometer is
located on the top of the box so that the operator can keep track of the functioning of the box.
In the open condition, which is not the operating condition, the Peltier cooler can be seen on the right with its power
supply, This compartment is separated from the cold side of the cooler system box with insulating material. On the
left is the Eheim pump which moves the cooled liquid and a reservour which de-gasses the coolent. The coolent than
moves through the instant disconnect couplings, through the connecting tubing to the camera cooler box located on
the telescope.
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Index for LX200 Mechanical Concerns
LX200 Flexibility of Mount
LX200 Repair of Declination Drive
LX200 Mount Vibrations
LX200 Mount Oscillations
Rebuilding the Declination System
Michael Hart's Major Rebuilding ofthe Dec Bearings
Declination Drive Adjustment (withoutrebuild)
Discussion of the Electrical Aspects of the Declination Drive
With Detailed Photographs of the Drive
PEC Correction and Training
Photos of the Disassembled LX 200 RA Base
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Index for LX200 Classic Electrical Systems
LX200 Main Computer Board Analysis
LX200 Keypad Operation
LX200 Keypad Code List
LX200 Keypad - Hot Plugging
LX200 Control Panel Ports, General Information and Design of a Guider Control Keypad
General Information and Concerns About Plugs and Cables for the LX200
Discussion About the Declination Drive With Detailed Photographs of the Drive
Discussions of Some Mechnical Aspects of the Declination Drive
Computer Control Code List for the LX200
An Extended Discussion PEC Operation and Training
LX200 Electrical Circuit Diagrams
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LX200 Saddle Mount
I have come upon a number of LX200 mounts that were languishing because the OTAs were taken off of them and put on a variety
of GEM mounts. The idea being that the OTAs were good and the GEM mounts much better than the original LX200 mounts. To a
considerable extent this was true. The Meade LX200 mounts were generally not precise enough for long exposure imaging,
especially with the full focal length of a 10 or 12 inch telescope. The focal lengths were in the 2000 to 3000 mm range. However,
with much shorter focal lengths such as used in wide field imaging these mounts could hold their own fairly well. It makes no
sense to reduce a 2000 to 3000 mm focal length telescope to under 1000 mm with a focal reducer since the optical quality of the
image suffers significantly. Thus many imagers use telescopes in the 500 to 1000 mm range for wider field imaging.
I have taken a number of the LX200 mounts and put a saddle like plate in place of the original OTA. This Saddle Plate (SP) allows
for mounting any number of short telescopes and their imaging accessories on the LX200 mount. This arrangement also
accommodates viewing as well as imaging. It is very nice to have a fine small refractor in a full GOTO mount. One can have this
GOTO mount carry almost any telescope of reasonable size. I have mounted TeleVue 3 and 4 inch scopes, like my TeleVue NP
101, as well as many others on my modified LX200 mounts. It is very nice to have a full GOTO mount for some of the smaller
refractors which are normally on non-GOTO mounts or very simple GEM mounts. The TV was originally on a TeleVue so called
Gibraltar mount. I call it the "pebble of Gibraltar." It is not very satisfactory for astronomical imaging at all since it has no drive of
any kind. The LX200 Saddle Mounted scope has, in both the Alt/Azm and Equatorial modes full tracking and guiding functions. The
Saddle Mount I made using 10" LX mounts will actually take a scope up to about 9 inches with no trouble at all. Thus one can
mount almost any refractor, small SCT, a MAK or even an SCT up to 9 inches on this saddle. Since the LX200 mount is designed to
handle a 12" SCT OTA, which weight about 40 pounds, it will easily handle a TeleVue NP101 or a Takahashi 106 or equivalent
telescope which weigh in the area of 15 to 20 pounds.
The photos below show several views of one of the Saddle Mounts in my shop with a TeleVue Ranger on the bottom for guiding
and a William Optics Megrez 80 II APO Triplet in the saddle. If smaller refractors are mounted, the saddle will take two of them
at the same time. Or if one chooses, a camera with a regular telephoto lens can be mounted along with a telescope. I have done
an under-over design with these mounts so a small telescope can be mounted underneath the saddle and be used as a dedicated
guider tube. This makes the LX200 mount and saddle essentially a pointing platform which can be locked to the sky. When
mounted on the wedge, of course, one has a functional imaging platform with no field rotation for any imaging use.
I want to thank Ted Agos <lester(AT)ziplink.net> <http://www.ziplink.net/%7Elester/saddle-1.htm> for the idea of
making these mounts and for providing two of the several saddles I have used. The saddle shown in the photos is one of his
design.
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Return to Observatory Design Index
Assorted Photographs from Construction
of the Doc G Observatory
The photos show some details of the construction. The pier and the deck posts were set in concrete after careful alignment with
each other and with true North. North being to the rear part of the deck. The bottom portion, which included the wheel structure,
was constructed next and put in place. The rail is mounted on solid twin 2 by 12 beams which are 20 feet long. There is a pair of
beams on each side of the deck which supports the entire structure. Eight posts set in concrete go down 5 feet into bed rock. The
deck is extremely solid and the building very heavy. It still rolls easily on eight "V" grooved wheels along the steel tracks. Then
rolled back, the telescope easily sees the area of the sky around polaris. North is to the left. In the background is the
conventional observatory building housing the C-11.
The building was constructed in situ on top of the base assembly. Details of the heavy duty rails and roller assembly can be seen
in several of the photos. The internal section was installed as a wind break and to hold shelves, charts, the computer and assorted
accessories. When the main doors are closed the building is very weatherproof, as it has to be in Wisconsin's harsh climate.
Never-the-less the building is easily opened, rolled back and the telescope ready to go in less than 5 minutes.
The inner wind break and work area has shelves for the computer and accessories and a comfortable place to sit and operate the
telescope from the computer. The pier is 16" in diameter and extends 6 feet into the ground. The lower 18" of the pier is sunk
into bedrock. A 3 foot square hole around the pier is filled with concrete. The pier structure weighs 4.8 tons. It is provided with
circular illuminated (red rope lights) shelves for accessories. The 12" LX200 is mounted on the Super Wedge which is so solid that
it is never suspect for vibrations of any kind. Numerous power outlets, red spot lighting and smaller shelves are provided for
convenience. One of the large doors is shown being folded back along the side of the building. The patio like deck around the
front area of the main deck is to prevent observers from falling to ground level.
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Doc G's Info Site
Home Page:
With Photos of Construction
Doc's Biography
Bibliography:
History and Biography
History of Astronomy
Biographies of Famous Astronomers
Applications
Telescopes and Equipment
Information and Pictures
Books with Photographs and Images
About the Objects in the Univers
Attachments:
Tubes and Adapters
Telescope Backplate Apertures
Vignetting by Adapters
Adapters for Camera Lenses
JMI Focuser with DRO
Parfocal Attachments for Imagers
Design of Guider Mount
Optec Filter Slider
Weighted Eyepiece Adapter
Discussion of 2" Diagonals
Optical Equipment
Design of Guider Mounts
ETX as a Guider Telescope
Parfocal Attachments for the ST-7
Focussing Accessories for Film
Cameras
Eyepieces and Field Stops Discussion
with Photographs
Design of a Projection Attachment
Video and Digital Cameras:
Sensitive Video Camera
Video Attachments
Viewing, Perception and Filters:
The Eye and Perception
Use of Filters for Viewing
Filters for Three Color Imaging
Solar Light
The Use of Solar Filters
Description of Optec Filter Slider
Astronomical Software:
LX200 Astro Programs
Using T-Point with modified LX200
Electrical Equipment:
Motors and Controls
Principles of DC motor Operation
Position Control Systems
Observatory Design:
Observatory Design
Thermal Problems
Control Rooms
Winch Design
Many Other Issues
NCRAL 2000 Paper:
The Role of CCD Cameras
in Amateur Astronomy
SPECIAL SECTION:
M. Hart on Astrophotography
Preface and Dedication
Choosing Color Filters
Thermal Effects in Guiding and
Focusing SCTs
Effective Use of PEC and Polar
Alignment
Imaging:
Film Photography
List of Interesting Objects by Size
Guider Design for Piggyback
Photography
3 Color Filters for Film & CCD Imagers
Focussing Film Cameras
Film and CCD Resolution
CCD Imagers and Accessories
3 Color Filters for Film & CCD
CCD Resolution Compared to Film-(Pixels Shmixels)
ST7 Long Cable
ST7 as a Guider
Cooler Box for Digital Camera
LX200 Information:
LX200 Mechanical Analysis
Flexibility of the Mount
Mount Vibrations
Mount Oscillations
Repair of the Declination Drive
Rebuilding the Declination Drive
Declination Drive Adjustment
Electrical Aspects of the Dec Drive
Focusing Mirror and Knob
LX200 Electrical Analysis
Main Computer Board Analysis
Keypad Operation
Keypad Code List
Keypad - Hot Plugging
Control Panel Ports
Notes on Plugs and Cables
Details about the Declination Drive
LX200 Mount Information
LX200 Saddle Mount
Doc G, shown above with the observatory is Emeritus Professor of Electrical and Computer Engineering at the
University of Wisconsin - Madison.
This roll-off building with a 12" LX200 telescope and accessories was donated to the Madison Astronomical Society
by Doc G in June of 1996. When rolled back, a windbreak remains which is supplied with various shelves for
computers and accessories as well as a comfortable place to sit to operate the telescope locally from the computer.
The telescope is also remotely controllable, by computer, from the nearby clubhouse.
In addition to the Meade 12" LX200 telescope in the DocG observatory, there is a second domed building shown in its
most recent incarnation below. The recently rebuilt ten foot building sports a new Pro Dome installed in June 2001.
The photo shows the installation of the dome almost completed. The Pro Dome with full automation using Digital
Dome Works and a second Meade 12" LX200 telescope will be operational in September 2001. The Pro Dome, the
LX200 telecope and numerous accessories including an SBIG ST-4 imager/guider were donated to the MAS by Dr.
Greiner. It is owned and operated by the Madison Astronomical Society.
The MAS now has two computer controlled telescopes which can be operated from the nearby club house.
Additionally there is a 17" Dobsonian in a roll-off building and a 16" CAT. The 16" CAT is a long focal length, 7900
mm, f19 designed mainly for planetary observation. The club house is a large all season building used for meetings
and additional equipment. It is heated and air conditioned. The dark site is the Yanna Research Station located near
Brooklyn, Wisconsin about 30 miles South of Madison, Wisconsin. It is owned and operated by the Madison
Astronomical Society.
More Pictures of the Observatory
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Books About Building Telescopes
The Creation of the Anglo-Austrailian Observatory, S.C.B Gascoigne et al., Cambridge University Press 1990
This is a very complete story about the agreements between two countries and the cooperative effort it took to create the
wonderful observatory that now exists in Australia. The personal, political and economic factors are described in detail. Anyone
interested in the creation of a great scientific facility will find this book fascinating.
James Lick's Monument, Helen Wright, Cambridge University Press 1987
This book takes on the tone of an adventure. The sub-title, "The Saga of Captain Richard Floyd and the building of the Lick
Observatory" gives the flavor of struggle, adventure and resolution of the many problems that had to be overcome to build this
famous observatory. The book outlines a personal adventure.
Yerkes Observatory 1892-1950, Donald E. Osterbrock, University of Chicago Press 1997
This book carries the sub-title "The Birth, Near Death, and Resurrection of a Scientific Research Institution." This is a true outline
of the major sections of the book. The long history of the observatory and mainly of the important astronomers who built it and
ran it from the beginning through its hard times and later glory years is portrayed with clarity and charm. Anyone interested in
the history not only of this observatory but in the influence that its many astronomer stars had on the development of astronomy
in America should put this book on the must read list.
The Perfect Machine, Ronald Florence, Harper Collins 1994
This book is about the building of the Palomar Telescope, an instrument that captured the imagination of a generation of
astronomers and of the public as well. This is a wonderful book which is full of facts about people and projects. It has the aura of
a detective story. It is loaded with personalities, technology and the long struggle to overcome the difficulties of creating this
great instrument over a period of almost 20 years. This book is one of the best, if not the best about astronomical adventures.
The Hubble Wars, Eric J. Chaisson, Harper Collins 1994
The saga of the building of the Hubble telescope has a very different flavor from most other books about the history and building
of great astronomical instruments. In this book the innards of Astro-politics is reveled in almost more detail than you might want
to know. Of especial interest is the exposition of the infighting among individual astronomers and astronomical groups about the
who, what and why of the design, about the construction and particularly about the use of this unique instrument. It is surprising
how personal and nasty scientists can be. This modern adventure should be read by anyone with an interest in the present and
future of astronomy.
A Short History of Observatories, Marian Card Donnelly, University of Oregon Books 1973. Here is a small book which is a
delight for anyone interested in how observatories got started and and how they took the shape they now have. Official
observatories, buildings devoted to astronomical observation, started in the 17th century with what were nothing more than
observing platforms. As instruments got larger and more demanding of the spaces in which they were installed, the buildings took
shapes that were dictated mor my engineering rather than esthetics. This book gives a fine set of diagrams and plates showing
some 70 or so buildings from the 17th century through modern times. A very nice book.
In Quest of Telescopes, Martin Cohen, Sky Publishing 1980. This book is about the travels of an astronomer from location to
locaton with comments about using a variety of telescope at a variety of locations. The comments are interesting in the sense
that they describe one persons experiences. The overall structure of the book is puzzeling. The locations are arbitrary and the
descriptions more like a personal journey that a coherent discussion. The pictures included are equally strange. They are not very
professional and give unusual views of equipment, some of which make no sense in the narrative. These are the meanderings of a
meandering astronomer with no clear goal that I can detect. Sort of a quest for a golden grail not found.
Unusual Telescopes, Peter L. Manly, Cambridge 1991. This is a really fun, fun book to read. It is, as might be expected, about
unusual telescope designs over the past two centuries. Though the designs vary from the bizare to the very strange indeed, there
are a lot of interesting ideas to peruse. A nice book which I found enjoyable.
Small Astronomical Observatories, Patrick Moore ed, Springer Verlag 1996. For anyone thinking about designing a small
observatory this book is an essential. It is full of ideas, pictures and descriptions of a great variety of small observatories. A lot of
nice ideas and experiences are outlined.
Reflecting Telescope Optics Vols. I and II, R. N. Wilson, Springer Verlag 1996-1999. These two volumes contain everything
you might want to know about reflecting telescopes. There are sections on the history, development, optical theory, optical
production, mechanics, baffling, attachments, active optics and more about reflecting telescopes. This is an incredible set of books
which will be of interest to those who are deeply interested in the details of what makes fine reflecting telescopes work.
Everything from the original telescopes of Newton and Herschel to the most modern Cassegrain telescopes and their variations,
even including the ESO, are discussed in detail. The book has extensive references. But be warned if you have problems
understanding mathematics and optical theory like the Hamilton-Seidel third order theory of aberrations, there will be sections of
the book that are not accessible. Even then there are descriptions of modern telescopes, especially the ESO, that are fascinating.
An expensive set ($100 each) but well worth the trouble if you do the digging required to reach an understanding of modern
reflecting telescopes.
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Biographical Works About Astronomers
American Astronomy: Community, Careers and Power 1859-1940, John Lankford, University of Chicago Press 1997
This is a fine book. It is not only well written but has extensive references and a thorough index. It is fascinating in terms of the
viewpoint it takes. This is not a presentation of facts but an analysis of the structures, the people and the powers that controlled
the development of astronomy in America.
Edwin Hubble, Gale E. Christianson Farrer, Straus and Giroux New York 1995
This is a complete and thorough biography of a great astronomer from the beginning. It is also a story about the quest for
astronomical truth in evaluating what structures are really out there, the size of the universe and the foundation for all of modern
cosmology. This is a gripping story not only of one man but of those about him who got astronomy on the right track to our
modern understanding of the universe. A great book about a great, if not one of the greatest, astronomers of modern times.
Explorer of the Universe, A biography of George Ellery Hale, Helen Wright, American Institute of Physics Press 1994
This a deep and thorough biography of one of the most influential telescope builders of the century. Hale was a complex person, a
scientist, a schemer and the driving force behind what are still three of the great telescopes ever built. The 40" Yerkes, the 100"
Hooker and the 200" Palomar instruments were created largely through his sheer willpower and his ability to raise large amounts
of money for their creation. This is indeed a wonderful biography filled with personal pathos as well as a history of how these
seminal instruments came to be. A must read book.
The Immortal Fire Within, Life and Work of Edward Emerson Barnard, William Sheehan, Cambridge University Press 1995
Barnard was one of the great astronomers of the late 19th century. This is a true biography with great detail about his personal
life. His work is covered in detail as well. He was a comet hunter, a pioneer in astronomical photography and a leader at the
several observatories where he held positions. These include Yerkes and Lick where he brought fame and eventually immortality
with him. This is a fine book.
Pauper and Prince, Richey, Hale and Big American Telescopes, Donald E. Osterbrock, University of Arizona Press 1993
This book is a fine intermingling of the lives and accomplishments of several of the influential persons who pushed forward the
building of great telescopes in the late 19th and early 20th centuries. Their lives are intertwined in complex ways with each
contributing their part to the success of completing these monumental scientific instruments. This is a fine read.
Russell W. Porter, Explorer, Artist, Telescope Maker, Berton C. Willard, The Bond Wheelright Company 1976. Porter was not
an astronomer but rather an engineer who learned how to build and design telescopes. He is largely responsible for the design
details of the great Palomar instrument. A genius in his own right.
Alvan Clark and Sons, Deborah Jean Warner, William Bell 1996
This is a nice little book about one of the most famous makers of telescope lenses of the past century. It has a short biography of
him. A large section includes descriptions of the instruments he made and the famous lenses he ground.
A compendium of all known Clark telescopes is included.
A Man Who Loved the Stars, John A. Brashear, University of Pittsburgh Press 1988. This is an autobiography written in 1912
through 1920. It is a thoroughly charming book. It is the tail of the life of a man who did optical work of various sorts for the
likes of Rowland, Draper, Michelson, Hale, Lick, Holden and a large number of the great telescope makers of the late 19th and
early 20th centuries. This is a very nice book to read since it gives such a personal account of what it was like to be an optical
pioneer in the early days of astronomical and spectrographic instruments.
The Astronomical Scrapbook, Skywatchers, Pioneers and Seekers in Astronomy, Joseph Ashbrook, Sky Publishing 1984.
This is a must book to have, to hold and especially to read. Ashbrook was one of the great editors of Sky and Telescope. This
book gathers together 91 of his best essays. The book is marvelous look into the lives and activities of astronomers viewed
through snippets from their lives. The stories are interesting, perceptive and amusing all at the same time. This is a wonderful
read, especially for those who are new to astronomy and did not read Sky and Telescope through the years. Thoroughly enjoyable
book.
William Herschel and the Construction of the Heavens, Michael A. Hoskins, Norton 1963. Hershel's writings with
discussions. There is nothing new to learn about the Heavens from this book, but it gives insight into the thoughts and the genius
of William Herschel as he tried to make sense of the structure of the heavens as he was able to define them. This analysis of his
writings may get a bit boring unless you imagine yourself in the middle of the 18th century and ignorant of the universe as we
now understand it.
The Shadow of the Telescope, A biography of John Herschel, Gunther Buttmann, translated by Bernard Pagel, Scribner's
Sons 1970.
John Herschel, the son of the famous astronomer William Herschel, let a charmed life. He was born to wealth and had a brilliant
mind. This biography is considered one of the best about him. John Herschel was a man for all seasons in the scientific world. He
contributed to mathematics, astronomy, chemistry and was a leading light in the general scientific advancement of the 19th
century. The book is a fascinating account of his scientific work, his travels and the people he knew as colleagues and friends.
This included everyone of renown in science in the 19th century. I found the book excellent. It is a bit hard to read since the
translation from the original German is a bit stilted. Never the less this is a book well worth the time spent reading it.
Lowell and Mars, William Graves Hoyt, U of Arizona Press 1976. This is a nice book about one very controversial astronomer. It
seems very fair and is quite interesting from a historical viewpoint. Lowell was an odd and driven person who has left a great
legacy to the astronomical world in the form of his enduring endowment to his observatory complex in Arizona. An astonishing
amount of good astronomy has come out of his passion.
Clyde Tombaugh - Discoverer of Planet Pluto, David H. Levy, University of Arizona Press 1991. This is a delightful, friendly
and just plain nice biography about the discoverer of the planet pluto. David Levy has researched the life and career of
Tombaugh, especially the Lowell Observatory days, with great thoroughness and sensitivity. He has extracted the earnestness
and excitement that was buried in the rather routine life of a fine but little known astronomer and breathed life into the hard work,
almost drudgery, that went into Tombaughs discoveries. I feel that this is a really nice book. I enjoyed reading it.
The Lord of Uraniborg - A Biography of Tycho Brahe, Victor E. Thoren, Cambridge 1990. This is a big book and a big tough
read about one of the founders of scientific method as applied to astronomical observation. It goes into great detail about
everything. Brahe had an unbelievably complex life in terms of his position among his relatives, kings and lords and his need to
have funds for his work, which was a major expense in an era of distrust of science in general and astronomy in particular. This is
all laid out in excruciating detail. There is also very great detail about his astronomical measurements, their influence on those
around him and various attempts to force accurate observations into a heliocentric model which simply did not work. He of course
was a direct influence on Keppler who in the early 17 century did cast some light onto the true structure of the universe. I found
the book alternately exciting and boring, never easy to read but in the end an enlightening look into the struggles, successes and
failures of the first great observational astronomer.
First Light - The Search for the Edge of the Universe, Richard Preston, Randon House 1987,1996. This is a book about a
small clutch of astronomers that worked at Palomar Observatory in the early 1980s. They are James Gunn, Donald Schneider,
Maarten Schmidt, and Eugene and Carolyn Schoemaker. The first three were looking for the edge of the universe and the latter
two looking for asteroids and comets. The edition I read was printed in 1996 and is an updated version of the original book
published in 1986. It is mainly a study of the people doing the research and their tactics. The in depth discussion of their
methods and habits is a wonderful if slightly light hearted treatment. This is a thoroughly enjoyable book about some of the
astronomical science that went on at Palomar a decade or so ago and the interesting characters that did it.
The Discovery of Neptune, Morton Grosser, Harvard University Press 1962. The book tells the tail of the principles involved in
the discovery of Neptune. Adams in England and Leverrier in France. The book is highly documented, easy to read and as
exciting as any mystery one could want. The interaction among the principles Adams, Airy and Challis in England and Leverrier, in
France, Encke, Galle and d'Arrest in Germany it truly fascinating. The blundering by the British and the fast action by the French
and Germans are placed in stark contrast. This is a great tale of astronomical discovery very well told.
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Books About Astro Photography
A Manual of Advanced Celestial Photography; Brad D. Wallis & Robert W. Provin; Cambridge University Press 1988
The first among good books on astronomical photography. The book concentrates on photographic techniques rather than
equipment. This is an extraordinary book and a must for anyone doing or wanting to do astronomical photography. Not for the
novice but a required book after digesting one or two of the following books.
High Resolution Astrophotography; Jean Dragesco; Cambridge University Press 1995 An excellent book with emphasis on
photography of the Sun, Moon and planets. Equipment is described in detail and numerous examples are shown. Not for the
novice.
Solar Astronomy Handbook; Beck, Hilbrecht, Reinsch, Volker; SONNE (1982 German) (first English Edition 1995) WillmannBell. Great detail about equipment and many examples of photographic results. Lengthily discussion of observations and
recording of all solar phenomena. Definitely for those with a deep interest in the Sun. Definitely not for the novice.
Astrophotography Ë An Introduction; H.J.P. Arnold; Sky and Telescope 1995 A nice, and simple, introduction mainly to
photography of the Sun, Moon and Planets. A fine book for the novice and the serious amateur.
Astrophotography II Ë Featuring the Techniques of the European Amateur; Patrick Martinez; (1983 French) (English Edition
1987) Willmann-Bell, Inc.; A fine small book jammed with important information that everyone should know about setting up
telescopes for photography. An excellent book for the serious amateur.
Astrophotography for The Amateur (revised 1991); Michael Covington; Cambridge University Press
A nice little book that covers the basics very clearly. The book tries to cover a bit too much ground for such a short book. Strictly
for the novice.
Covington's book has been revised. The 1999 edition is larger and crammed full of very good information. While my original
review (above) was a bit cool, I now find the new and more detailed coverage is really excellent. The intermediate amateur
should have this edition at hand all of the time as a prime reference. Many new photographs are included and the array of
equations for optical calculations is great. They are very well explained and accurate. An enlarged section on film photography
and a very fine section on CCD imaging are also well written. This book has risen to become a top recommendation for everyone
interested in the whole array of astrophotographic techniques.
The Cambridge Eclipse Photography Guide; Jay M. Pasachoff and Michael A. Covington; Cambridge University Press 1993
A very nice book about eclipses with detailed tables covering eclipses through 1999. This is a timely book for the eclipse of 1998 in
the Caribbean and 1999 in Europe. Excellent discussions of observing and photographing these events.
Astrophotography - Second Edition, Featuring the fx system of Exposure Determination, Barry Gordon, William-Bell 1985
A collection of details about photographing objects. The book has a lot of charts and means for calculating exposures. I have
never warmed up to the book because I feel it spends too much time on details.
Advanced Amateur Astronomy, Gerald North, Cambridge 1997
I did not find this book very advanced though it covers some unusual equipment in some detail. The images are only so so and
may reflect the equipment involved. Not what one would expect from a book revised in 1997.
Splendors of the Universe, A Practical Guide to Photographing the Night Sky, Terence Dickinson and Jack Newton, Firefly 1997.
A good, even wonderful, book about photographing the night sky. Images from the near to far objects are included with practical
advice about how to photography them. This book is not just a picture book but a fine how to do it book. I recommend this one
highly for anyone who wants to try this most difficult type of photography. The book will be helpful.
Kodak - Photographic Filters Handbook, Eastman Kodak Company 1990. This is the complete and essential handbook on the
Kodak Wratten filters. Excellent in every way, this is a necessary book for anyone who wants to use filters for any photographic or
imaging purpose.
Photographic Atlas of the Stars, H J P Arnold, P Doherty and P Moore, Kalmback Books 1997. This is one of the must have
books. It is a sky survey done by taking the sky in large chunks. The photographs are excellent and are on a scale that enables
the viewer to relate to what you actually see when you look up at night. The 45 maps of the heavens show the stars and objects
that one might see with the naked eye under ideal conditions. Each photograph is accompanied by a hand drown map which puts
names of the outstanding objects of interest directly on the opposite page. This is a perfect way to find and name objects. I have
used this book many times to track down an object that needed a name or number. A beautiful and useful book.
Atlas of Deep Sky Splendors, Hans Vehrenberg, Sky Publishing 1983. Splendid is the only way to describe this book of
splendors. One of the best collections of super fine images available. This is a big book, hundreds of big plates, many in color
with and excellent text to go with the images.
The Hubble Atlas of the Galaxies, Allan Sandage, The Steinhour Press 1961-1984. This is the famous Hubble galaxy book
completed by his student and colleague Allan Sandage. An outstanding collection of images of galaxies and the systematic
classification of them using the Hubble methodology. A book of great historical interest as well as a text for learning about
galaxies. This is a big, big book which is a joy to the eyes and mind.
Astrophotography with the Schmidt Telescope, S. Marx and W. Pfau, Cambridge 1992. This book is a detailed history of the
development of the Schmidt telescope and its application to sky mapping and surveying. Design theory is covered and there are
dozens of beautiful images of familiar objects taken with these instruments. A very fine book.
Observing and Photographing the Solar System, Dobbins, Parker and Capen, William-Bell 1992 (rev). This is subtitled "A
Practical Guide for the Amateur Astronomer." In every way this is a superb book. The details about all of the equipment required
and how to use it are covered at length. Numerous fine images of the planets are included. There is an extensive discussion
about the planets, how they appear and how to observe them intelligently. The techniques are important to amateur
photographers since relatively modest array of equipment is required and well within the reach of many amateurs. This is a very
fine reference book.
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Books Relating to CCD Imaging
Unfortunately the current books relating to CCD imaging are, in my opinion, not very useful and are mostly out of date. One can
find some information in the following books but you must look carefully.
CCD Astronomy, Christian Buil, William-Bell 1991
This is about the best of the early books about CCD cameras and image processing. It covers everything from building a camera
(out of date) to discussing the principles of imaging and image processing.
The Art and Science of CCD Astronomy, David Ratledge (ed.) Springer 1997
This book is a collection of CCD imaging experiences related by a dozen amateur imagers. It is rather inspirational to see what
some amateurs have accomplished. The book is quite over priced in my opinion. $40 for 150 paperbound pages. I found most of
the articles of some value but a bit disjointed because the several authors have not been edited together carefully. There are
several interesting appendices. Probably a book to get, for the tidbits scattered about in it, but then again somewhat of a
disappointment.
A Practical Guide to CCD Astronomy, Patrick Martinez and Alain Klotz Cambridge University Press 1998
This is a recent book with detailed discussion of the design of CCD imagers. The imagers discussed seem to me to be somewhat
arcane. There is also a long discussion of image processing. There is a certain sense of authority in the writing. The images used
as examples are quite terrible. The book gives the overall feeling of and old book in a modern binding. I found it useful but not
exciting to read. Lessons given on the SBIGUSER group are much more up to date and applicable to use of the SBIG ST imagers.
Astronomical Image Processing, Richard Berry, William-Bell 1991. A nice pamphlet on the basics of image processing. This
material is most readable and has some nice examples. Additionally it has some software for the PC. Unfortunately, it is well out
of date.
Electronic Imaging in Astronomy, Ian S. McLean, Wiley 1997. This is an astonishing collection of about everything you could
want to know about electronics as applied to astronomical imaging, photometry and measuring techniques. Written for the
professional. The book is essential to those who want to know much about the practice and theory that underlies electronic
astronomy. Still after a thorough reading I found that it did not help me much with understanding how to use my current
equipment to do imaging. I felt that is was useful to know all of this stuff but I still am looking for a book that helps with the day
today problems of imaging with a modern CCD imager.
At this time I await a really definitive book on CCD Imaging. (December 1999 still waiting)
The situation is not much better with signal processing for Astronomical images.
The handbooks that come with the SBIG and Bisque software are better than most of the books at this time.
Additionally, there are a number of web sites belonging to members of the SBIGUSER group that are very, very informative.
The handbook that comes with MIRA is also quite helpful.
One of the best sources of lessons on using a CCD imager is the SBIGUSER group which has informative posts from day to day.
One can join this group through the SBIG web pages.
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Books about Viewing
Burnham's Celestial Handbook - Revised and Enlarged Edition 1977, Dover. This book needs no comment from me. It is
still one of the best books with detailed information about everything you might want to know about and view. The set of three
books is a must for every amateur who wants a reference to observable objects in the sky. This is a must have set.
365 Starry Nights, Chet Raymo, Fireside Books 1982. This is a really neat book. It has a year around and night by night set of
charts that give good information on what is up there that night. It is a fun book which has useful application to your everyday
viewing habit. While designed for the novice, it is of value to anyone who has the habit of regular sky watching.
Visual Astronomy of the Deep Sky, Roger N. Clark, Sky Publishing 1990. Now this is a serious book about visual observing for
anyone. It discusses the function of the eye, how the eye is affected by viewing and the things to do to get the most out of you
eyes. I feel this is an essential book for the viewer. There is a wonderful atlas of deep sky objects and a realistic assessment of
what one can see. It holds the key to understanding why we see gray smudges in the sky instead of the glorious images in the
picture books. I recommend a reading of this book for all visual astronomers.
Photographic Atlas of the Stars, H J P Arnold, P Doherty and P Moore, Kalmback Books 1997. This is one of the must have
books. It is a sky survey done by taking the sky in large chunks. The photographs are excellent and are on a scale that enables
the viewer to relate to what you actually see when you look up at night. The 45 maps of the heavens show the stars and objects
that one might see with the naked eye under ideal conditions. Each photograph is accompanied by a hand drown map which puts
names of the outstanding objects of interest directly on the opposite page. This is a perfect way to find and name objects. I have
used this book many times to track down an object that needed a name or number. A beautiful and useful book.
NightWatch, Terence Dickinson, Camden House 1989. This book has, for good reason, gone through many printings and
revisions. It is a fine book for the starting amateur. It has good advice about everything from getting to using a small telescope.
Everyone, beginning amateur and advanced amateur should read this book at least once.
The Backyard Astronomers Guide, Terence Dickinson and Allan Dyer, Camden House 1994. This is the big brother version of
NightWatch. It is a comprehensive book full of good advice. For the starting amateur especially it is the first primer on all aspects
of astronomy. Highly readable, it gives good and vice and should be a first read by anyone interested in astronomy with a small
telescope.
Spherical Astronomy, Robin M Green, Cambridge 1985. Heavy duty book. Only for those who eventually want to do some
serious positional calculations. Most amateurs will want to let others do this really tough stuff. A good book never-the-less.
Textbook on Spherical Astronomy, W. M. Smart, 1931 to 1990. This is it. The authoritative textbook. It is a good place to
start. If you are smart enough you will learn a lot. Really, really tough going.
Astronomical Algorithms, Jean Meeus, William-Bell 1991. This is a classic book on algorithms for astronomical calculations. A
good reference book. But, the descriptions of the algorithms and how they are to be applied are brutally brief. Very difficult to
apply the algorithms unless you already know how.
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Books About Optical Systems and Telescope Mechanics
Practical Optical System Layout, Warren J. Smith, McGraw Hill 1997. This is a nice book about basic optical systems which
explains important optical systems with overwhelming mathematics.
Telescope Optics Evaluation and Design, Harrie Rutten and Martin van Venrooij, William Bell 1988. If you want to understand
your telescope optics this is the book to help. It is a very thorough discussion of telescope optics of all kinds. It is comprehensive,
discussing everything from the most common designs to the more rare types. Good book.
Star Testing Astronomical Telescopes, Harold Richard Suiter, William Bell 1994. A manual for optical evaluation and
adjustment of telescopes. This is a very thorough discussion of optical aberrations in telescopes. It describes in detail how to find
out that your telescope is a mess. These tests are complex and the descriptions are just as complex. Not a necessary book, but
one that will help you understand just how complex a simple telescope optical system really is.
Advanced Telescope Making Techniques Vol. 1, Allan Mackintosh, William Bell 1977, 1986. A rather incoherent collection of
articles about the Optics of telescopes. I found it confusing and not of much value.
Advanced Telescope Making Techniques Vol 2, Allan Mackintosh, William Bell 1977, 1986. A companion to the first volume.
All about the mechanics of making optics. Equally confusing I think.
Telescope Control, Mark Trueblood and Russell Merle Genet, Willmann-Bell 1997. This is a book that is the first I have seen
about telescope control. It gives the reader a lot of information about existing systems and quite a bit of information about
systems that have been built in the past few years. It is very descriptive and rather wordy I think. It describes several actual
systems for control and has some nice discussion about the kinds of problems that a telescope control system faces. In many ways
it is a good book but in many ways a disappointment in that it describes basically obsolete systems. It is not a waste, but I had
hoped for more. It is simply too wordy and lacks hard case substance.
Reflecting Telescope Optics Vols. I and II, R. N. Wilson, Springer Verlag 1996-1999. These two volumes contain everything
you might want to know about reflecting telescopes. There are sections on the history, development, optical theory, optical
production, mechanics, baffling, attachments, active optics and more about reflecting telescopes. This is an incredible set of books
which will be of interest to those who are deeply interested in the details of what makes fine reflecting telescopes work.
Everything from the original telescopes of Newton and Herschel to the most modern Cassegrain telescopes and their variations,
even including the ESO, are discussed in detail. The book has extensive references. But be warned if you have problems
understanding mathematics and optical theory like the Hamilton-Seidel third order theory of aberrations, there will be sections of
the book that are not accessible. Even then there are descriptions of modern telescopes, especially the ESO, that are fascinating.
An expensive set ($100 each) but well worth the trouble if you do the digging required to reach an understanding of modern
reflecting telescopes.
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Books About the Solar System
Jupiter - The Giant Planet, Reta Beebe, Smithsonian 1994. This is a fine and readable description of Jupiter and its moons.
There are numerous excellent color plates. The narrative is clear and well written. Just the right technical level to be challenging
but not overwhelming.
The Planets - Portraits of New Worlds, Nigel Henbest, Penguin 1994. Pictures, pictures and more pictures. This is a very fine
collection of images of the planets. There is some nice text to go with them.
The New Solar System, J. Kelly Beatty and Andrew Chaikin Des, Sky Publishing Third Edition 1990. This has got to be the best
book on the solar system to date. Very up to date pictures and a text of excellent authority and detail. If you are interested in
the planets, this book is a must have.
The Planet Observer's Handbook, Fred W. Price, Cambridge 1994. As the title suggests, this is a handbook for observers. How
to details about observing by an observer are excellent in their detail and authenticity. A fine handbook.
The Great Comet Crash - The Collision of Comet Shoemaker-Levy 9 and Jupiter, John R. Spencer and Jacqueline Mitton,
Cambridge 1995. This is the greatest book I have seen on the big crash. Full of wonderful images, excellent text and carries an
authenticity that is deep and true. A fine book and a must for those interested in this event.
Atlas of the Moon, Antonin Rukl, Kalmback Books 1992. Map, maps, maps and images as well. Everything you ever wanted to
know about the moon. This is the guide for moon observers.
The Understanding of Eclipses, Guy Ottewell, Astronomical Workshop 1993. A nice small booklet about recent and coming
eclipses. Full of excellent and easy to understand diagrams. Just in time for the February eclipse. Nice book.
The Giant Planet Jupiter, John H. Rogers, Cambridge 1995. This is the giant book about the giant planet. This is a complete
book about Jupiter. There is a lot of everything you might want to know. Very excellent scientifically, very authoritative and
almost more that you want to know. But for Jupiter enthusiasts and real must have book.
Observing Comets, Asteroids, Meteors and the Zodiacal Light, Stephen J. Edberg and David H. Levy, Cambridge 1994. This
is a fascinating and well documented book about a topic that is much larger than I had imagined. Thoroughly interesting and
informative. Even though my main interest is not these objects, I find this book an excellent one. I suppose one should be more
interested since one of these objects is the most likely to bonk into us.
Pluto and Charon - Ice Worlds on the Ragged Edge of the Solar System, Alan Stern and Jacqueline Mitton, John Wiley and
Sons 1998. This is a nice book. For those who have an interest in the solar system and particularly for those that have enjoyed
the Tombaugh biography it is a must read. This book is all about Pluto and its companion. It is well written and tells possibly
more that you wanted to know about this "minor" planet. Never-the-less a good and relatively easy read. Pluto will always be a
planet, and one with a special place for me, even though some think otherwise.
Volcanoes of the Solar System, Charles Frankel, Cambridge 1996. A really thorough discussion of volcanoes on earth and other
bodies of the solar system. A fine and authoritative book on volcanoes. While most are on earth, those on other bodies have a
great resemblance. Interesting book.
Meteorite Craters and Impact Structures of the Earth, Paul Hodge, Cambridge 1994. This is a true encyclopedia of impact
structures. All known impacts are discussed in detail. photographs, maps and detailed descriptions are included. An interesting
book about the structures and impacts that might just do us in sometime.
Everybody's Comet - Comet Hale-Bopp, Alan Hale, High-Lonesome Books 1996. This is a little book in paper designed for the
layman. It starts with what is a comet and includes are rather detailed exposition of how the comet was found through predictions
of how it will do in the year to come. (1997) A quickly put out and rather shallow book.
The Comet is Coming!, Nigel Calder, Viking 1981. This book is about Halley's comet of 1985-6. This is a book for the layman
as intended. It has all the usual stuff about dirty snowballs and a few of the too close encounters with the earth. It is ok for the
layman but not much else.
Comet of the Century, Fred Schaf, Copernicus 1997. This book is a bit more comprehensive that most others on comets. It is
heavy on the narrative with few diagrams or images. But it does discuss many comets since 1700 and through to Hale-Bopp,
including a section on Hyakutake. The book is interesting but somewhat of a bore to read since it is so poorly illustrated.
Observing and Photographing the Solar System, Dobbins, Parker and Capen, William-Bell 1992 (rev). This is subtitled "A
Practical Guide for the Amateur Astronomer." In every way this is a superb book. The details about all of the equipment required
and how to use it are covered at length. Numerous fine images of the planets are included. There is an extensive discussion
about the planets, how they appear and how to observe them intelligently. The techniques are important to amateur
photographers since relatively modest array of equipment is required and well within the reach of many amateurs. This is a very
fine reference book.
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Books About the Milky Way Galaxy
The Milky Way, Bok and Bok, Fifth Edition, Harvard 1981. This is the fifth edition of a book of historical stature. The first edition
appeared in 1941. The book has been a standard reference for those many decades and was once again revised in 1981. This is
a true textbook and bears the authenticity required of such a book. This is not a casual read but one to have as a reference for
almost anything you ever wanted to know about our own galaxy.
Galaxies, Howard Shapley, Third Edition, Harvard 1972. This edition of the book, first published in 1943, was revised by Paul
W. Hodge in this latest edition. This is a text about not only the Milky Way Galaxy, but about the nature of galaxies in general. It
is most interesting to read about galaxies in the words of one of the great astronomers of all time.
Galaxies and the Universe, David Eicher ed, Kalmbach 1992. Subtitled, An Observing Guide from Deep Sky Magazine. What
can I say but this is a really nice collection of articles from Deep Sky. Nice images, nice discussion and just a nice book.
Beyond the Solar System, David Eicher, Kalmbach 1992. Anothe great collection of images with a brief but informative text
with each. Really nice section of color images. This is another nice volume put together by Eicher.
A Photographic Tour of the Universe, Gabriele VAnin, Firefly Books 1996. Now this is a major book of images from the solar
system to deep space. This is a collection of images to thrill anyone interested in astronomy. One of the best. This marvelous
collection of images is accompanied by a modest but quite informative text.
Hubble Vision, Astronomy with the Hubble Space Telescope, Carolyn Collins Peterson and John C. Brandt, Cambridge 1995.
There have been many Hubble images and compilations. Non has been as good as this one. The book has a story of the design
and development of Hubble as well as a collection of images that is simply great. The book concentrates on the images of great
importance in advancing our underatanding the the universe. This is a must have book.
Hubble's Universe, A Portrait of Our Cosmos, Simon Goodwin, Penguin Studio 1997. A smaller book that the above and a bit
less ambitious. It must be considered a complement to the bigger Hubble Vision book. It is a nice book but pales compared to
the bigger work. I would consider it in addition to but not a substitute for the above.
Stars and Galaxies, Astronomy's Guide to Exploring and Cosmos, David Eicher ed, Astronomy 1992. Another of Eicher's efforts
proves a winner. This is really the book for persons who want to view the Cosmos as the title suggests. Excellent pictures and
maps of the major objects in the sky. I recommend it highly. Anyone can learn a lot from this marvelous presentation.
The Guide to the Galaxy, Nigel Henbest and Heather Couper, Cambridge 1994. This book means by Galaxy, THE Galaxy. That
is the Milky Way Galaxy. It is the best I have seen in terms of both text and images of our galaxy. The presentation is clear, the
science correct and up to date and the images quite inspiring.
The Milky Way Galaxy and Statistical Cosmology 1890-1924, Eric Robert Paul, Cambridge 1993. This is a high technical and
highly detailed book about how views of the structure of our galaxy changed over the years in question. Only those with a deep
interest in how our sun's position in our galaxy has changed, certainly an important topic, will bee able to plow through this book.
It is tough going and at the same time a brilliant example about hard work changes our view of the universe.
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Books About the Universe
Galaxies, Howard Shapley, Third Edition, Harvard 1972. This edition of the book, first published in 1943, was revised by Paul
W. Hodge in this latest edition. This is a text about not only the Milky Way Galaxy, but about the nature of galaxies in general. It
is most interesting to read about galaxies in the words of one of the great astronomers of all time.
The Crab Nebula, Simon Mitton, Scribners 1978. Here is a little book about one star. Or, what was a star at one time. This is a
good book for those who want to really study the history of a star and its remains. It is quite technical. Never-the-less it is a
worth while study for the amateur even if it is a bit advanced. It demonstrates how much information can be gained from an
event, even a thousand years after it occurred.
The Quest for SS433, David C. Clark, Viking 1985. This book is subtitled, Discovery of the Astronomical Phenomenon of the
Century. Indeed it is. The discovery and analysis of this event is in many ways like that of the Crab nebula. It is a constant
amazement that one event, a very special one at that, can yield so much information and change the view of stars formation and
destruction. An interesting book for those interested in super stars.
Galaxies and the Universe, David Eicher ed, Kalmbach 1992. Subtitled, An Observing Guide from Deep Sky Magazine. What
can I say but this is a really nice collection of articles from Deep Sky. Nice images, nice discussion and just a nice book.
Through the Eyes of Hubble - The Birth, Life and Violent Death of Stars, Robert Naeye, Kalmbach 1998. This is a
wonderful book. The images are the very latest from Hubble and concentrate on the supernova. It is a large format book, not
very thick, full of the most startling and magnificent of the Hubble images. There is an excellent text to go with the images. I
recommend this book highly even to those who already have several books of Hubble images.
A Photographic Tour of the Universe, Gabriele Vanin, Firefly Books 1996. Now this is a major book of images from the solar
system to deep space. This is a collection of images to thrill anyone interested in astronomy. One of the best. This marvelous
collection of images is accompanied by a modest but quite informative text.
The Astronomical Companion, Guy Ottewell, 1979-1993. This is an interesting booklet. It is large format bu thin and in
paper. It take on a unique way to look at the universe. It is highly oriented toward viewing the universe in spatial dimensions.
Large maps of the solar system, galaxy, clusters of galaxies and stars, going out to the edge of the known universe are presented
in a spatial perspective. One of the most different and interesting booklets I have ever found. It is a must for those who like to
take an overview of the big plan of the universe.
Hubble Vision, Astronomy with the Hubble Space Telescope, Carolyn Collins Peterson and John C. Brandt, Cambridge 1995.
There have been many Hubble images and compilations. None has been as good as this one. The book has a story of the design
and development of Hubble as well as a collection of images that is simply great. The book concentrates on the images of great
importance in advancing our understanding the the universe. This is a must have book.
Hubble Vision, Further Adventures With the Hubble Space Telescope, Carolyn Collins and John C, Brandt, Cambridge
(second edition) 1998
Sort of a second edition but also a complete rewrite of the first edition reviewed just above. The book has been totally revised to
include the latest images and latest astronomical discoveries from Hubble observations. Still full of the most beautiful and
amazing images, it has much more text and theory about recent discoveries and theories. Again, a must have book.
Hubble's Universe, A Portrait of Our Cosmos, Simon Goodwin, Penguin Studio 1997. A smaller book that the above and a bit
less ambitious. It must be considered a complement to the bigger Hubble Vision book. It is a nice book but pales compared to
the bigger work. I would consider it in addition to but not a substitute for the above.
Stars and Galaxies, Astronomy's Guide to Exploring and Cosmos, David Eicher ed, Astronomy 1992. Another of Eicher's efforts
proves a winner. This is really the book for persons who want to view the Cosmos as the title suggests. Excellent pictures and
maps of the major objects in the sky. I recommend it highly. Anyone can learn a lot from this marvelous presentation.
The Guide to the Galaxy, Nigel Henbest and Heather Couper, Cambridge 1994. This book means by Galaxy, THE Galaxy. That
is the Milky Way Galaxy. It is the best I have seen in terms of both text and images of our galaxy. The presentation is clear, the
science correct and up to date and the images quite inspiring.
Hubble - A New Window to the Universe, Daniel Fischer and Hilmar Duerbeck, Springer 1996. Of the many books about the
Hubble Space Telescope, this one is definitely in the running as one of the best. It has good textual material and the same
collection of the best of the Hubble images. There is a well organized discussion of the design and building of Hubble as well as a
vast collection of great images. The text is intelligent and informative. One of the best of the Hubble based books.
Splendors of the Universe, A Practical Guide to Photographing the Night Sky, Terence Dickinson and Jack Newton, Firefly 1997.
A good, even wonderful, book about photographing the night sky. Images from the near to far objects are included with practical
advice about how to photography them. This book is not just a picture book but a fine how to do it book. I recommend this one
highly for anyone who wants to try this most difficult type of photography. The book will be helpful.
The Hubble Atlas of the Galaxies, Allan Sandage, The Steinhour Press 1961-1984. This is the famous Hubble galaxy book
completed by his student and colleague Allan Sandage. An outstanding collection of images of galaxies and the systematic
classification of them using the Hubble methodology. A book of great historical interest as well as a text for learning about
galaxies. This is a big, big book which is a joy to the eyes and mind.
The Universe and Beyond, 1992 revision, Terence Dickinson, Camden House 1995
This book is to a strange collection of fine images and rather simplistic text. It is a clear popularization of complex ideas and
concepts. It is ok but has never thrilled me.
The third edition of this book (1999) is an improvement over the 1992 edition. It is modern in that it includes significant
Hubble images. Some of the text and general structure have been revised. The author has a fine technique for describing complex
astronomical concepts in understandable terms. While the book is still for the beginner, it is a good introduction to astronomy of
the solar system and deepest space. This is a welcome and very nice revision. In the new edition, the publisher, Firefly Books of
Canada, has done an excellent job of
producing a good looking book.
The Supernova Story, Laurence A. Marschall, Plenum Press 1988. This is a wonderful book about Supernova 1987A and also
about superstars in general. This is a moving, interesting discussion about the useful information that these uncommon
superstars can provide. It is thoroughly authoritative. A find book.
Supernova-The Violent Death of a Star, Donald Goldsmith, Oxford 1990. Paperback. Devoid of charts and images though with
a fair text. This book can be well overlooked.
The Big Bang Never Happened, Eric J. Lerner, Vintage 1992. Says Who! A paperback which expounds some unusual theories
of the universe. If he is right he will be the only one. An amusing little book to read.
Pathways to the Universe, Francis Graham-Smith and Bernard Lovell, Cambridge 1988. This is a book about everything. It has
a different viewpoint from most books on everything about the universe that is quite charming. The discussion is about divers
thing from radio astronomy and the moon to deep space objects and what they tell us about the design of the universe. The
illustrations are excellent and the text very readable. This is a fine book for someone interested in an overview of astronomy.
This overview is especially interesting and written with a tone of contagious enthusiasm.
Stars, Nebula and the Interstellar Medium, C. R. Kitchin, Adam Hilger 1987. This is a text like book. It is heavy duty, full of
equations, graphs and complex discussions. It does tie together the results of observational astronomy with astrophysical theory.
Not for the novice.
Planetary Nebula, Steven J. Hynes, William-Bell 1991. This book is subtitled, A Practical Guide and Handbook for Amateur
Astronomers. Well it may be for amateurs, but only those with a deep, deep interest in planetaries. It is practical, it has
extensive tables, extensive drawings and a few so, so photographs. About half the book is devoted to tables and locator charts .
Essential to those pursuing planetaries.
The Messier Album, Mallas and Kreimer, Sky Publications 1978. A nice, brief description of Messier and his work in compiling his
list of objects. At the same time a great collection of good, but not spectacular, images of all of the Messier objects with a short
description of each. Nice set of locator charts with a realistic assessment of imaging the objects. Most of the images are made
with a 12 1/2" reflector, well within reach of the serious amateur. A nice book for those interested in the fail fuzzies out there.
Messier's Nebula and Star Clusters, Kenneth Glyn Jones, Cambridge 1991. This is a big book which is sort of like a grown up
version of the "The Messier Album." It has essentially all of the things you might want to know about the astronomer, Messier,
and his friends. There is a really fine set of charts for locating the object and an authoritative discussion of them with good
references. The objects are ordered, classified, cataloged and described in relation to each other. There is also a set on nice
photographs of them. Of special value is a paragraph on each describing how to observe them.
Modern Cosmology and the Dark Matter Problem, D. W. Sciama, Cambridge 1993. As a lecture in physics and on a new and
complex theory in cosmology, this is not an easy book. In fact it is a tough treatment of the observations that have forced
cosmologists to recognize the existence of dark matter. It is a high level, difficult book.
A Short History of the Universe, Joseph Silk, Scientific American Library, 1994. This book is part of a large series of books on
scientific subjects from the Scientific American Library. It is a fine, authoritative and beautifully produced book. It is a modern
overview of the evolution of the universe that anyone can read with ease.
Stars, James B. Kaler, Scientific American Library 1992. This book is beautifully produced, as are all the books in this series. It
discusses the role stars play in the evolution of the universe, the creation of elements and the structure of the universe. A nice,
not too difficult book which I found enjoyable.
Cosmic Clouds- Birth, Death and Recycling in the Galaxy, James B. Kaler, Scientific American Library 1997. This book goes
into some depth on the topic of the material development of the galaxy. The physics and chemistry of star formation and the
cycling of material in the galaxy are covered with authority. The book is beautifully produced.
Gravity's Fatal Attraction, Mitchell Begelman and Martin Rees. Scientific American Library 1997. This book gives a rather
detailed but understandable discussion of the effect that gravity has on the formation of stars and other objects in the universe. It
is a nice book, beautifully produced, which rounds out the series on astronomy within the Scientific American Library series.
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Books With an Emphasis on the History of Astronomy
Blind Watchers of the Sky, The People and Ideas that Shaped Our View of the Universe, Rocky Kolb, Addison Wesley
1996
A rather light weight treatment of the history of astronomy and our understanding of the universe. Sort of a tour of astronomy
from Tycho to the big bang.
Skywatchers, Shamans and Kings, E. C. Krupp, Wiley 1997
The influence of astronomy on archaeology, or perhaps vice versa. Interesting light reading.
In Search of Planet Vulcan - The Ghost in Newton's Clockwork Universe, Baum and Sheean, Plenum Trade 1997
This is a nice little book and an easy read about the search for a planet supposedly causing perturbations of Mercury's orbit. Even
though we know the result, it is a nicely detailed account of the predictions of Le Verrier and the attempt by his followers to find
this elusive planet. The hundred year search is a study in personalities, factions, fabrication and stubborn refusal to give up the
futile search. There is a large section on W. Watson an astronomer with connections to the Universities of Michigan and Wisconsin
who was a diligent searcher for Vulcan..
Pluto and Charon, Alan Stern and Jacqueline Mitton, Wiley Interscience 1998
This is a delightful book. It gives a history of the discovery of both Pluto and Charon, a very detailed and scientific discussion of
analyzing the size and atmosphere of these objects and a nice discussion of the formation of the planets and our solar system.
This is a readable book but has solid science underlying it.
Starseekers, Colin Wilson, Doubleday and Company 1980. This is a history of mankind and his looking to the stars. Starts with
Stonehenge and ends with a whimper sometime in this century. A so so book that adds very little to our understanding of the
history of astronomical activities.
The Astronomers, Companion book to the PBS television series. Donald Goldsmith, St. Martin's Press 1991.
This is a book that defies being categorized. It goes with the television series. That's it. It's ok if you like the series.
Greenwich Time and the Longitude, Derek Howse, Philip Wilson Publishers 1997
This is a delightful book for anyone interested in the establishment of time over the centuries. The emphasis is on the Greenwich
observatory and their roll in setting time standards over the world. The chronicle covers time measurement from the hourglass to
the cesium clock. It is loaded with information about the technology of time measurements and the lives of the people who
improved the measurement of time over the past several centuries. This is a very nice book.
The End of the Dinosaurs, Charles Frankel, Cambridge University Press 1999. This in small but chock full of information book
about the latest theories of how the dinosaurs vanished. It establishes ties between many known major impact craters on the
earth's surface with major changes in the earth's development. Major astronomical catastrophes seem to have caused major
catastrophic steps in the development of the flora and fauna of the earth over the past 500 million years. A fascinating book, well
written, authoritative and easy to read. Highly recommended.
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Return to Tubes and Adapters Index
Discussion of Back Plane Opening
on the LX200 (10" and 12")
Included here, at the end, is a discussion of Diagonals, the connecting tubes and other
considerations: (click here for Diagonals)
For a related discussion of Eyepieces and field stops, see elsewhere on this web site:
Field of View for Eyepieces
I have made precise measurements on the back plane aperture and the sizes of several attachments to my 10" LX200. (same
dimensions for the 12") I have also carefully viewed the image with a low power eyepiece to get an estimate of the actual
amount of vignetting. The purpose is to get an estimate of the available image size and how it is limited by image quality and
vignetting.
Here are the dimensions: The size of the aperture in the back plane is 57 mm. The normal adapter plate supplied has a thread for
Schmidt adapters. The aperture in the Schmidt thread adapter is 38 mm. The large aperture on the rear of the tube is thus
greatly constricted by the normal Schmidt adapter opening. This is because the Schmidt thread, while somewhat standard on the
back of SCTs, was originally designed for small telescopes. Several manufacturers provide attachments for the larger telescopes
which have a larger apertures. The JMI focuser can be obtained with an adapter that has a 51.2 mm aperture. They recommend
this adapter when mounting their focuser on the larger SCT. Their focuser itself has an aperture of 50.6 mm. (basically 2 inches)
The aperture on a 2 inch eyepiece is slightly smaller. Generally 48.5 mm. A 48 mm filter has a clear aperture of 43 to 44 mm. A
"T" adapter tube (photographic style) has an aperture of 37 mm. The 0.63 focal reducer has a clear aperture of 41 mm.
So, what do all these figures mean? It is clear from the numbers that all so called 2 inch equipment attached to the rear of the
SCT has its light path considerably constricted by the 38 mm Schmidt thread aperture. So there seems to be good reason to open
up the back plate to a full 2 inches as many accessory manufacturers have done. Another question arises when doing 35 mm
photography at prime focus. Is there any vignetting at the corners of the image? The 38 mm aperture of the "T" thread is quite
marginal when one realizes that the diagonal of the 35 mm frame is 43 mm. In fact the "T" adapter was from the beginning
recognized as marginal in size.
Incidental dimensions: The thread of the Schmidt connection is 50 mm diameter with a 1 thread per mm (1.00 pitch). The
thread on the T adapter is 42 mm diameter with 1.333 threads per mm (0.75 pitch) The tread on a 48 mm filter is of course 48
mm diameter.
To judge vignetting one must take the following viewpoint. Think of looking from a point on the film plane toward the lens.
Vignetting will not occur as long as the entire rear exit pupil of the lens is visible. Since any 35 mm camera lens is a very thick
lens, the rear exit pupil is well toward the rear of the lens and often quite small (an inch or so). Thus even from the corner of the
film plane, all of it is generally visible through the "T" mount aperture. If the rays that form the image on the film plane came from
a great distance, the aperture would cast a shadow that cut off the corners of the image. The imaging rays from a telescope come
from the secondary mirror through the center tube which forms the limiting stop. Because the rays come in very nearly parallel,
the aperture in the "T" adapter is slightly too small and there will be a some significant vignetting of the corners of the 35 mm
image.
For total freedom from vignetting, an adapter should be used which opens up the constrictions described above. Several
manufacturers supply such adapters. Lumicon being one. I inspected the real image at the back of the telescope with an eyepiece
just as one normally does when viewing but without the adapter plate. When inspecting the exit pupil it is clear that slight
vignetting starts to take place with any deviation from the optical axis at all. This vignetting is very small. I estimated it to be a
half stop to a whole stop at a distance of 25 mm from the optical axis. Moving to the very edge of the aperture, 28.5 mm from the
optical axis, was unsatisfactory since the primary showed an image of the corrector plate. Allowing these rays into the imaging
chamber would cause severe flare. It is very encouraging that a full 50 mm diameter of real image at the focal plane looks quite
free of vignetting. (possibly one stop at the most) Some other significant optical aberrations seem to occur at about the same
distance from the optical axis. But I would judge that a full 50 mm of real image is useful. We can be comfortable that this image
easily covers a 35 mm frame if it is not restricted by adapter tubes that are too small. It also means that a full size focal reducer
of 50 mm (0.63 reduction assumed) will give a 32 mm image which covers most of the 35 mm frame. The regular focal reducer
with a 41 mm clear aperture only yields a 25 mm image circle which is not enough to cover the 35 mm frame. My experience is
that with this reducer significant vignetting takes place on 35 mm film.
It makes sense to use larger diameter focal reducers for both 35 mm film and especially for 6 X 6 cm. size film. Lumicon makes
such a focal reducer. Additionally, large rear plate adapters are made by JMI and Optec. Optec in fact makes a filter slider
adapter which takes full aperture 2" filters. Additionally, when it is so clear that the real focal plane image is so generous in size,
it makes good sense to use 2" eyepieces where the full real field of view of the telescope can be enjoyed. This is true mainly at
lower powers of course. (Generally with 30 mm and longer focal length eyepieces which have a field stop nearly 48 mm in
diameter.)
Diagonals and Their Connection to The Telescope
I have always been concerned about the connection of diagonals to the viewing back of the SCT telescopes because of the limited
size of the tubes used in their construction. In the photograph below are shown two 2" Meade diagonals. One has the attachment
tube with the ring for the Schmidt thread back plate. The plate is shown on the bottom right. The second diagonal has the
attachment for insertion into a 2" tube. This is used with the JMI focuser for example. Both adapters are shown dismantled.
That is, the diagonal is shown at the top, the tube and and ring below and in the case of the 2" tube a small adapter ring that is
used to connect the 2" tube to the diagonal. The small ring is used so that both forms of the diagonal can use the same main
diagonal body.
The reason for showing this detail is to show the internal diameters of the tubes. Both of the tubes for the diagonal are 41 mm
clear internal opening. The tube shown at the right is a Tele Vue Schmidt to T-thread adjustable adapter tube. I show this tube
because it has several interesting applications. It is intended to connect a camera with T-thread to the Schmidt thread on the
back plate adapter of course. But it also has exactly the correct dimensions so that it will accept a 2" tube. The front part is
shown below the complete adapter. Notice that it has a Schmidt thread on the front and a nice large set screw to hold any 2"
tube.
Now consider the inside dimensions of the tubes. The opening on the Schmidt adapter plate has an opening of only 37 mm. This
is one of the serious flaws in the use of the Schmidt thread. It was originally intended for small SCTs such as the 8" which itself
has a limited opening in the back plate as described above. So when a 2" diagonal is attached to the back plate with a Schmidt
adapter, the cone of illumination is limited by the 37 mm opening in the back plate. This is unfortunate with a larger telescope like
the 10" or 12" SCT since they have a larger opening in the back which would take a larger adapter tube. This is why JMI and
Optec provide back plate adapters with larger openings, to reduce vignetting.
Now the short part of the Tele Vue adapter could be used to take the diagonal with the 2" tube directly. This would enlarge
(improve) the opening from 37 mm to 41 mm. Not much, but a difference of 20% in light passage and less vignetting. This might
be helpful with the larger SCTs. Additionally, the use of the TeleVue tube provides a very nice way to adjust the angle of the
diagonal for convenient viewing. Note also that the size of the field stop with both the 40 mm and 56 mm eyepieces is about 46
mm. Thus for these longer focal length eyepieces with the large field stops, the cone of illumination is definitely limited by the
connecting tubes. Now the ideal situation would be to use the JMI back plate which has a full 50 mm opening and connect the
diagonal, or better the eyepiece directly to the back of the telescope with an appropriate sized connecting tube. The Tele Vue tube
is ideal for this application. If you use the JMI focuser you also have just this arrangement.
In summary, I suggest that the sizes of the diagonals and the connecting tubes is a restricting factor and that a larger diagonal
and connecting tube set would be better when 2" long focus eyepieces are used. I do not at this moment know of diagonals which
are slightly larger with appropriate tubing which would not vignette on the 2" eyepieces. Both the Meade and Tele Vue diagonals
are quite good however.
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Return to Tubes and Adapters
Vignetting Caused by the Limiting Aperture
of Various Tubes and Adapters.
An SCT telescope has a very complex baffling system. This is necessary to prevent light from the corrector plate from leaking directly through
to the image focal plane. Such leakage would cause the image, which is a real aerial image, to be diluted and lowered in contrast. Unfortunately
the geometry and mirror placement in a folded optical system is so restricted that these baffles also cut off some of the rays that should form
the image a distance off of the optical axis. The image is brightest on axis, at the center of the field of view, and dimmer off axis. This is a
classical case of vignetting. Note that this is not a sharp cut off of the image, but a gradual dimming of it. Unfortunately, it is a feature of folded
telescope optics which cannot be avoided. It is thus important to not exacerbate the vignetting by restricting the image forming rays
additionally with connecting tubes which are of insufficient size at the output end of the telescope. Unfortunately, all too many adapters added
to the back of an SCT do just that. This is partly the result of a decision made many years ago to use an output thread onto which most
adapters are fitted. When amateur SCT instruments were first designed they were often 4 inch or 8 inch optical assemblies. Thus their output
light beam was only slightly over an inch in diameter. The typical eyepiece was a 1 1/4 inch (32 mm). This so called Schmidt adapter thread
and its associated tube has a typical diameter of 38 mm. This is adequate for a 1 1/4 inch (32 mm) eyepiece. In the intervening years the SCT
became very popular and was made in sizes of 10, 12, 14 and even 16 inches. Such telescopes can have and indeed often do have much larger
beams of light that are useful for image formation. Most current SCT instruments have a full 2 inch (50) and some even a 3 inch (75 mm)
diameter circle of full illumination. Thus the early standard output adapter is significantly too small in diameter for the newer larger telescopes.
It both limits the size of the actual field of view of the telescope and vignettes the light beam reaching the image focal plane.
First consider the cone of illumination of the raw telescope system as shown in Illustration 0. Here we see what is apparently a satisfactory
illumination system. In fact, If you hold a piece of transparent paper behind the telescope with it pointing at a bright flat surface at some
distance, you will see and ever increasing circle of illumination as you move the paper back away from the telescope. Unfortunately, this is a
serious illusion for several reasons. As the illustration shows, only the central region of the cone of illumination is fully illuminated. The regions
outside of A and B is only illuminated by some of the rays that transit the tube.
The illumination falls off to zero at points C and D. At an intermediate point between A and C or B and D the illumination may be reduced by a
magnitude or more. Another problem is that when doing the transparent paper experiment, it is not only possible but likely that light rays
directly from the corrector plate are entering the baffle tube and striking the sheet of paper. This light is totally undesirable since it adds
background flare to the entire image. It is not appropriate to do this experiment in bright light since the eye is not as sensitive to intensity
variations at high light levels as it is a very low light levels. At first it looks like there should be no trouble illuminating a 2 inch eyepiece. But,
the above situation is greatly modified by the typical back plate on the telescope.
The above problems are indicative of the difficulty in baffling a folded optical system. The baffle tube has to be made big enough to accept the
light from the secondary mirror, but not so big as to allow rays to pass directly from the corrector plate. This is a tough mechanical design
problem and is the source of some vignetting in an even well designed folded optical system.
The vignetting phenomenon is described below in some detail for several common optical setups in the following discussion. . In Illustration 1,
the light coming toward the image is restricted only by the central baffle tube. This tube is shown for illustration to be 2 inches in diameter.
Only in larger SCT instruments will the central tube be as large as 2 inches. In the SCT, this tube is the central structure that holds the primary
mirror. The light is directed to the left from the secondary mirror (not shown). Notice that when all tubing is of at least the same size as the
baffle tube, as shown, all of the light which enters the baffle tube at the right can reach the focal plane at the left. The size of the focal plane
that is well illuminated is about the same as the baffle tube. For a focal plane larger than the baffle tube size, full illumination strikes the focal
plane on the optical axis of the system, but not all of the image forming light makes it to regions of the focal plane that are well off axis of the
focal plane. As the diagram shows, the focal plane is vignetted for regions beyond the points A-O-B. That is, the green rays reaching point O
come from the entire baffle opening at the right. Also, the red rays reaching points A and B come from the entire baffle opening. Again, the
light is not cut off abruptly, but gradually fades out to darkness.
Most often when an off axis position where the light is greatly reduced is reached, the optical aberrations also are so severe that the image is
poor to useless. But it can be seen that a telescope with a 2 inch diameter baffle tube (50 mm) can fully illuminate the field stop of a 2 inch
eyepiece. The field stop of a long focal length 2 inch eyepiece is just a bit smaller than 2 inches. The situation shown in diagram No. 1 is ideal
since the eyepiece will have a bright field over its entire apparent field because it is fully illuminated by the light beam coming down the baffle
tube..
Now let us turn to the situation in diagram No. 2. Here a stop has been placed in the light path. The stop in this case is drawn to be
approximately the size of the Schmidt threaded visual back commonly used on SCT telescopes. This tube has an internal diameter of 30 to 38
mm. The stop might also represent the tube used to connect a 2 inch diagonal with this thread to the back of the telescope. In this case, it is
clear that the center of the image at point O can still see the entire aperture at the right end of the baffle. Thus it receives all of the light rays
going down the baffle, the green rays. But as one moves off axis toward the edge of the focal plane, such as points A or B, it is clear that these
points can only see a restricted portion of the aperture at the right end of the baffle tube. This is shown by the red rays. Clearly some of the
rays are intercepted by the stop which is too small. Not all desired light rays can get to focal plane regions near A or B. Thus regions of the
focal plane away from the optical axis are not fully illuminated. They are what is called in optical parlance vignetted. The severity of the
vignetting depends on the details of the geometry of the baffle and the tubular restriction. But it is clear that any tubular opening that is
smaller than the full size of the baffle tube will cause some vignetting. In the case shown, it looks like the light will be reduced by about a factor
of 2 or 3. This means that the stars at the edge of the field will be about a magnitude dimmer than they should be.
So, rule number one is to use only full sized, in this case 2 inch, tubing everywhere in the optical path if vignetting is to be avoided. This
problem is well recognized and a number of manufacturers make adapters which provide full illumination of a 2 inch eyepiece on a typical 10 to
16 inch SCT. Notice also that if a photographic film is to be fully illuminated, a 2 inch system is essential. The 35 mm format has a diagonal of
43 mm. A system with full 2 inch tubing will cover the film properly. Also note that with small CCD chips, where the diagonal is much smaller
the problem is less severe. The chip only occupies the central region of the focal plane and is easy to fully illuminate it.
An aside to this rather obvious optical situation is that it is also wise to keep the surfaces of the tubing away from the axis of the optical system
as much as possible. The reason is that if light rays strike a surface at near grazing angles they are well reflected. The use of baffles, or
reflection reducing treatment is essential. Baffling is described in another article on this web site. Even the smaller 1 1/4 inch eyepieces can
benefit from larger connecting hardware.
Diagram No. 3 shows the same baffle tube, but with a 1 1/4 inch (32 mm) eyepiece. It is clear here that the vignetting problem is much less
severe. Again, it must be emphasized that vignetting of the illumination is not an abrupt cut off of the image. It is a gradual dimming of the
image as one moves away from the optical axis.
Insertion of a Lens in the Optical Path
The question often arises about what happens when an auxiliary lens is installed in the optical path. These lenses are of two types. One is a
negative lens, Barlow lens, which spreads the light beam coming down the baffle tube. This type of lens does not generally cause vignetting
since it spreads the incoming light beam over a larger diameter. The negative lens can, in fact, improve the uniformity of illumination of at the
focal plane. The result of using a negative lens is to create a dimmer image, but one that is more spread out. This results in a larger image and
an apparent extension of the focal length of the optical system. Such lenses are often used to image planets so as to obtain a larger image.
Since the planets are generally quite bright but very small compared to deep space objects, they benefit from a larger but dimmer image.
A critical and less well understood case is that which happens when a positive lens is introduced in the light beam. The positive lens is the well
known focal reducer. This lens concentrates the image into a smaller circle, making it smaller, and of course concentrates the beam of light as
well, making it brighter. This is a great advantage when trying to fit an extended astronomical object onto a small CCD chip. The focal reducer
decreases the effective focal length of the telescope and because of the concentration of the light beam, increases the effective brightness of
the image. This is a considerable advantage when trying to fit a large very dim object on a small CCD chip. With an eyepiece that has a given
diameter field stop the reducer also forces more image to fall within the field stop. Thus the actual field of view of the telescope is increased
with use of a focal reducer. This can be desirable for viewing where a larger actual field of view is often attractive. But because the focal
reducer concentrates the beam of light coming down the baffle tube it often causes quite perceptible vignetting. This effect is shown in diagram
No. 4. If the original beam of light is about 2 inches in diameter as shown in the diagram. The beam of light after passing through the focal
reducer will be less than that diameter. The full intensity beam might be only an inch or inch and a half instead of 2 inches in diameter. In the
case shown, the beam will not fully illuminate a 2 inch field stop. A long focal length, 2 inch eyepiece has a field stop nearly 2 inches (generally
about 46 mm) in diameter. There will be a severe falling off of the light in the outer parts of the field. Again note that the light is not sharply
cut off, but gradually dims as one moves away from the optical axis. This dimming can easily amount to several magnitudes light reduction.
This amount of reduction of the light intensity may not bother many viewers, but it is anathema to many others.
The above situation is even a bit worse than it appears at first. Focal reducers that screw onto the back of the telescope, on the Schmidt
thread, have an aperture of only about 43 mm. This means that the original beam is reduced from almost 50 mm to 43 mm. It is this reduced
beam which is additionally reduced by the focal reducer.
The focal reducer will generally fill the field stop of a 1 1/4 inch eyepiece. Much depends on the strength of the focal reducer. The stronger focal
reducers, like a 0.33 reducer will not even fill the field stop of the smaller eyepieces. The dilemma of this situation is clear. One can, with a not
too strong focal reducer get a fairly well illuminated field stop with the smaller eyepiece. This is nice in some cases. One can get a larger field of
view with the larger 2 inch eyepiece, but the image will be vignetted with the focal reducer in place. Still the larger field of view obtainable with
the 2 inch eyepiece is often desirable even if the illumination is not perfectly uniform over the whole apparent field of view. A 0.63 strength
focal reducer will often give a nice looking wider field despite some modest vignetting.
The best way to get the biggest actual field of view, with excellent illumination, is to use an eyepiece with a full 2 inch field stop and tubing with
full 2 inch openings. Several manufacturers make suitable adapters and/or focusers for this purpose. Examples are: Lumicon, Peterson
Engineering, Optec and JMI. However, it is also useful to use a modest strength focal reducer, say 0.63, to get a bit more actual field of view
with a 2 inch eyepiece of long focal length. The situation is quite different for a refractor, where there is not a restrictive baffle such as that
used in the folded telescope.
The amount of vignetting one gets depends on the exact optical geometry and strength of the focal reducer. But in general, a telescope that
will just fully illuminate the field stop of a 2 inch eyepiece will not do so with a focal reducer in place. The stronger the focal reduction taken,
the worse the vignetting will be. Focal reducers are meant to be used with a relatively small CCD chip where a smaller circle of illumination may
well be satisfactory. They are not satisfactory when trying to illuminate a 35 mm film frame. In this case it often looks like the picture was
taken through a port hole.
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Attaching Camera Lenses to
CCD Imagers and Other Adapters
This note describes the problems which arise when trying to attach standard camera lenses to CCD imagers and spme solutions.
Other ideas for convenient attachment of optics, cameras and imagers to telescopes are discussed as well. Photographs of some
of the adapters described here are in other articles on this web site.
A table of film to flange distances for many cameras is given at this page. Camera Film/Flange Distances
There are several problems that must be solved. One is obtaining a suitable camera flange to fit the lens mount. Modern lenses
generally are of bayonet mount type and thus require an adapter ring which mates to the lens. This ring is like the flange on the
camera. Additionally, there must generally be a mechanism that allows the lens to be stopped down. If the lens is to be used at
full aperture, the stop down mechanism is not required. Some lenses can be stopped down manually with or without the stop
down mechanism. In the case of the Canon FD lenses, which I use, such a mechanism is required.
Another problem is to adapt the camera flange to a T-thread adapter which fits the CCD imager and still keep the total thickness of
the adapter short enough so that infinity focus of the lens will allow it to focus on the CCD chip. This requires a very short adapter
in most cases.
A knowledge of the camera flange to film (focal) plane distance is necessary and a way to make a male T-thread on the back of
the adapter is required. This is because CCD imagers generally have on their front surface a female T-thread. Unfortunately, the
imaging chip in the CCD imager is some distance behind the front surface of the T-thread. In the 416XT this distance is 15.5 mm.
In the ST-7 it is 23 mm. In the 216XT it is only about 10 mm since the 216XT has no shutter mechanism. This means that the
adapter can only be a thickness equal to the camera flange to film distance minus that required for the CCD imager.
I have made a number of adapters for Canon lenses. These are among the most difficult since the Canon (FD type lenses) flange
to film distance is only 42.1 mm. This is among the slimmest of all modern 35 mm SLR cameras. (See list attached at the end of
this note.) It is so short in fact that even a Nikon lens to Canon camera adapter can be made. This adapter is only 4.4 mm thick
and has both the male Canon bayonet and the female Nikon bayonet within its structure. This is a real tour de force in adapter
ring design. I have one so I can put a Nikon 500 mm f 5 mirror lens on a Canon FD camera.
Most adapters for lenses for other cameras are not quite as difficult as those for the Canon because the flange to film plane
distances are larger and thus the lenses shorter. The easiest lens to adapt is a T-mount lens. The standard T-mount to film
plane distance is 55 mm. Thus all that is required is a straight through female T to male T tube. T- thread lenses are now quite
rare because the T-mount cameras are obsolete. There were never many really high quality lenses of this type made that I know
of. They were mostly cheap and of marginal quality. But you might want to search out camera stores and be lucky enough to find
one of the few good ones made. Do not underestimate the quality of the lens required for CCD imagers. A 10 micron pixel CCD
can resolve 100 lines per mm. So you want a lens significantly better than this resolution and longer focus lenses that do this well
even on axis were not all that common until the last 15 years or so.
Usually, it is desired to place a good quality 35 mm lens on the CCD imager. I will describe below a method to do this which
produces, in my opinion, a professional quality adapter. It is so difficult and time consuming to produce the bayonet mount for
any good lens, such as Canon or Nikon, that this part, the bayonet mount itself, must be purchased. The preferable product to
purchase is an adapter which has a quality bayonet mount and the mechanism for stopping down the lens manually. The rear of
the adapter would ideally have a T thread and the problem would be solved. Unfortunately, such adapters are not made to the
best of my knowledge. I have not been able to find them. I have more recently heard that some CCD imager manufacturers do
supply such adapters for a limited number of lenses. The reason is that all camera flange to film distances are much less than the
T-thread distance of 55 mm. Of course, the 55 mm was chosen so that T-thread lenses could be adapted to any SLR.
However, it is quite possible to obtain adapters for specific lenses which adapt the lens to a standard C-mount which is used on
many movie and video cameras. This is the adapter type to purchase and if it is made right, the C-thread will be on a removable
element. Such is the case for an adapter made by NPCM and available through almost any good camera dealer. They will most
likely have to order it for you.
This adapter has a removable C-thread piece on the back leaving a flanged hole 38.2 mm in diameter and 3.9 mm deep with three
set screws in its rim. Into this flanged hole a turned piece can be attached which carries the male T-thread required to mate with
the CCD imager female thread.
This piece needs to have a thread which is 42 mm diameter by 0.75 mm and 5.0 mm thick. (the T thread) And then a step down
to 38.2 mm for a distance of 3.8 mm. This small stepped (donut shaped), threaded ring is inserted in the flanged hole and
tightened with the three set screws. And there you have it. A perfect adapter for the selected bayonet lens to T-thread. This
small part can be made of brass which is very easy to machine.
I believe the above described adapter is designed and made in about the easiest way possible. Anyone with a small lathe can
make it easily. I have made several for myself and friends. I use another adapter with the original C-thread to attach Canon
lenses to my video cameras. One can also make the bayonet camera mount the basis for several adapters for CCD, video or
standard cameras to the telescope by using the Schmidt to T-thread tube with a standard T-thread to camera adapter ring.
These are quite common. Then any of several receptors, camera, CCD, video camera or other can be quickly bayoneted to the
telescope. Of course all the receptors should be made par-focal.
For example, I have made up several par-focal eyepieces by mounting an eyepiece in an appropriate length tube and adding to the
front a camera bayonet flange. Then one can focus visually, bayonet off the eyepiece and bayonet on the receptor. This is
especially convenient with the Canon bayonet mount since it is of the breach mount type. That is, it has a bayonet flange which is
tightened with a rotating ring similar to the breach of a cannon. But other bayonet mounts are also very good. I made most of
the attachments described when I got tired of having to work with Schmidt style rings in the dark.
There are many photographs of these adapters included in other articles on this web site.
Here are some flange to film plane distances.
Leica (screw)
28.8 mm
Leica (M bayonet)
27.8 mm
Canon (screw)
28.8 mm
Canon (FD and earlier)
42.1 mm
Nikon
46.5 mm
Minolta
43.5 mm
Pentax K
45.5 mm
Exacta
44.7 mm
Alpa Bayonet
37.8 mm
Contarex
46.0 mm
Contax RTS
45.5 mm
Ikarex BM
44.7 mm
Konica Autoreflex
40.5 mm
Miranda
31.5 mm
Olympus OM
46.0 mm
Praktica/Pentax*
45.5 mm
Petri Bayonet
45.5 mm
Rico Bayonet
45.5 mm
Rollei 35
44.7 mm
Topcon DM
44.7 mm
Voigtlander
44.7 mm
Yashika FR, FX
45.5 mm
* also Alpa 2000 Si, Argus, Chinon, Contax D and S, Cosina, Edixa, Fujica, GAF, Ikarex TM, Mamiya/sekor, Petri, Pentacon, Ricoh,
Spiraflex, Vivitar, and Yashica SLRs with M42 Universal mount.
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Rotation of Star Fields & the Use of De-Rotators in
Alt/Azm Mounted Telescopes
The following is a long note on guiding and de-rotating. Also discussed is the use of two or more tubes for guiding and imaging on
the same mount including considerations of wedge mounting, de-rotating and piggy back camera mounting. The several possible
cases are discussed in detail starting with simple cases and going to the more complex. These thoughts are based on experience
with my two LX200 telescopes over the past three years but apply to any telescope.
Several ideas for guiding are presented for cases where a camera (imager) is used piggyback and the main tube is used for
guiding, the main tube is used for imaging and a separate tube (telescope) fastened to it is used for guiding or some similar
combination. This does not preclude using several imagers on several tubes with a guider on another tube all on the same mount
and possibly all in use simultaneously. The advantage of using a separate guider is apparent to anyone who has ever used an off
axis guider. The separate guider tube can be of fast focal ratio so as to get a bright guide image, its axis can be adjustable and so
be moved a bit from that of the imaging tube so as to center the guide star on the guider chip and an independent, stand alone
guider chip can be used in its independent operational mode. It is very convenient to have guider and imagers independent. Of
course the ST 7/8 offer additional options. This arrangement is not discussed here.
To start, I assume accurate POLAR alignment of the RA axis. I assume that the guider, whatever type is used, finds a star and is
working correctly. These assumptions insure that the guide tube is pointed at a star and locked onto it. This is the most straight
forward setup and is relatively easy to establish when a reasonably fast guide tube is used. I use a C5 on both my 12" and 10"
LX200s. If polar alignment is not excellent, some rotation of the field will take place. (The case of Alt/Azm mounting is considered
as well later.) I also assume for this discussion that the atmosphere causes no distortion of the celestial sphere. This assumption is
not quite correct, so this issue will be discussed later.
For the ideal case, the declination is fixed and the telescope needs only to track the RA perfectly. There will be no de-rotation
required and a de-rotator is not needed. If the telescope moves precisely in RA no guider correction is required either. This
situation could happen accidentally but is not typical by any means. Now assume that the telescope does not move precisely in RA.
Typically worm drive defects are enough to cause significant RA rate deviations. The LX200 has a fine system for correcting
periodic worm drive defects which can reduce them by 10 or 15 to one. For example 50 arc seconds worm induced wobble and be
reduced to 3 or so through a training program. Even with a well trained drive a guider will be required to reduce RA wobble to 1
arc second for high precision imaging. This has been done manually in the past and now, with a guider, can be done automatically.
If there is no distortion of the celestial sphere by the atmosphere, then the guider tube can be pointed at a declination or RA other
than that of the imaging tube and guiding will still be perfect. This is true since the undistorted celestial sphere in its entirety
moves with the same angular rate in RA everywhere. Even with atmospheric distortion, a guider pointed within a few degrees of
the imaging tube will provide excellent guiding. Also note that this assumes that the guide tube and the imaging tube are rigidly
held with respect to each other. Considerable care must be exercised to insure mechanical rigidity of the tubes.
A counter example can be imagined by suggesting that this scheme will not work with the guide tube pointed at the pole star. And
sure enough it will not. So what is wrong? It is this. The effective length of the guider telescope is longer when it is pointed at a
guide star that is moving the greatest linear amount for a given angular motion of the celestial sphere. This linear amount is
proportional to the sine of the angle from the pole to the declination being guided upon. When pointing the guide telescope at the
pole star, the motion of the star is nearly zero and the guider fails. To get the best accuracy with the above conditions, the guider
should be pointed to a declination 90 degrees from the pole. Then, the guider will be locked to the celestial sphere where the
linear motion of the guide star is great and the guiding will be accurate and all will be well.
Even if the polar alignment is perfect and the guider is perfect there is a complication. That is the atmosphere. Except at the
zenith, the atmosphere distorts the position of the stars in the celestial sphere as it is seen from the telescope position. Thus as a
star moves from near the horizon to a position higher in the sky and again toward the horizon, its motion is not perfectly regular in
angular rate of RA. nor does it maintain exactly the same declination. The amount of this deviation while quite small requires
keeping the guide star near the star field being imaged. The LX200 has a first order correction for this effect in its computer. This
is why the correct latitude and longitude must be entered into the computer to insure refraction correction that relates exactly to
the local horizon and thus insure pointing accuracy. The two tubes do not have to be exactly axial aligned. Usually, imaging is
done away from the horizon because of light pollution so the atmospheric distortion is usually not a primary consideration. In
summary, polar mounting reduced or eliminates principle tracking defects and is thus very attractive for imaging. Several tubes
can be pointed from the same platform and all will function well with a single guider.
A separate but important consideration is the focal length (more importantly, the effective focal length) of the guider tube
compared to that of the imaging tube. The guider nominally cannot point more accurately than the angle subtended by one pixel
within the guider image. Sophisticated software might improve this resolution in some cases. The focal length of the guider tube
should be similar to that of the imaging tube. It is usually recommended that the guider optic be at least 1/2 the focal length of
the imager. For example, I use a C5 (fl = 1300) for my 10 inch f6.3 (fl = 1200) and also on my 12 inch f10 (fl = 3000). I feel this
is an adequately long guider focal length. Any shorter focal length imaging lens such as might be used on a typical piggy back
camera would be guided easily.
Now, what about the Alt/Azm mounting for which celestial field rotation is a problem when imaging. Normally a de-rotator is
necessary. The rate of de-rotation is a complex function of the altitude and azimuth to which the telescope is pointed. A table of
de-rotation rates is attached at the end of this note. The table is normalized and so the numbers must be multiplied by the local
rotation rate which is 15.2 times the cosine of the latitude. Values above 80 degrees are not given since rotation becomes very
large near the zenith. The formula for the rate of rotation of the star field is rate = (const.) X cos(latitude) X cos(azimuth) / sin
(zenith distance) The rotation goes to zero at azimuth 90 and 270 degrees (due East and due West) and on a line connecting
these points. Even on this line the rotation becomes singular (infinite) at the zenith. At the pole the rate is 360 (approx.) degrees
per 24 hours. At other points the constant must be calculated for the observers location.
The rate of rotation is seriously large, so that only very short exposures are possible. Depending on your desire for perfection,
exposures of only a few seconds up to a minute are possible without serious rotation errors. If a de-rotator is working correctly is
will take care of all the calculations and turn the imaging surface at the correct rate as long as it knows where the telescope is
pointing. I am told that the de-rotator on the 16" LX200 does this operation very well. ( I cannot give personal confirmation since I
have not use one.) The telescope must be leveled and the correct location entered into the computer so that the telescope knows
exactly where the pole star is. This is because the de-rotation is calculated on the basis of the known declination and azimuth..
This implies that the Alt/Azm setup must have the same precision as that required for polar alignment.
There are severe limits to the use of a de-rotator. The guide star must be in the field of the de-rotator and be de-rotated with the
field being imaged. An off axis guider will work. But off axis guiding is already difficult and with the guider rotating as well may
require a triple jointed neck. The ST 7/8 cameras solve this problem by mounting the guider chip next to the imaging chip Thus it
rotates with the field and will correctly guide when using a de-rotator. This does not solve the problem of field rotation in any
other tubes or piggy back imagers that may be on the same platform.
If a series of short exposures is satisfactory, a single chip could be used as a part time guider. Appropriate software would be
required and short exposures would be necessarily acceptable. But single long exposures have better signal to noise ratios. So
using a chip to share functions as a guider and imager does not seem like a good idea. The shift and accumulate method used by
SBIG is a good, if partial solution to this problem. There are also software image processing techniques that might be applied to
combine a series of rotated images. If the guider points the telescope to the correct RA. and declination and tracks a guide star
accurately and the telescope computer knows the value of the coordinates, it can calculate the correct rate of de-rotation. The
guider tube and the imaging tube need only be aligned to the accuracy and with the rigidity required for the polar guiding case.
But conversely, they need to be set at least with this accuracy and not less. Considerations for atmospheric distortions and cetera
are the same for either setup.
Neither Polar nor Alt/Azm setups are much simpler, each than the other, to achieve the same accuracy. In one case you need the
wedge and in the other the de-rotator. One case is a well known solution the other, currently, somewhat unknown. It is my
opinion that for a permanently mounted telescope the polar mount is certainly the simplest and the least problematic. That is
because it is a simple, well understood and versatile solution to imaging guiding. For a moveable telescope that has to be reset for
each imaging session I still think the polar wedge solution is the best. Alignment, leveling and the like are not that much more
demanding for polar as for Alt/Azm setup I have chosen not to use a de-rotator and have opted for a wedge for both my
permanent and trailer mounted telescopes. Thus the following opinions my be somewhat biased.
The de-rotation is limited to the main tube for which the de-rotator is designed. The piggyback image is not de-rotated. Use of a
separate guider telescope is not possible. De-rotation does not work well near the azimuth because the rate of rotation becomes
very large. The clearest part of the sky, at least 30 degrees wide, is lost to imaging because of mechanical problems of the derotator clearing the fork. Of course the pole region is lost to some polar mounted telescopes for the same reason. But the polar
region is not of as much interest and it is at the zenith that the sky is the clearest and imaging gets the best results. The derotator is just another complex mechanical mechanism with bearings, motor, computer software and the like which can fail to work
precisely.
My conclusion is that a de-rotator should be the system of last resort for imaging. Since this was written, a de-rotator for the
LX200 telescopes has become available. I have chosen to pass on it for some of the above reasons and others mentioned below.
There is considerable added extension to the rear of the telescope (about 75 mm). The attachment uses the standard Schmidt
thread which is of small diameter (internal tube opening of about 34 mm) which vignettes 35 mm format. It would be fine for CCD
chip sizes. For imaging that requires guiding, it must be done with an off axis guider that rotates with the imager or with the ST
7/8 two chip imager. In the case of the LX200 series, the PEC does not work in Alt /Asm mode. The angle of exclusion at the
zenith is stated to be as large as 40 degrees from the zenith limited by the physical arrangement of the tube and or fork. .
Those interested in the field rotation problem when using a telescope in Alt/Azm mounting might find the following calculations
interesting. I have calculated the rotation rates using the formulas given in Meeus. The reference is Meeus, "Astronomical
Algorithms", Chapter 13. Using the concept of the Parallactic Angle this reference explains rotation. The discussion is quite brief
and thus not a clear as it might be.
A convenient formula which is very easy to tabulate is:
Angular rate of rotation = (a constant) X cos (azimuth angle) / cos (altitude angle)
The constant is the angular rate of rotation of the earth times the cosine (local latitude). For my location, 43 degrees, the constant
is about 11.1 degrees per hour. One must be careful with this equation since there are several singularities. i.e. points where the
cosine goes to zero. The singularity causes a line of zero rotation going from 90 to 270 degrees azimuth, which is due East and
West. This line intersects the zenith but at the same time the values of rotation at the zenith are infinite since the cosine at zenith
is zero as well. The tables, calculated with MATLAB, are attached at the end of this note. The table is for altitudes up to 80
degrees. Above 80 degrees, the rotation gets very large. Conclusions from the tables follow.
The rotation rate is smallest pointing East or West and largest pointing North or South for a given altitude. When pointing at an
altitude of 60 degrees, the rate of rotation can get to be 2 times normal (the constant in the equation above). At 80 degrees, the
rate can go to 6 times normal drift rate. That is why, I believe, de-rotators are not generally used at pointing angles closer to the
zenith than about 20 degrees.
The following table is for rotation rates below 80 degrees altitude. The values are for 43 degree latitude. Note that the values get
very large at 80 degrees and above that angle they get larger still.
Columns are Altitude ---- Rows are Azimuth
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
310
320
330
340
350
360
0
11.1
10.9
10.4
9.6
8.5
7.1
5.6
3.8
1.9
0.0
-1.9
-3.8
-5.6
-7.1
-8.5
-9.6
-10.4
-10.9
-11.1
-10.9
-10.4
-9.6
-8.5
-7.1
-5.6
-3.8
-1.9
0.0
1.9
3.8
5.6
7.1
8.5
9.6
10.4
10.9
11.1
10
11.3
11.1
10.6
9.8
8.6
7.3
5.7
3.9
2.0
0.0
-2.0
-3.9
-5.7
-7.3
-8.6
-9.8
-10.6
-11.1
-11.3
-11.1
-10.6
-9.8
-8.6
-7.3
-5.7
-3.9
-2.0
0.0
2.0
3.9
5.7
7.3
8.7
9.8
10.6
11.1
11.3
20
11.8
11.7
11.1
10.2
9.1
7.6
5.9
4.0
2.1
0.0
-2.1
-4.0
-5.9
-7.6
-9.1
-10.2
-11.1
-11.7
-11.8
-11.7
-11.1
-10.2
-9.1
-7.6
-5.9
-4.0
-2.1
0.0
2.1
4.0
5.9
7.6
9.1
10.2
11.1
11.7
11.8
30
12.8
12.6
12.1
11.1
9.8
8.3
6.4
4.4
2.2
0.0
-2.2
-4.4
-6.4
-8.3
-9.8
-11.1
-12.1
-12.6
-12.8
-12.6
-12.1
-11.1
-9.8
-8.3
-6.4
-4.4
-2.2
0.0
2.2
4.4
6.4
8.3
9.8
11.1
12.1
12.6
12.8
Below is a graphical representation of the data given above.
40
14.5
14.3
13.6
12.6
11.1
9.3
7.3
5.0
2.5
0.0
-2.5
-5.0
-7.3
-9.3
-11.1
-12.6
-13.6
-14.3
-14.5
-14.3
-13.6
-12.6
-11.1
-9.3
-7.3
-5.0
-2.5
0.0
2.5
5.0
7.3
9.3
11.1
12.6
13.6
14.3
14.5
50
17.3
17.0
16.3
15.0
13.2
11.1
8.6
5.9
3.0
0.0
-3.0
-5.9
-8.6
-11.1
-13.2
-15.0
-16.3
-17.0
-17.3
-17.0
-16.3
-15.0
-13.2
-11.1
-8.6
-5.9
-3.0
0.0
3.0
5.9
8.6
11.1
13.2
15.0
16.3
17.0
17.3
60
22.2
21.9
20.9
19.3
17.0
14.3
11.1
7.6
3.9
0.0
-3.9
-7.6
-11.1
-14.3
-17.0
-19.3
-20.9
-21.9
-22.2
-21.9
-20.9
-19.3
-17.0
-14.3
-11.1
-7.6
-3.9
0.0
3.9
7.6
11.1
14.3
17.0
19.3
20.9
21.9
22.2
70
32.5
32.0
30.5
28.1
24.9
20.9
16.3
11.1
5.6
0.0
-5.6
-11.1
-16.3
-20.9
-24.9
-28.1
-30.5
-32.0
-32.5
-32.0
-30.5
-28.1
-24.9
-20.9
-16.3
-11.1
-5.6
0.0
5.6
11.1
16.3
20.9
14.9
28.1
30.5
32.0
32.5
80
64.0
63.0
60.2
55.4
49.0
41.1
32.0
21.9
11.1
0.0
-11.1
-21.9
-32.0
-41.1
-49.0
-55.4
-60.2
-63.0
-64.0
-63.0
-60.2
-55.4
-49.0
-41.1
-32.0
-21.9
-11.1
0.0
11.1
21.9
32.0
41.1
49.0
55.4
60.2
63.0
64.0
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For a discussion about projection attachments go to: Projection Attachments
A lot of fun with video imaging of the moon, sun and planets can be had with an appropriate video camera and lens attachments.
With a sensitive camera and a fast photographic lens, brighter stars can be imaged as well.
I have chosen the GBC 505E camera as the imager and have made a number of lens attachments. The camera with the control
box I made and a 2" projection lens adapter is shown in the figure. The four projection lenses I use are small, fast and very high
quality microfiche projection lenses. These are ideal for projection magnification of the aerial image created by the telescope. The
four lenses are 10 mm, 17 mm, 22 mm and 32 mm. They are placed 250 mm from the image CCD with the tube shown. These
give magnifications of 25X, 11X, 15X and 7.8X respectively. These magnifications are just right to place planets on the 12 mm
chip in the camera. This is a very nice camera with 800 by 600 pixel resolution and extraordinary sensitivity for a video camera of
0.01 foot candle and a usable image at 0.001 foot candle. It runs on 12 volts and puts out an S-VHS signal. I have built a separate
power supply for it with full voltage regulation and a gain control. The camera can be operated at the end of 100 feet of remote
cable easily.
The camera can be used with four different lens arrangements. One is the projection tube shown in the first picture. The 2" tube
gives excellent rigidity to the structure which is placed directly into the opening on the JMI focuser. Rigidity is essential because of
the large magnifications involved in planetary imaging. The four projection lenses shown fit into the tube at the location shown by
the set screw. The lenses are inserted from the camera end and locked into place with the set screw. They are of course used in
the retro projection orientation to ensure the best possible projection quality. True projection lenses give superior quality to
eyepiece lenses which are not designed for projection use. Some of the photographic references given on this web site describe
the use of projection lenses in more detail. The projection tube itself is designed to do triple duty as a projection imager for a
Canon camera and for the ST-7 as well as for the Video camera. It is fun to image planets in real time on a video screen while
recording them on an S-VHS VTR. There are moments of very clear viewing that can then be captured and still framed.
Another mode for the video camera is to use any canon lens with it. Adapters have been made to use the standard Canon bayonet
mount on the ST-7 and the Video camera. All adapters are par focal with the Canon camera. On the right above the video camera
is show with an 85 mm f 1.2 Canon lens. It is mounted in a custom made holder that holds it firmly balanced and allows it to be
placed on a piggy back mount on the telescope. I use the Losmandy camera adapter. With a fast lens, a bright star field can be
seen and the video camera used as a finder scope. With longer telephoto lenses, the lenses themselves typically have mounding
frames built into the lens.
Finally, the Video camera can be used with standard C mount lenses. The last two pictures show the camera with a standard
Canon lens, focal length 135 mm f 2.5 and with a C mount lens of focal length 75 mm f 1.4. Faster, slower, longer and shorter
lenses are all quickly interchangeable and par focal using the adapters shown.
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A Collection of Lens Mounting Techniques
with Comments About Their Stability
This note is about mounting lenses on an OTA. In this case all lenses are mounted on a 10" LX200 using Losmandy rails and
accessories. The basic ideas are conveyed in the photographs and comments. There are of course many ways to get the rigidity
needed and one does not have to use the Losmandy system. I find it so versatile however that I strongly recommend the
Losmandy system. The real issues are to take care that the mounting is rigid, free of not only flexure but of vibration as well. In
some cases the 10" would in my opinion be overloaded with the lenses shown. But, I have modified the 10" dec bearings so they
operate smoothly even with rather heavy loads.
The photographs also show the telescope mounted only in the Alt/Azm mode. (This is only for convenience in photographing the
equipment in my basement laboratory.) This is not a big issue for the photographs, but the mounting of large lenses in the Polar
mode requires careful balancing of the entire structure. This is sometimes not easy with what are rather large and asymmetrical
loads but it can be done. For long exposure piggy back photography of course one needs to use the Polar mounting mode.
I might add, that I have been generally satisfied with the mounting methods for all of the lenses except for the 400 mm f2.8. This
lens weighs 11 pounds and really overloads both the 10" and 12" LX mounts. General dissatisfaction with mounting heavy lenses
and cameras on the OTA is one factor that has lead me to design a new mount to be used entirely for photography and imaging.
Below left is shown a 200 mm f 2.8 lens with the camera body mounted directly on the Losmandy camera adapter plate DCM.
This is a satisfactory mounting provided that the camera body is strong and has a very flat bottom so that it mounts without
rocking motion on the plate. Some cameras, like the Canon shown are very good in this respect while others are not. I would not
recommend mounting a lens longer that a 200 mm in this way.
On the right is shown a Canon 200 mm f 4.0 lens with the lens mounted on the adapter plate and the body mounted on the lens.
Note that is this case, the lens itself has a very fine rotating mount so that the camera can be held in vertical or horizontal
positions. This way of mounting a good quality 200 mm lens in fine for piggy back photography. Again the mounting requires a
mounting ring with a generously sized base plate and one that is flat so it does not rock. The Canon lens is very well designed and
built in that respect. Interestingly, the 200 mm f 2.8 lens which is shorter and heavier does not have the integral mounting
ring. It was designed as a high speed hand held lens while the f 4.0 lens is an older and possibly more conservative design. Two
reasons for the integral mounting ring are to balance the lens/camera better and to reduce stress on the camera body mount for
long heavy lenses.
It should also be noted that Losmandy makes a second camera mounting adapter called the DCM1. This plate holds the camera
higher off the rail and it has an adjustable pivot to allow the camera to be adjusted vertically. This bracket is useful if one wants
to mount much shorter focal length lenses or even wide angle lenses. Such lenses are of such light weight that they are easily
held by the camera body and taller Losmandy plate.
Below is shown a 300 mm f 4.0 lens mounted again on its own integral mounting ring on the DCM mounting plate. The camera is
shown in two positions. One of the nice features of the integral mounting ring is the rotational adjustment provided. This
mounting is again quite good, but getting to the point that consideration might be given to an additional mounting ring to
strengthen the flexural integrity of the mounting system. The lens and camera are together very well balanced with this
combination.
As one moves to longer, larger and heavier telephoto lenses, it becomes more and more important to pay attention to flexure of
the lens with respect to the optical tube which it is assumes is being used to do the guiding. The 400 mm f 4.5 lens shown directly
below is a fine example of optical design and has its own integral mounting ring. Notice that the ring is forward enough to
perfectly balance the lens/camera combination on the mount. While excellent for normal photography, this is a lens that really
should have added to it a bit more stability.
Shown immediately below is just such a simple means to stabilize the long heavy 400 mm lens. One can now see the advantages
of the Losmandy rail system. In an instant, one can add a medium sized mounting ring at the front of the lens because the rail
extends the full length of the OTA. This assures that the lens will not flex during long exposures. I have easily taken one hour
exposures with this setup that show no differential flexure between the lens and the OTA.
Then there is the monster, and a monster it is. This is the 400 mm f 2.8 Canon telephoto. All 12 pounds of it. Frankly, this lens
is almost too large and heavy to mount on an LX OTA. It can be done, as shown below by placing a nice little wedge under the
front cell of the lens for stability. Again the Losmandy rail system shows its mettle by making placement of the lens mount and
the wedge easy and versatile. I have used this lens on both a 10" and 12" LX200 but not without problems. Even though the
declination bearings on both telescopes have had the Hart transplant, these mounts are basically not strong enough to carry this
lens well. Incredible stress is placed on the RA bearings in the Polar mount mode. Remember, that the lens has to be balanced
with almost as much weight as it carries. It is to use such large lenses that I am now designing and building a mount that will
take 150 plus pound loads with ease. By the time two imaging systems and a guider are mounted on the drive, the weight easily
gets to the 50 pound range and then is doubled for the balancing weights. In polar mode that takes a compact (short), strong
mount. A discussion of mount design and building is elsewhere on this site.
Another interesting lens is the 500 mm f 5.0 lens that was made many years ago by Nikon. The lens is shown below mounted on
its integral foot. This mounting is not satisfactory. Additional braces, wedges or rings would have to be used. Since I do not use
this lens for astrophotography, I do not show a suitable mounting method. This is a small mirror lens designed mainly for sports
photography. It can be had held. I show it just for the sake of curiosity.
Many smaller mirror lenses are available. Usually 500 or 600 mm f 8. These I have felt are not suitable for astrophotography.
They are designed mainly for sports photography. They are very light and can be hand held.
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Focusing a Film Camera -Designing and Using a Focusing Device
for Cameras with Removable Pentaprisms
Suggestions about focusing in general, the Olympus camera, Astrofocuser, Takahashi and knife-edge methods provided by Michael
Hart are now in a section called "M. Hart on Astrophotography" elsewhere on this web site. Information on the Canon camera and
several focusing devices for it is included below.
A major problem with most standard focusing screens is that they are too dim for focusing already dim astronomical images.
Generally the standard screen must be replaced with a brighter version. A variety of interchangable screens are provided by Nikon
and Canon among others. While these special screens help for framing and course focus, they still do not easily allow for the
critical focus necessary for astrophotography. Nikon and Canon provide a focus screen with a small clear spot in the center that
has a cross pattern etched on the focusing surface. This helps concentrate the eye so as to insure that focusing is being done on
the front surface of the screen. They also both have available a 6 or 8X vertical tube-like magnifiers (called chimney magnifiers)
that provide an enlarged image to aid focusing. But these magnifications are not quite strong enough to insure critical focusing
except with relatively bright objects.
At magnifications of 15 to 20X it is generally possible to focus by object contrast as well as focus by point source. Many find
focusing by object contrast very difficult simply because of the lack of contrast in extended objects due to the brightness of the
sky background. Then, point source focusing is more useful. Using a simple magnifier of power as high as 20X requires an
eyepiece lens of approximately 12 mm focal length. Power in a magnifier is considered to be 250 divided by the focal length of the
magnifier lens in millimeters. Such a lens when used directly at the surface of the focusing screen is not very convenient with any
camera because it has to be within 12 mm of the screen surface. This mechanical/optical arrangement is inconvenient at best. A
magnifier with a relay lens to get long focus relief can be devised for these cameras. But the most common method is to use a
long focus microscope as described below.
It is essential to focus exactly on the front surface of the focusing screen. The ability to do this will depend on the type of surface
used. Finely divided ground glass screens or clear screens with cross hairs are available for some cameras. Several of these will
work. In order to do a good job of focusing, one must be able to concentrate the eye on this surface, keeping it in focus. It is
necessary to critically focus the viewing magnifier on the pattern on the front of the focusing screen first so as to hold the
attention of the observers eye to that focal plane. It is sometimes possible to use the defocused light of a bright star to illuminate
the screen and thus effect eye focus. The image of the object being observed must be focused on the the front surface of the
focusing screen. At 20X or so it is quite easy to focus since a star will flair a little just as it approaches focus, then become very
small and flair again outside focus. With extended objects the contrast will sharpen distinctly at the sharp edges of the image.
Generally, if there are suitable stars within the frame, point source focusing will be easier and more accurate.
It is generally felt that a magnification of at least 15X and preferably 25X, or even more, is required in order to focus a star image
on the screen of an SLR camera. Getting this amount of focusing magnification while retaining the maximum brightness of the star
image is difficult. A long operating distance microscope has been intensively investigated as an aid to focusing a film camera for
astrophotography. This device enables focusing at relatively high magnifications while maintaining great ease of use. It is
described here as applied in two ways to the Canon camera. Clearly similar applications are possible with other cameras that have
removable pentaprisms.
It is assumed in this discussion that the reflex viewing focusing screen is parfocal with the actual film plane so that the front
surface of the focusing screen represents the focal plane of the actual image that falls, during exposure, on the film plane. This is
a reasonable expectation with a high quality SLR camera that is in good adjustment. I have checked many Canon and Nikon
cameras an have not found any that fail this requirement.
The long working distance microscope will have both a good working distance and good retention of the brightness necessary to
effectively focus on a star image. Basically the optical system used for this application consists of a small telescope and a transfer
lens which forms a virtual image of the object which can then be observed through the telescope. Such a device is diagrammed
in the following figure.
The above diagram is easily understood by observing that the object, the camera focusing screen, is at a distance from the
transfer lens equal to the focal length of the lens. This positioning causes the transfer lens to create a real image of the focusing
screen at infinity. The optical rays emerging from the lens toward the right are in fact parallel. This type of image is exactly the
image that the telescope is capable of observing. The telescope is focused at infinity and forms an image that the observers eye
can see. The total magnification of this arrangement is that of the transfer lens combined with that of the telescope. In the
particular device shown below, the telescope has a magnification of 8X and the transfer lens has a magnification of 3.3 times. This
gives a total magnification of about 26 times.
This total magnification is about right to effect careful focusing on the focusing screen of the camera. The lenses in the device
shown are quite generous in size and thus deliver a bright image to the eye. The structure of this form of focuser is really quite
clever. It provides a significant magnification, a good working distance and and a bright image. All of these features are desirable
for focusing dim images. Additionally, the fact that viewing is at right angles to the telescope OTA and with good clearance
between the OTA and the observers head is a considerable advantage.
The optical device is shown mounted with a custom made tube on a Canon camera. This adapter was devised from a spare Canon
viewfinder and a small intermediate tube which holds the optical device at an appropriate distance from the focusing screen. To
give the best possible focus and brightest image the focusing screen is of the type that has a clear central spot and a cross hair
engraved on the bottom of the screen exactly in the focal position of the film.
The small telescope and transfer lens is available from Edmund Scientific Industrial Optics Division. It is called a "portable direct
measuring microscope." The particular unit used has an adjustable eyepiece lens which enables critical focusing on a graduated
scale and a course/fine focus adjustment at the position of the objective lens of the telescope. The position of the transfer lens is
also adjustable with respect to the telescope. It is a very nicely designed and well made unit. With all of the various adjustments,
the magnifier can easily be set to give a very sharp image of the front surface of the camera focusing screen. In addition to the 75
mm working distance, the microscope has a 9 mm diameter field of view. (this is a generous and adequate size)
An alternate optical system can easily be devised using the small telescope and the magnifying viewer normally available for the
camera. In the case of the Canon, there is available a very fine 6X viewfinder with adjustable focus. This is not just a simple
magnifying lens but is a compound, color corrected lens which gives a magnified view of the entire focusing screen. I have thus
made another focusing system which uses the Canon 6X viewer and the small Edmund telescope without it own transfer lens..
Since the magnifier has a power of 6X and the telescope has a magnification of 8X the total magnification is 48X.
This device is shown on a Canon camera at the left and removed from it on the right in the two photographs below. The device is
especially easy to use since it has a very deep mounting cup which sits firmly on the top of the finder. It stays in place and thus is
a hands free attachment After focusing, the telescope and its adapter can just be lifted off of the finder and the entire screen
seen at 6X magnification for framing.
I have found both of these focusing attachments to be totally suitable for accurate focusing of astronomical images for film
photography. It should be possible to make devices similar to these for any camera that has a removable pentaprism.
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Go to Fixing the Main Mirror Flop
Go to Fixing the Flip Mirror
Meade ETX Application as a Guider and
Modification of the Mirror Assembly
Addendum: In the year or so that has passed since the information here was prepared, it has become apparent that carrying out
the practice of using a separate guide scope is not at all easy and some have reported difficulties. Therefore when attempting to
do this, be sure you have read all of the guidescope related articles on this website and all of caveats. This is not a trivial
mechanical system to get right.
The ETX can make an good guider scope for use on a larger SCT. I have mounted an ETX on both a 10" and a 12" LX200 for use
as a guider scope. A separate guide tube is, in my opinion, much easier to use than an off axis guider. And, it can be just as
accurate when mounted and adjusted carefully. I later I switched to a Celestron C-5 for the 12" telescope. This was principally to
gain the added light gathering capacity of the C-5. However, this switch required the addition of an external flip mirror at
considerable extra cost. The ETX is not only suitable but better sized, for the 10" telescope because of its lesser weight its lesser
cost and the very convenient build in flip mirror. I believe that there are two or three limitations and slight problems with the ETX
that do need to be addressed. One is that the ETX is optically a bit slow (f14) so that brighter stars need to be selected for good
guiding. I believe that one can live with this limitation by using appropriate eyepieces and suitably light adapting one's eyes.
Optically, the ETX is excellent. In fact, I had a chance to compare three ETXs directly against two Questars. The ETXs were in
every case superior. The mechanical structure of the ETX is not quite as elegant as the Questar but it is very good. The ETX is
also about one fifth the cost of the Questar. There are two mechanical issues in the ETX design that need attention if it is to be
used as a guider telescope.
The guider which has been used with the ETX is a 216XT which is used in stand alone mode or on one of the COM ports on a
computer. The guider can be placed in operation after the telescope has been pointed to the correct location. The guider takes
over automatically when it is engaged and guides the LX200. The ETX is a very good guider telescope. It has a focal length of
1250 mm which is long enough to guide a 10" LX200 for imaging at prime focus (2400 mm) and especially suitable then telescope
is used with a Meade 0.63 reducer or a MAXfield 0.33 reducer. Additionally, the ETX has a built in flip mirror which gives full view
of the part of the sky to be guided upon and makes it easy to choose the desired guide star and place it in the field of the guider
CCD. The two ports on the ETX are easily made parfocal if the 9 mm eyepiece is used with the 216XT. Thus, with the ETX,one
gets a full view imager and a built in flip mirror. The savings on the latter item alone is significant. It would be hard to find a
better combination.
It is very important to have the ETX firmly mounted on the main telescope tube. Piggy back mounts are never adequate for
precision mounting of accessories. (piggy back mounts will not hold a camera with al ens longer than about 100 mm rigid enough
for any use in my opinion.) Thus I have installed a Losmandy rail on the 10" and a pair of Losmandy rings to hold the ETX.
Remember that to maintain 1 arc second alignment of the main tube and any attachment to it, the ends of the auxiliary tube may
move no more than 0.05 thousandths of an inch in a distance of ten inches. Flexing of the mount as the telescope moves across
the sky must be minimized. The Losmandy rail and rings is one combination I have found that does the job well.
The ETX is shown below mounted in 125 mm Losmandy rings, which fit perfectly. Notice that the ETX is nicely proportioned
compared to the 10" LX200. It is light in weight and of suitable focal length. The ETX is shown without the 216XT guider or 9 mm
eyepiece which are normally used with it for guiding purposes. All in all, the ETX is a good and even handsome guider telescope.
It looks like it belongs on the LX200.
Checking and Fixing the Mirror Flop.
One of the main problems with using a second SCT as a guider telescope is that you now have to worry about two mirrors flopping
about. Thus I carefully checked the main mirror flop in the ETX and modified/adjusted it to eliminate the mirror flop. Fortunately
the guider telescope can have the mirror fixed essentially at infinity focus and every thing, including the guider imager firmly
locked in place. First, the optical tube must be opened and the focus mechanism inspected. The open tube is shown in the
photograph below. Several photos also show close ups of the focus mechanism as well.
As can be seen, the focus mechanism is not very robust. The end of the focus screw is loosely retained in a plate mounted to the
rear of the mirror. Considerable slack exists at this point. Additionally, the point at which the focus rod penetrates the back plate
has a relatively loose bearing. When the mirror is near infinity focus the mirror is quite a way toward the back plate. and thus
the spring under considerable tension. Fortunately the movable slide is quite tight on the center mounting tube. I have not
completely solved the issue of ensuring that the mirror is absolutely tight on the center mounting tube. Removing the existing
lubrication and re-lubricating it with beeswax seems to hold the mirror securely enough for most applications. Since the guider
telescope is used only at infinity focus, it would be possible to fix it firmly with small springs or set screws. I have not done that
at this point but will certainly do so if there appears to be any residual mirror flop in the guider telescope.
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Fixing the Flip Mirror in the ETX
I found that the flip mirror, when in its 90 degree position, was not as stable as I would have liked. Each time the mirror was
flipped, the centering of the guide star in the eyepiece changed considerably even though the star was usually on the guider chip.
But in some cases the flip mirror was badly enough set that a star in the center of the eyepiece did not fall on the chip at all. This
problem needed to be fixed before the ETX could be used with the tiny 216XT chip.
The reason for this problem is that the up position of the mirror is established by a spongy pad on the back of the mirror which
does not firmly establish a stopped position. The fix allows for precision adjustment of the angle of the mirror in its up position.
(90 degree position) The angular accuracy required for the mirror is quite high. A good fix can be effected in the following way.
Remove the black case from the body of the telescope. Done by removing three screws. You will see the mirror mechanism
shown below in two views. One with the mirror out of the path and the other with the mirror in the 90 degree position. There is a
small rubber pad on the paddle like extension sticking out on one side of the mirror mount. Scrape the rubber pad off and scrape
the paddle clean. Drill a No. 3 tap drill size hole through the back of the casing immediately next to the attachment ring and in line
with the paddle. This location is shown in one of the lower photographs immediately next to the thread on the back adapter
tube. As can be seen, there is just enough room to do this. Be careful to not ruin the thread on the rear mount ring. Tap the hole
and insert a long No. 3 set screw. Turn the screw inward until it engages the paddle. This screw can now be used to set the mirror
angle very precisely. The mirror will now come up and into contact with the set screw with a nice firm click. A dab of cement can
be applied to hold the screw firm.
If the screw is too short it will fall through the threaded hole and if too long it will interfere with attachments to the rear thread. If
the screw is too short, use a longer one and grind or file it to the correct length. It needs to engage the paddle but not stick out
the back of the hole. To set the mirror to the correct angle first move the mirror to the down position (so it is out of the way of the
light beam) and center a target (star) on the chip. Then, without moving the ETX remove the camera and put the mirror in the up
position (so it intercept the light beam and directs it at 90 degrees). Insert the screw and turn it until the target is centered in the
eyepiece. The mirror is now aligned so that it directs the target into the center of the eyepiece. Glue the screw in place from the
inside. This operation can be seen in the photographs.
In the final photograph, the telescope is shown with a standard adapter the converts the small thread on the back of the ETX to
standard Schmidt thread. This adapter also covers the set screw hole. I prefer to have the Schmidt thread rather than the small
thread so that standard accessories fit easily.
The chip and the eyepiece will now have concentric images which will repeat reliably when the mirror is flipped. If there is a very
slight lateral offset of the image it will be stable so it can be corrected with the moveable cross hairs in the 9 mm eyepiece. If the
lateral offset is too great, the mirror will have to be shimmed. I did not have to do this. You will find that the mirror mechanism
has a detonate spring that is supposed to hold the mirror against the set screw when it is in the 90 degree position. It might be
necessary to adjust this spring to insure that it pushes the mirror paddle against the set screw firmly. I had to adjust the spring on
the ETX shown.
I have found that with the two adjustments described above made, the ETX makes a very suitable guider telescope.
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The Off-Axis Guider and Its Applications
The Off-Axis Guider (OAG) has a long history. It is the guiding method used from the beginning of astrophotography by all
astronomers. Images of overheated or freezing astronomers, their eyes glued to the telescope guiding eyepiece fill biographies
about these brave pioneers. There are reports of eyelashes frozen to the eyepiece and worse. Today, the OAG is still one of the
most popular guiding methods. However,generally it is a CCD imager used as a guider that does the guiding job.
There are, unfortunately, a large number of OAGs and CCD guiders that are terribly difficult to use and do not work well. They
have given, in many cases, the use of the OAG a bad name. With a high quality guider and a high quality CCD imager/guider it is a
very reliable way to guide telescopes for film or CCD imaging. At the very start, I will say that the only guider that I know of that
does the job well is the Lumicon giant OAG. The CCD guider of choice is the SBIG ST-4. There are other CCD guiders that do very
well also. I think of the Meade 216XT or even the ST-7 imagers. I have used the Lumicon OAG and both of these CCD imagers
with good results.
Idea of the OAG is to guide on a section of the star field that is created by the main imaging telescope, but just far enough out of
the image field so that the "picked off" part of the field does not cut into the field being imaged. This is often a bit tricky to
accomplish since it generally means that a deflecting mirror or more commonly a prism is placed at the very edge of the imaged
field and directs a portion of the image off to one side. The tolerances are quite critical. It is not only off the main axis of the
image but off far enough to avoid cutting into the desired image. Unfortunately, telescopes often have such a small image field of
high optical quality that the picked off star images are somewhat distorted. This is generally coma distortion. Refractors are much
better in this respect than SCT type telescopes. Nevertheless, it is not too hard to pick off a section of the star field that is totally
adequate for guiding. This is where the quality of the CCD guider comes into play. Some CCD guiders are much better at capturing
and locking onto the slightly distorted star images than others.
Also the quality and versatility of the OAG comes into play. It must be easy to rotate the OAG so that its prism can be set to
intercept a good guide star. When the OAG is rotated of course, the star field rotates and the CCD guider has to be oriented so
that it still guide the telescope properly. Thus, it is not trivial to adjust the OAG and guider combination to make it guide well.
These factors are what have made the OAG difficult to use and have discouraged many amateur astronomers from using them.
I must admit that I have had all too much experience with crummy OAG/CCD combinations and thus grew to hate them
thoroughly. But because the CCD guider is guiding on a part of the actual star field image being captured,it gives excellent
guiding. It is probably closely equal to that obtainable with the SBIG ST type self guided imagers. The features to look for in the
OAG are a large pickoff prism which can be easily adjusted to find a good guide star. There is none better than the Lumicon unit.
Another feature is to have the takeoff tube for the guider that is very solidly built. And finally, the entire OAG should be very
solidly built and have a convenient way to hold a focal reducer or expander is that might be used. The Lumicon OAG has all of
these features.
The Lumicon unit has adapters on both ends that can be changed or modified to fit any telescope and imaging device. This
includes small and large format film cameras as well as CCD imagers of any type. I can attest that it is extremely well built and
that it operates exactly as expected. Considerable technique is required to operate even this fine OAG. Descriptions about these
techniques are given by Philip Perkins at <http://www.astrocruise.com/guide.htm>. He is without doubt a premiere
astrophotographer and is willing to share his excellent techniques with everyone. I defer to this site for the operational details.
Below is shown the Lumicon giant Off Axis Guider. Notice its giant size. It is a full 4 inches in diameter. It is very solidly
constructed. The adapter ring, lower right, screws directly onto the back plate of atypical SCT. It is very firmly held by three set
screws with the grooved ring. An adapter for any telescope is either available or can be easily made. The supplied 2" adapter tube
is shown to the left of the main body of the OAG. Internally there are stepped retainers and set screws to hold the large diameter
focal reducer also available from Lumicon. The focal reducer is at the top right. This focal reducer can be adjusted to several
positions and is a full 80 mm in size. This provides several levels of field reduction or none if the lens is not used. The pickoff prism
is large in size and can be easily positioned by rotating the entire OAG about the mounting ring. A side tube is provided for the
mounting of the CCD guider, in this case a 216XT. An eyepiece for setting up the guide star can be substituted for the CCD guider.
This tube is very rigidly mounted but still has a good range of adjustment to aid in finding a suitable guide star. The back side of
the giant OAG has other adapters available for almost any camera or imager. (including large format cameras)
I have made a few additional adapters for the unit to suite my own needs. One is an insert to hold a smaller and I think optically
superior focal reducer Generally I use the Meade 0.63 reducer or it will take any other with a Schmidt thread. I also have made a
similar adapter that will hold a projection lens for high magnification or a focal expander(Barlow lens). I also have made a 6 by 9
cm film and plate adapter with a fold away focussing screen and magnifier attachment. This is shown at the far left bottom. The
latter substitutes for a medium format camera but is very much lighter. For exposing the film, the hat trick is used instead of a
shutter since film exposures are usually a half hour or longer. A final attachment, a sort of coup de gras I think, is the flip mirror
mounting I have made to use in place of the side tube. This allows finding a guide star and flipping the mirror to the guider
quickly. It is shown at the upper left. All in all, the Lumicon is a fine OAG with or without the accessories. If you want to do guiding
with an off axis guider, this is the one to use. I have found it a most desirable and useful accessory which works very well.
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Information on the Optec Filter Slider
The Optec filter slider is an attachment of exceedingly high quality that holds and automatically places filters in the optical path for
imaging or photography of almost any sort. It can also be used to hold filters for viewing. I am very happy with this attachment
for a number of reasons which will become apparent in the following discussion. Thus I have stated my prejudice and my biases
will follow.
The biases are basically as follows. I believe the filter should be a large distance from the imaging surface, whether it is film or a
CCD chip, so that any defects or dust are well out of focus. I also feel that the filter should be large enough so that there is no
possibility of vignetting on a full 35 mm format or when focal reducers are used with CCD imagers. It is also desirable to have
easy exchange of filters and use of filters which are standard size so that a great variety of filters can be used. Finally, the
operation of the filter slider should be fully automated for use with standard CCD imagers.
Here the Optec filter slider is shown on the back of a 10" LX200 with the JMI focuser including the DRO. Details of the adapters
are shown in the following discussion.
The Optec filter slider adds only 25.9 mm to the back focus requirements. It mounts on an adapter ring of its own and has the
correct sized ring on the back to take the JMI directly. This arrangement insures full 2" clear openings internally. The top view
shown here demonstrates the small thickness of the combination. It is very strong mechanically as well.
Here are shown the Optec filter slider and the Optec adapter tube along with the JMI adapter tube. The Optec tube is 60.5 mm
outside diameter and 53.1 inside diameter and screws directly on the back of the 10 or 12" LX200. The JMI adapter is 55.8 mm
outside diameter and 51.2 inside diameter. It is no longer used with the Optec filter slider in place. The back side of the filter
slider has a 55.8 mm outside diameter tube which takes the JMI directly as seen in the photos above. This makes for a very
strong mechanical connection and gives a full 2" clear internal opening. The actual opening is 50.5 mm. While giving statistics,
the JMI weighs 1 1/4 pounds and the Optec 1 3/4 pounds. This weight is of course very close to the declination axis and easy to
balance. In fact, if you use the Meade dew hood, as I do, you need all of this and more to balance the hood. The small motor
drives the filters with great positional accuracy to the three positions. The system is fully automated and compatible with the
SBIG imager filter commands. One slight disadvantage is that there is no clear filter position to facilitate focusing.
Another view of the Optec filter slider shows the filter tray that in this case is provided with a three color filter set for CCD color
imaging. The Optec filters are all exactly the same thickness so focus is maintained when changing filters. Separate blank trays
can be had into which any color filter set desired can be mounted. The openings in the trays are 50 mm with a 47 mm shoulder
and thick enough to take standard 48 mm filters. For example, I have a tray loaded with filters for use with monochrome
imaging. Additional trays can be set up with viewing filter sets or with color filter sets that have different densities and pass
bands. See a discussion of filters for use in color imaging elsewhere on this web site. (Attachments/Filters/Three Color
Filters) The little door on the end allows for quick interchange of filter sets. (it takes less than a minute to do this operation)
The filter slider comes with a control box and cable with manual selection buttons. There is an eject button for changing filter
trays on the end of the tray structure itself. The entire Optec filter setup is well build and works as advertised. It is not
inexpensive but it provides versatility that is hard to find elsewhere. For example, it can be used for 35 mm film photography
when filtering is desired and for direct viewing when band pass filters are desirable and need to be exchanged quickly.
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Weighted 1-1/4" to 2" Eyepiece Adapter
Shown is a weighted adapter to convert 2" to 1 1/4 " tube size for using small eyepieces and large eyepieces without disturbing
the telescope balance. A commercial adapter made by TeleVue is shown on the left and a home made version is shown on the
right. As it turns out, when the adapter is made from brass to the correct size, it is also just about the right weight to make small
eyepieces and large ones weigh the about same. This allows the telescope to remain in balance when eyepieces are changed. The
adapter should weigh somewhere between 12 and 14 ounces. One could tune the weight for a particular large/small eyepiece
set, but that is not necessary. The heavy adapter reduces the weight difference from about 12 ounces to within a few ounces for
almost any large/small pair. The TeleVue adapter is beautifully made as one expects. It weighs 12 ounces. One might note that
a small 1 1/4" eyepiece might weigh about 4 ounces while the 20 mm Nagler weighs 48 ounces. This extreme small/large pair
weight difference may require re-balancing the telescope even with a weighted adapter.
Here is shown the homemade adapter with the standard 26 mm Meade eyepiece. Note that the adapter has, at the left end, a
threaded ring which will take standard 48 mm filters.
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Discussion and Comparison of
2" Diagonals: Meade and TeleVue
Note: Sunce this article was written, there have been a large number of comments that the newer high reflictivity diagonals are a
considerable improvement over older diagonals. A number of companies, including TeleVue, have made these "super reflective"
diagonals available. They should be considered for the highest quality results.
The following is a discussion of the design of the Meade verses the TeleVue 2" diagonals. Considering how simple this device is, it
is hard to believe that the two diagonals could be designed so differently. The TeleVue is shown on the left and the Meade on the
right. Superficially they are very similar and both do the same job. Interestingly, and amazingly to me, the TeleVue is actually
made from a single piece of material. (not the mirror and mirror mounting plate of course) To do this, a large amount of
machining has to be done. This makes the diagonal expensive. It is however beautifully machined. The Meade is made more
conventionally. The housing is one piece and the two tubes screw into the housing. There is one slight advantage to this design.
The 2" tube can be removed and a Schmidt adapter screwed in place. Thus one design can serve for two types of adapters. The
Meade is available as shown with the 2" tube and alternately with at Schmidt tube and locking ring that fits directly on the back
plate of most SCTs. With the TeleVue an additional adapter tube is required to fit it to the standard SCT Schmidt thread. Both
diagonals have a full 2" internal opening and the mirrors are full size. I do not have information on the optical quality of the
mirrors or details about the coatings that may be provided.
I do not use the 2" diagonal with a Schmidt threaded tube at all since I consider the inside diameter of the Schmidt tube to be too
small. I use only the full aperture 2" tubing for the 2" diagonal and the 2" eyepieces. (I also use the JMI or Optec rear plate on
the SCT to insure a full 2" opening at the back of the telescope. (This is only possible with the 10" and 12" optical tubes.)
Below is shown the Meade diagonal with the 2" tube removed. Note that I have modified the 2" tube by cutting a groove to
secure it better into the JMI focuser (or alternately a 2"size SCT rear plate adapter tube). Additionally, I have inserted a 2" filter
adapter ring so that standard 2" filters (48 mm) can be used. The TeleVue diagonal has a thread for the 2" filters machined as
part of the tube. At the right is the Meade diagonal with the weighted adapter and a 26 mm eyepiece. There has been no hard
evidence that I know of that the two diagonals function differently from an optical viewpoint. The TeleVue is more finely made in
my opinion but is considerably more costly.
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Focal Reducers (Field reducers)and Focal Magnifiers (Barlow lens)
Go to Focal Magnifiers
PART I Focal Reducers
February 1999 update note: Meade has released a new focal reducer/flattener as of February 1999. This is said to be a three
element reducer which will fully cover chips as large as the Kodak1600 size. (1616XT or ST-8 size) This is encouraging since the
Optec will not cover the larger chips. The Meade reducer is also quite reasonably priced. I have not received the Meade focal
reducer, but it is reported to have a focal length of 88 mm. This is slightly longer that the 55 mm of the Optec and thus requires a
slightly greater distance between the reducer and the image plane than the Optec. The calculated distance for a reduction of 0.33
is 59 mm. Since the chip is behind the front of the imager body surface by 43 mm for the ST-7, and adapter tube of about 15 mm
is about right. I understand that Meade provides a variable adapter of 15 to 30 mm. SBIG has a reducer designed specifically for
their ST-5C which I have not seen.
There are significant restrictions on the positioning of focal reducers used with SCT telescopes. These restrictions and some
applications of focal reducers are discussed in this note. Focal reducers are precision optical elements you add to the optical path
and should be used according to the instructions provided with them. They should be used at their design distance from the
imaging plane and should not be stacked to get greater optical effect. Most 0.63 focal reducers are intended for use with an f 10
SCT but will work with an f 6.3 SCT as well. Stronger focal reducers, like the 0.33 reducer are designed specifically for f 10
telescopes. Correct optical application of these reducers will give the best optical results.
The telescope produces a real image some distance behind the rear plate of the telescope. This real image is normally viewed
with an eyepiece or intercepted by a photographic film or CCD imager. For objects at infinity, like stars, this image can be moved
closer to or farther from the back plate by moving the mirror. This is called focussing. As the mirror is moved back, the real image
moves closer to the back plate and as the mirror is moved forward toward the secondary, the image moves farther away from the
back plate. Focus can be accomplished by moving the mirror or by moving the eyepiece or CCD.
Focal reducers are positive lenses which do two things to the real image. They move the focal point forward (toward the back
plate) considerably and they reduce the image size. It is the reduced image that is desired since it is both smaller and brighter.
Thus more image fits on the film or CCD imager, which has the effect of decreasing the effective focal length of the telescope. It
also increases the surface brightness of the real image thus decreasing the effective f number of the telescope.
Focal reducers are designed, optically, to work at a fixed and specified distance from the film or CCD imager. They are best
corrected for this one distance. For the Meade 0.63 focal reducer this distance is 87 mm from the rear of the lens (96 mm from
the exit pupil) and for the Optec MAXfield 0.33 it is 29.5 mm. (from the rear of the lens structure) In practice the MAXfield
distance of 29.5 mm is measured from the rear element of the reducer but is actually about 35 mm from the nodal point. The
MAXfield is a very thick optical system. Note that all measurements are slightly approximate since the the focal reducers are thick
lenses and do not have their nodal point marked on the lens mount. In this discussion, I am using the simple thin lens formulas
so approximate locations for the lenses and images are sufficient. Additionally, the 0.63 reducers have considerable variation
allowed for their placement. The 0.63 reducers work well from about 80 to 110 mm but with slightly different reduction ratios.
The focal reducers will give the specified focal reduction and the best optical performance when used at the design distance. They
can be used at other distances to get slightly different reduction ratios. This property in not different from other optical systems
such as the SCT itself which will give the best correction for objects at infinity and with the mirror toward the back of the optical
tube. This position of the mirror gives a real image quite near the back plate of the telescope. As tubes, focusers and focal
reducers are added, the mirror must generally be moved forward in the tube to extend the real image backward. The maximum
distance that the real image can be moved backward from the back plate is usually called the maximum back focus distance.
There is a design crunch caused by using focal reducers. When a relatively strong positive lens is placed in the optical path of the
telescope near the real image, it moves it forward significantly. Thus, the original real image, without the reducer in place, needs
to appear a considerable distance behind the back plate.
The approximate distances are easily calculated from thin lens formulas. The following two formulas are used for all calculations.
The assumption is that the telescope is to the right and the reducer and imaging surface to the left. Thus the light rays are moving
right to left.
Magnification = 1 - Fd / Fl
and 1 / Fl = (1 / Fd) + (1 / Ff)
Where Fl is the focal length of the reducer
Fd is the distance between the reducer and the CCD, film or imaging surface
Ff is the distance between the reducer and the original image produced by the telescope with no reducer in place.
Notice that for this lens system, since the initial image, that is, the real image produced by the telescope falls to the left of the reducer lens and
so does that of the final image, the sign on the value of Ff is negative. This diagram should be kept in mind since it shows that what really
happens with a focal reducer is that the image is pulled forward, toward the telescope and is at the same time made smaller.
Examples: For the Meade 0.63 reducer, the distance between the rear of the reducer and the CCD should be 87 mm. (or about 96
mm from the lens pupil itself) This means that the original real image produced by the telescope should be about 148 mm to the
left of the reducer. For the MAXfield 0.33 reducer, the distance from the rear or the lens structure to the chip should be about
29mm. For this reducer the real image produced by the telescope will be about 150 mm. to the left of the reducer.
The total distance that the original real image must be to the left of the back plate is the calculated distance plus the distance that
the reducer is to the left of the back plate. This considerations require the mirror to be moved toward the secondary enough to
place the real image far enough to the left to satisfy the optical conditions for focus. To meet this condition one needs to "have
enough back focus." For most SCTs the real image for a distant object can be moved from about 25 mm to about 250 mm behind
the back plate of the telescope. These numbers depend upon the exact design of the SCT of course. Fortunately, this means that
both the Meade 0.63 and the MAXfield 0.33 reducers can be focussed easily.
However, when too many focusers, filter wheels, flip mirrors and the like are used the required back focus may not be available.
You can easily calculate your own optical situation using the formulas supplied above. Note that using several reducers will not
only cause back focus problems but may seriously deteriorate the image due to aberrations and non-optimal lens placement.
Additionally, a minimum amount of tubing (length) should be used in the optical path so that the primary mirror does not have to
be moved more than necessary toward the secondary to attain focus.
Examples of three focal reducers are shown below. The large, 82 mm, reducer is that from the Lumicon Giant OAG, the center
one the standard Meade 0.63 reducer and on the right is the Optec 0.33 reducer. The focal lengths of the Lumicon and Meade are
about the same, 260 mm, while the Optec is a focal length of about 55 mm. The Lumicon lens is a simple cemented doublet (two
elements), the Meade a dual cemented doublet (four elements) and the Optec a more complex thick lens design. While all field
reducers have a limited field of illumination, designs like the Optec are very strong reducers and have a field, in this case, limited
to the K 400 size chip.
It should be noted that focal reducers with long focal lengths like the Meade and the Lumicon (260 mm) can be used at reduction
ratios that are somewhat different from their design centers without serious degradation of the image. It has been reported that
even at 0.4 reduction setting the images are quite good but may be best at the design center. Lumicron allows for a generous
adjustment of the distance settings. On the other hand, the Lumicron is a much simpler lens design and may not have
appropriate quality for the small imager chips where its size is not needed in any case. It is designed primarily for the 35 mm film
format, I believe.
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PART II Focal Magnifiers (Barlow lens)
Focal magnifiers are probably more familiar to most astronomers. Their purpose is to increase (magnify) the size of the image.
This is most often done to squeeze higher power out of a telescope, often for planetary viewing. The magnifier lens, which is in
this case a strong negative lens, is placed near focal plane and expands the cone of light thus expanding the image size. It s
placement seems to be not a critical as the placement of the focal reducer. This is an illusion that must be tamed. In fact, the
exact same optical equations apply to the focal magnifier as to the focal reducer.
These equations are given once more for reference and a diagram of the focal magnifier is provided as well directly below. The
approximate distances are easily calculated from thin lens formulas. The following two formulas are used for all calculations. The
assumption is that the telescope is to the right and the magnifier and imaging surface to the left. Thus the light rays are moving
right to left.
Magnification = 1 - Fd / Fl
and 1 / Fl = (1 / Fd) + (1 / Ff)
Where Fl is the focal length of the reducer
Fd is the distance between the reducer and the CCD, film or imaging surface
Ff is the distance between the reducer and the original image produced by the telescope with no reducer in place.
Notice that for this lens system, since the initial image, that is, the real image produced by the telescope falls to the left of the
reducer lens and so does that of the final image, the sign on the value of Ff is negative. Notice also that the sign of the focal
length of the magnifier lens is also negative. This diagram should be kept in mind since it shows that what really happens with a
focal magnifier is that the image is pushed backward, away form the telescope and is at the same time made larger. This exactly
the opposite of what the focal reducer does to the image. When the formulas are used, with the appropriate signs for the
distances and the lens, the magnification and the distances will be calculated correctly.
Thus it is clear that the amount of magnification that a given focal magnifier will give is dependent on the spacing between the
magnifier element and the focal plane. Magnifiers, like reducers, are designed to work at a specified magnification and thus a
specified distance from the image focal plane, but their positioning is not as critical as that of reducers. Magnifiers are most often
used for added power when the eyepiece is not of short enough focal length. The optical quality of the resultant image may suffer
if too much magnification is demanded.
DISCUSSION: Vignetting and Illumination
In the case of the image magnifier there is no problem with vignetting since the image is enlarged. However it must be realized
that the total illumination available is still confined to the cone of illumination striking the front surface of the magnifier and that
this illumination is spread out over a larger surface thus reducing the intensity of illumination. This makes the image dimmer and
increases imager exposure as would be expected.
On the other hand, the focal reducer concentrates the light from the telescope and produces a brighter image thus shortening the
exposure time required. Reducers are used to image objects which are normally too large to fit on the surface of the imager,
generally a CCD. There is a problem with vignetting when a focal reducer is used. Only those light rays which enter the front
pupil of the reducer will reach the image plane. Thus if the initial image, before reduction, is not uniformly illuminated over its full
surface by the cone of light from the telescope, then neither will the reduced image. This is usually not a severe problem with a
focal reducer that is properly designed and used at its design distance. But when using a larger field receptor like 35 mm film, the
telescope will generally not illuminate the full initial image and will thus not illuminate the full reduced image. For example, with a
reduced image that is to fill a 35 mm frame, and a 0.63 reducer power, the initial image would have had to fill a frame 41 by 57
mm. This is a circle of illumination of 71 mm. (almost 3") Small Cassegrain telescopes simply do not have a circle of
illumination that large. Thus some compromise may need to be made when using 35 mm cameras with a reducer. Usually a
square area the size of the short side of the frame is covered fairly well. In a more flexible setup, one can adjust the distance of
the reducer from the imaging surface to take slightly less reduction and obtain more uniform illumination of the film.
The size of the focal reducer and the size of the tubing used to attach it to the camera is also of some concern as described
elsewhere on this web site. Adapter tubes with "T" threads are generally not of sufficient size and will cause vignetting on 35 mm
format film in almost all cases.
Focal Reducers Used With An Off Axis Guider
When using a focal reducer with an off axis guider, it is often the case that the focal reducer is used to reduce the desired image
but not the guider image. This arrangement is shown here.
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Movement of a Star in the Image Field Due to Rotation
When imaging, it is essential to track the star field being images with very great accuracy. Both Film and CCD imagers can resolve
to about 0.01 mm. Thus if the telescope has a focal length of 3000 mm the telescope has to be pointed and held to an angular
accuracy of about 1 arc second. In addition to pointing the telescope to a given star in the field being imaged, the telescope has to
be set up so that the star field does not rotate in the image plane. There are two ways of doing this. One is to rotate the image
plane to match the rotation of the field and the other, more traditional way, is to mount the telescope axis to be parallel with that
of the earth. In the latter case, it is possible to attain very excellent polar alignment, especially with a permanent pier, but very
hard to attain perfect alignment. The issue of the actual motion of a star in the image plane of a polar mounted telescope is
considered here.
The motion of a star in the image field depends upon three factors. One is the amount of actual field rotation which is caused by
the fact that the RA axis of the telescope is not exactly aligned with the earth's axis. Call this the angle of misalignment, A. This
misalignment causes a rotation rate, R. The rotation rate is 4.37E-3 radians per minute times the number of radians of
misalignment. This can be shown from a detailed study of the spherical geometry of the celestial and observing spheres. The
numbers used here are for simplicity the worst case but with the 1/cos (declination) set to 1. The rotation will be worst at
declination = 90 degrees.
The problem is that the rotation is a strong function of the pointing declination and RA. (see another article on this web site that
gives the exact formulas)
Then we have R=(4.37E-3 * A/57)/ cos (declination) Where A is in degrees of misalignment.
To simplify the calculation I assume we point well away from the pole and assume the 1/cos (declination) < say 4. Thus the
examples are a bit optimistic but generally within a factor of a few and close enough to give the idea. I have ignored this factor in
the examples by assuming declination = 0 and cos = 1.
The actual number of minutes of exposure possible for a given allowed amount of drift, d, is dependent upon the distance between
the guide star, which is the center of rotation, and the star of concern. Call this distance, D. D is the actual distance measured at
the imaging plane.
It can be found in two ways. The simplest is when the guide star and the star of concern are both on the field being imaged. If the
guide star is in the center of the image for a 35 mm size image, the most distant star is in the corner which is 21.5 mm distant.
For a CCD chip of 400 size, the distance from the center to the corner is 4.15 mm. If the guide star is outside the image frame its
effective distance must be calculated from the angular separation of the two stars and the focal length of the telescope. For
example, with a 1200 mm telescope and a separation of 1 degree, the star separation in the image plane is 1200 times 0.017 or
20.4 mm. (0.017 is the radian angle corresponding to 1 degree)
The actual number if minutes of exposure allowed to reach the limits described is then:
Minutes of exposure = ( d/DA) * 13E3
For example with 1 degree of misalignment, an allowed 0.01 mm of star trail, and a distance of half the diagonal of a 35 mm
frame, we get:
Minutes = 0.01/(21.5 * 1) * 13E3 = 6 minutes
For the same situation with a size 400 chip (416 or ST-7) we get:
Minutes = 0.01/(4.15 * 1) * 13E3 = 31.3 minutes
For the same CCD chip conditions but with the guide telescope pointed 1 degree off the axis of the center of the chip and using a
1200 mm telescope, we get, the effective distance, D = 21 mm between the guide star and the center of the chip. Then the center
of the chip will move the specified amount of 0.01 mm in a time given by:
Minutes = 0.01/(21 * 1) * 13E3 = 6.2 minutes
Thus we see that a significant penalty is paid by guiding on a star too far away from the center of the imaging film or chip. It is
worse by a factor of 1/cos (declination) as described above. At my location (about 43 degrees) the zenith is at declination 47
degrees and the cos is about the square root of 2 or 1.4. So the rotation at the zenith for me is worse than the example by 50% or
so. At some hour angles the problem is not as bad since this depends on the RA and the direction of the misalignment. So you can
see that exact calculations are somewhat convoluted but the principle is to get the alignment with the pole as close as possible so
as to reduce rotation as much as possible.
It has been reported to me by several persons setting up their telescopes that they are usually satisfied with polar alignment to 2
to 10 arc minutes. With a permanent pier mounting it is possible to get polar alignment to 10 arc seconds. (with care) The
problem with trying to hold accurate alignment is that the compliance of the LX mount is such that an angular change in pointing
introduced by a 1 kilogram load change is about 50 arc seconds. So it is important to align the telescope with a load similar to that
used under actual operation.
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Projection Imaging
See an example of a projection attachment for a video camera elsewhere on this web site: Video Camera and Attachments
See details of a projection attachment at the end of this article. For Photo of attachment press here.
The topic of eyepiece projection comes up from time to time when a very high magnification is required for objects that are quite
bright. Planetary photography especially comes to mind. A few comments about this topic is appropriate because projection
imaging is often not well described. For many reasons the results are quite often rather poor. I have done quite a bit of projection
imaging with microscopes and a bit with telescopes as well. Only positive projection is described here. Negative projection, such as
with a Barlow, is generally limited to low projection powers (typically 2 to 4 times image magnification). It does work well with a
good Barlow (negative) lens. My telescope experience has been mostly with projection of planets onto a video camera chip. While
it is certainly possible to get good images by projection with an eyepiece, I think they are used mostly because they happen to be
available rather than being chosen for their optical effectiveness.
When setting up an optical system for image projection, it helps to think about the differences between visual observation of the
telescope's real image and projection of that image. Eyepieces are designed to be magnifying glasses for the eye and not
projection lenses. They are designed to let you look at the real image that the telescope objective creates at the eyepiece position
(actually at the field stop of the eyepiece). They are basically quality magnifying glasses which are designed to have a large field
of view as seen by the eye. This is called the apparent field of view. The eyepiece has special properties that observers seem to
covet. They are designed to have a large apparent field of view and a large field stop so that the actual field of view is as large as
possible. The actual field of view depends only on the focal length of the telescope and the size of the field stop. They need to be
bright and free of reflections. They often have many thick lenses in them and are designed to control flatness of field and off axis
distortions as seen by the eye.
As an aside, one should note that the largest field stop possible is a bit smaller than the diameter of the eyepiece tube. This means
that a 1 1/4 inch eyepiece usually has a maximum field stop of about 28 mm. The reason for going to a 2" eyepiece is to get a
larger field stop and thus see a larger part of the sky with a given focal length telescope. Of course this large field stop also
requires large lenses in the eyepiece and the costs rise rapidly. You can estimate the diameter of the field stop by looking into the
back side of the eyepiece and hold a millimeter scale against it.
The compound (and complex) lens part of the eyepiece is designed to provide a view of the real image that the telescope objective
creates at the field stop. This requires a design that has a large and flat field and projects a virtual image at some convenient
distance in front of the eye. The power of the eyepiece is usually considered to be the focal length divided into 250 mm. The sharp
edge of the field stop can provide a reference point for focus. But since the eye has considerable accommodation there is
sometimes a cross hair or reticule located at the field stop upon which to fix the eye's attention.
The point of describing these factors is that the optical design of the eyepiece requires a large field and a curvature of field that fits
that of the telescope. The real image is usually slightly within the focal length of the eyepiece and thus the projected image is a
virtual image that the eye can see. (Note the eye cannot see a real image unless it falls on a ground glass or impinges on smoke.)
Additionally the eyepiece must cover a large field. That is, it must cover the entire area of the field stop.
Real images that need to be magnified by projection are not large in size. They are generally planets which have tiny but bright
real images. One often looks at these with a short focal length eyepiece (a high power eyepiece). This type of eyepiece has a small
field stop but that is fine since the image is small.
Her is shown the basic difference between projecting a real image formed by the telescope onto a film (or CCD) surface (top) and
viewing the real image with the eye through an eyepiece. (bottom)
¾ The idea of image projection is to focus the real image (called the object in the projection system), which is formed by the telescope
objective into another real image (called the image) which is a significantly enlarged version of the object. In this case, the object lies outside
the focal length of the projection lens by a small amount and the projected image is outside the focal length by quite a bit so it is substantially
larger than the original. To do this the lens used should be a projection type lens designed for a real object to real image magnification of
about the desired amount. (usually 3 to 20 times) Lenses made for this purpose are microscope objectives, enlarger lenses, copy lenses,
microfiche lenses, small format lenses, movie lenses. video lenses and the like.
Some of these lenses are designed for specific magnification ratios and for flat but not necessarily wide fields. They are usually
designed specifically for high resolution and high contrast. The size of the lens needs to be somewhat larger than the object, that
is the real telescope image, so that it intercepts the entire cone of illumination of the object. This is so that there will be no
vignetting of the real image. Since the largest real image of a planet, Jupiter or venus, is only 0.4 mm with a 2000 mm telescope,
we are looking at very tiny images; even compared to a CCD chip. This is where positive projection works the best. Magnification
of 3X to 10X will fill the CCD chip or video camera chip or even give a useable image on 35 mm film. Most books on photography
of planets recommend, or hint at, using a lens other than an eyepiece for positive projection. (See references at the end of these
comments.)
Ideal lenses for this application are small copy lenses or microfiche lenses. Short focal length photographic enlarger lenses and
photomicrographic lenses are also very good. If an eyepiece is used, a simple orthoscopic type is better than a complex and costly
wide field type. That is because the complex wide field eyepieces have too many glass surfaces and too much of the design is
directed to factors that are important for visual work but not important for projection applications
The desirable lenses mentioned above are usually designed specifically for image magnification ratios of about 10 to 1. They are all
of a speed of f4 or faster and thus will easily accept the object light bundle without vignetting. Edmund Scientific is a source for
high quality lenses of this type. There are other sources such as surplus optical suppliers. Some photo shops have selections of
used lenses that are also suitable. You may have to have an adapter tube or some sort of holder ring made to mount them in the
tubes you are using.
Typically the microscope objectives are designed to work at a projection distance of 160 to 250 mm. So a 10X objective will be
just about right for many purposes. Such an objective has a focal length of 16 mm. I personally use surplus microfiche lenses
which can be found for only $20 or $30 each in focal lengths of 10 to 30 mm. They are very excellent for this purpose. You will
find them in surplus optics catalogs.
Basically, the optical arrangement you want to use is to place the projection lens a distance of 150 to 250 mm (6" to 8") from the
imaging plane, that is the film or CCD chip and focus the telescope to place the real image in front of the lens at a distance slightly
greater than its focal length. Believe me that for a very modest cost for the projection lens you will obtain projection results better
that most eyepieces can ever deliver.
Projection Attachment
Here is shown an example of a projection attachment. The attachment is a 2" O.D. tube which is blackened inside. The tube has
on the right a standard 48 mm filter adapter ring to hold an IR filter of other filter as necessary. There is a position setting ring
which allows consistent setting of the tube in a standard 2" focuser. In this case a JMI focuser. The lens is slipped into the tube
inside a retaining ring mounted inside the tube. The lens shown is one of a set of four microfiche lenses of different focal lengths
that can be inserted. This provides for a variety of magnifications. The lens is held in place by the set screw shown.
The left end of the tube is terminated in a standard T-thread. To the T-thread in turn any number of camera attachments can be
appended. In this case a Canon breach mount is shown. This mount in turn can be bayoneted to the second adapter shown, a
Canon to C-type adapter, or to other adapters designed to fit a CCD imager, or a standard Canon camera for that matter. The
projection side spacing chosen for the particular lenses used in this projection tube is approximately 6 inches. Generally 6 to 8
inches is appropriate for the projection side of micro lenses or microscope objectives. The position of the adapter in the photo
corresponds to the above diagram.
I have found that using the Canon bayonet mount as an intermediate adapter has been very convenient. In this fashion all tubes
lenses, cameras and imagers can be made parfocal and easily changed. Because of the breach mount, which can be made very
tight, there is no slack in the entire structure.
Several books that give good advice on these issues are:
Astrophotography for the Amateur - M Covington - Cambridge rev. 1991
Astrophotography an Introduction - HIP Arnold - Sky and Telescope 1995
Astrophotography II - P Martinez - William Bell 1987
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Field of View for Eyepieces
Go to Table of Eyepiece Specifications
Note: In addition to the discussion following about eyepiece properties, there is the question of ensuring that the field stop is fully
illuminated by the telescope. This very important issue is discussed in a brief addendum at the end of this article. See: Field Stop
Illumination
This very simple topic seems to be misunderstood by many. The confusion comes from the rather loose terminology related to
eyepieces. There are two fields of view that are normally described. One field of view is the angular field of view that is seen in
the field stop of the eyepiece and is the actual angular field seen through the telescope of a section of the sky. This field of view is
determined only by the focal length of the telescope and the diameter of the field stop in the eyepiece. The second field of view is
that which the observer sees when looking into the eyepiece. This is usually called the apparent field of view. This has nothing to
do with the telescope. It is a property of the eyepiece.
The eyepiece presents to the eye an enlarged view of the field stop located in the eyepiece. This field stop is determined by the
desired eyepiece field of view, the magnification of the eyepiece and the focal length of the eyepiece. It is limited only by the size
of the mechanical tube holding it and of course the optical characteristics of the eyepiece. Eyepieces come in a great variety of
optical types. They range from very simple Kellner eyepieces which have simple optics to highly complex super-wide types. With
the wider type designs, the observer sees a very large apparent field of view. This can be quite impressive, but, it must be
emphasized that this large field is only that of the field stop and is only indirectly related to the actual size of the star field
observed.
Let us start by looking at a typical set of eyepieces. Shown below, in the top row, are the Meade 56 mm, 40 mm, 32 mm, 26 mm
and 24.5 mm.
In the bottom row are the 18 mm, 14 mm, 13 mm, 8.8 mm, 6.7 mm and 4.7 mm.
Now the actual field of view of the telescope is determined only by the size of the field stop and the focal length of the
telescope. The actual angular field of view is the diameter of the field stop divided by the focal length. This measure is in radians
and can be converted to degrees by multiplying by 57. This field of view is what is called the actual field of view of the telescope.
There is naturally a relationship between the actual field of view, the focal length of the eyepiece, the magnifying power of the
eyepiece and the apparent field of view that the observer sees. It is because of all of these factors and the way they are often
discussed in books that confusion arises. Let us try to clarify these issues.
The Meade descriptions for these eyepieces are given in the table. Descriptive terms will vary among manufacturers. (Some get
more glitzy for sales appeal)
Basically eyepieces fall into categories of normal, wide and ultra wide. This refers to the APPARENT field of view (APPAR). The
wider eyepieces look to the eye like they have a wider field of view. The actual field of view depends on factors described above
and is listed in some detail with examples below. The wider eyepieces generally have more glass in them, are heavier and more
expensive. (some excessively so in my opinion) The ACTUAL field of view (ACTU) is dependent upon the telescope focal length. It
is given for a 10" f 10 telescope (fl = 2450 mm) The Magnification (MAG) is also given for the 10" f10 telescope.
56 mm Super Plossl
40 mm Super Plossl
32 mm Super Plossl
26 mm Super Plossl
24.5 mm Super Wide
18 mm Super Wide
14 mm Ultra Wide
13.8 mm Super Wide
8.8 mm Ultra Wide
6.7 mm Ultra Wide
4.7 mm Ultra Wide
52
67
52
52
67
67
84
67
84
84
84
deg.
deg.
deg.
deg.
deg.
deg.
deg.
deg.
deg.
deg.
deg.
APPAR
APPAR
APPAR
APPAR
APPAR
APPAR
APPAR
APPAR
APPAR
APPAR
APPAR
1.15
1.06
0.67
0.54
0.66
0.48
0.47
0.37
0.30
0.23
0.16
deg.
deg.
deg.
deg.
deg.
deg.
deg.
deg.
deg.
deg.
deg.
ACTU
ACTU
ACTU
ACTU
ACTU
ACTU
ACTU
ACTU
ACTU
ACTU
ACTU
MAG
MAG
MAG
MAG
MAG
MAG
MAG
MAG
MAG
MAG
MAG
45
63
78
96
102
139
179
181
284
373
532
Field
Field
Field
Field
Field
Field
Field
Field
Field
Field
Field
Stop
Stop
Stop
Stop
Stop
Stop
Stop
Stop
Stop
Stop
Stop
49 mm
45 mm
29 mm
23 mm
28 mm
21 mm
20 mm
16 mm
13 mm
9.8 mm
6.8 mm
It is also apparent that the actual field of view is also the apparent field of view divided by the magnification. The diameter of the
field stop can be measured by placing a mm scale across the bottom of the eyepiece tube and estimating the size of the field stop.
The size of the field stop is limited by the size of the tube or course. This is why 2" eyepieces can see more of the sky. They can
have larger field stops. But notice that several of the 2" eyepieces are actually dual use. That is they can be placed on a 2" or a 11/4" tube. Thus the field stop is less than 1-1/4 inches of course.
Keep in mind that the actual field of view of the star field is dependent on the focal length of the telescope and the diameter of the
field stop. And that the apparent field of view is dependent only on the design of the eyepiece. But it is not glitzy enough to give
the size of the field stop. The eyepiece maker gives the focal length of the eyepiece, which is certainly important, the apparent
field of view of the eyepiece and sometimes the magnification (POWER). These are important pieces of information, but now to
find the actual field of view one has to go through an obscure and not obvious calculation. A fundamental way to calculate the
magnification is to take the focal length of the telescope and divide it by the focal length of the eyepiece to get the magnification
of the combination. POWER is an important term from a sales viewpoint. But it is not a characteristic of the eyepiece alone.
I feel it is important to think about optical elements in the most appropriate physical terms because one can then calculate other
things. For example, suppose one has a reticule at the position of the field stop. It will be in focus and will have some pattern on
it, often divisions of tenths or hundredths of a mm. (or of an inch) Suppose we want to know the angular arc subtended on the
star field of one of these divisions. We need know nothing except the distance between the divisions and the focal length of the
telescope. The distance between the divisions divided by the focal length of the telescope gives the angular measure on the sky in
radians. Multiply by 57 to get degrees.
For example, for the familiar Meade 9 mm eyepiece and with a 1600 mm telescope focal length, the actual angular view of the
outside circle is 83", that of the inner circle is 37" and that between the parallel lines 8.2". If the concept of actual field of view
were more commonly used, it would be clearer to observers why they can't see M31 no matter how high the POWER of their
telescope. Meade gives a nice table in their operating manual showing their eyepieces, their focal lengths, their apparent fields of
view and the actual fields of view when used with a variety of telescopes.
A better idea about the issues of the relative sizes of the glass elements and the field stops can be had by looking at the following
photographs.
Below are the field stop side of the 56 mm, 40 mm, 32 mm, 26 mm and 24.5 mm. All of these eyepieces have field stops about as
large as is possible for the tube size in which they are mounted. In order to have a larger actual angular field, the tube simply has
to be larger. Amateur telescopes typically have a largest tube of 2". A few have a tube of 3". On the other hand, 4" diameter
eyepieces are not uncommon on professional telescopes. Such eyepieces get very large, heavy and expensive.
Below are shown the field stop side of the 40 mm, 24.5 mm and the 8.8 mm eyepieces. The difference in the sizes of the field
stops are quite apparent.
Finally, below are shown the field stop side of the 26 mm, 18 mm, 13.8 mm 6.7 mm and 4.7 mm eyepieces.
A very nice book, which describes a great variety of optical features and simple calculations for telescope systems is
"Astrophotography II" by Patrick Martinez. It is published by Willmann Bell (www.willbell.com) It has a wonderful variety of
useful information for anyone interested in astrophotography.
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Field Stop Illumination Limits Caused by the Telescope
The above analysis helps the user select an eyepiece that has a large enough field stop so as to attain an actual field of view that
is desired. Another closely related issue is that of the ability of the telescope to fill the field stop with a real image of the sky that
is both sharp over the field stop and also uniformly illuminated. For refractor telescopes this is usually not a problem since the
objective lens throws a cone of light directly down the tube and is limited only by the size of the eyepiece holder and focuser
mechanism. If the telescope will take a 2" eyepiece, the focusing mechanism will generally be open enough to accept the
necessary cone of light.
The problem is not so simply with a more complex, folded optical system like the SCT. In the SCT, there is a secondary which
projects a cone of light down a rather restrictive tube to the final opening in the back plate of the telescope. It is easily possible for
this tube and the size of the opening in the back plate to limit the cone of light. With smaller SCTs it is almost certain that the
cone of light will show some vignetting at diameters will under 2". Thus, with the larger diameter eyepieces, the field stop may not
be fully illuminated. For example, a 2" 56 mm eyepiece has a field stop diameter of 49 mm. That is as large as a field stop can
possible be in a 2" tube. Note however, that if the 2" eyepiece is connected to the back plate of a SCT with an adapter of the type
that carries a Schmidt thread, the inside diameter of the opening in the adapter will be only 38 mm. It is clear that this type of
adapter will vignette the cone of illumination at the edges of the field stop in a 2" eyepiece. In order to get full illumination with a
field stop of 49 mm one should use an adapter with a full 2" diameter opening. This size is provided, for example, by the JMI
focuser, which has a clear opening of 50.6 mm when it is used with the special adapter plate that they provide. I must recommend
that when a 2" eyepiece is used, care be given to opening the connecting tube to the full 2" with appropriately large adapter
tubes. JMI, Optec, and Lumicon provide adapters which take these matters into consideration in their designs. (Remember that the
Schmidt Thread was designed for small SCTs and is quite marginally sized for 10" and larger telescopes.)
For a discussion of back plate opening sizes, see elsewhere on this website:Telescope Back Plate Aperture Sizes
This comment and warning is given because it is too bad to spend a large amount of money for a 2" eyepiece when the field stop
cannot be fully illuminated by the telescope. The use of a field reducer is especially problematical since they compress the cone of
illumination even more. The un-vignetted light cone for the 0.63 reducer is only about 26 mm. This circle of illumination is suitable
for a 1-1/4 " eyepiece only. In such a case, the longer focal length eyepiece will decrease the magnification without increasing the
actual field of view.
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Olympus D600L -- A Fine Digital Camera
I have been one to denigrate digital cameras for some time. See my infamous pixels shmixels note elsewhere on this web site. My
basic argument is that while the size of the pixels is small enough to compete with the grain of fast color films, the chips have
simply not been large enough to all for what I consider reasonably large final prints or displays. I have felt that the 800 by 600 PC
screen is minimal resolution and that a 1200 by 1000 pixel screen would be much more satisfactory. Such screen resolution is
now common on PC monitors.
To this time the only attachments for standard cameras have been available for Nikon or Canon cameras for the most part. And at
a price of $10,000. I have recently purchased an Olympus D600L which meets some of my expectations. This camera has a
chip of 2/3 inch size with 1265 by 1024 pixel resolution. This gives a file of 1.410 Meg. The resolution can be selected to be lower
to save file space. Medium resolution is still 1280 by 1024 pixels but is significantly compressed on storage to give a file of about
380 pixels. There is a lower quality setting which gives a resolution of only 640 by 512 pixels. The images are stored on a small
cards, 4 Meg, which can be quickly interchanged. In high resolution the card stores 4 images, in medium resolution 12 images
and in lower resolution 48 images. The lower resolution will give very nice 4" by 6" snapshot prints while the highest resolution is
satisfactory for full screen display on a good monitor with very good image quality.
The camera has a very fine 3:1 zoom lens, auto focus, built in flash and extensive editing capabilities within the camera itself.
There is through the lens viewing and a 1.8 inch LCD screen on the back which shows the image just taken for a few seconds in
full color so it can be reviewed by the photographer and others. Additionally, the screen cam be used to review any of the stored
images and they can be erased to make room for new images if desired. The entire data set can be quickly downloaded to a PC
using a serial port built into the camera with a cable provided. From there, with provided software, the images can be edited as
necessary. The downloaded format is standard JPEG. Thus any image editor that will handle JPEG can be used.
I have found the medium and especially the high quality images to be very fine. They are of much more than sufficient quality to
publish on a web site or in a newsletter. The only flaw I find with this camera is that it is best used in a fully automatic mode.
Thus it is really a point and shoot camera. This is a mode that almost guarantees an acceptable image but is a limitation for a
serious photographer who likes to have more control over the composition and lighting of the subject being photographed. While
the camera will take close ups to 1 ft. in the Macro mode, its performance is somewhat limited. The lens is relatively slow with the
aperture depending on the length of the zoom setting and varying from f 2.8 to f5.6. The effective ISO speed of the imager chip is
100.
Thus, in summary, this is, in my opinion, a nice little point and shoot camera with a lot of good features and a resolution that is
finally adequate. But it is just that, a point and shoot. The next step must surely be a professional level camera with this chip or
a larger one that will have interchangeable lenses and other features that the professional demands but which will still be in a
price range that is accessible to the average photo enthusiast.
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Color Filters Used for Three Color Imaging
and Related Concerns
Note: Since this article was written, a great deal of work has been done using negative color filters with CCD imaging. This topic is
not discuss here but will be the subject of a future article. Also, new filters are available from several sources that have the infrared rejection required for CCD imaging built in. See the SBIG web site for the latest information on these filters.
For 50 years I have used Wratten color filters for photography and have done everything from minor color correction to color
separation photography. This includes everything from minor color correction for color films to three color separation photography.
I believe that I understand color correction and three color photography quite well. I have reviewed the literature regarding color
correction and three color work for astronomical imaging and summarize what I have found in the following. The topic of color
filters has become of interest as a result of questions about the thickness of filters and the exposure times required in order to
obtain correct color balance. A list of the thickness of some filters is appended at the end of this note.
I became interested after I purchased a set of color filters from Optec and found a rather large ratio of exposure times for the
red, green and blue filters. Additionally, I have seen rather wide ranging recommendations for the sets of Wratten color filters to
use one of which included the choice of an 80A. I was quite astonished by this recommendation since the 80A is a color correction
filter and not a color separation filter at all. It must be noted that the choice of filters depends upon the application including
elements about the spectral sensitivity of the film in the case of photography and of the CCD chip in the case of electronic
imaging. There is probably no best set of color filters to use in general but there may be a best set to use when all factors are
known.
In a recent experience, I heard a talk at the Florida Keys Winter Star Party in which astronomical images very much more blue
that I have ever seen were presented with the comment that these images were more accurate than most of those seen in the
past. They were quite beautiful. We do not know, in terms of what the eye might see exactly what color the many objects in the
sky really are. Most dim extended objects appear gray to our eyes. I recently saw M57 through a 40" scope and for the first time I
saw slight color. For bright objects, like stars and planets, we do of course see color. The lack of color is simply that the light from
most extended objects is too dim for the eye to register color. The objects certainly have color and it may be captured with film
and CCD imaging.
Color filters may be of two types. Multi-layer dichroic filters can have very sharp and well defined pass bands. Wratten filters are
dye controlled filters and have pass bands which are in general much less sharp. As a starting point, and to limit this discussion,
we will assume that a three color system of imaging will be used. This will require a set of three filters that are broadly Red,
Green, and Blue. This contrasts to false color systems in which a band of the light spectrum may be assigned any color desired.
There are numerous sets of RGB filters that are traditionally used for three color imaging. The reconstitution of the image might be
on a computer screen with RGB colors of their own hue and saturation or by printing processes which all have their own color
representation and balance.
Common sets of RGB filters for photographic purposes are Wratten filters: Some possible sets are: (in order RGB)
Separation filters 29, 61, 47
Tricolor filters 25, 58, 47
Because the filters will be represented by numbers, a brief note is interjected at this point to describe the filter colors as they are
described in the Kodak manual:
Red filters:
23A, Light red.
25, Medium red. Red tricolor. For color separation work.
29, Deep red for use with No. 58
Green filters:
56, Light green.
57A, Medium green. (but lighter than the 58)
58, Medium green. Green tricolor. For color separation and tricolor printing.
61, Deep green tricolor. for use with Nos. 29 and 47
Blue filters:
80A, Light blue. Color correction to convert from 3200 K to 5500 K light
(tungsten to daylight)
38A, Medium blue. Absorbs red and some green.
47, Deep blue. Blue tricolor. For color separation work with Nos. 29 and 61.
Because of the way these filters, all of which are pass band filters, (Except the 80A which is a correction filter used with color film.)
behave in the pass and stop bands and particularly how rapidly they cut off, they will produce distinctly different color balance in
the final image. Astronomical, printed color images rarely give all of the information necessary to be able to judge the accuracy of
the color displayed. Often though the type of film, if color film, and the exposure times are given as well as the technique used to
generate the image. Malin for example, gives great detail about these matters in his book. But even then the variety of
photographic techniques used is largely directed to make nice looking images. That is, nice detail, nice color and the like.
Photometric color accuracy is not usually the goal.
An example of deviation from the normal (traditional) filter set is suggested by Wallis and Provin, whose color images are quite
wonderful. They suggest using the Wratten filter set for photography of 23A, 57A and 47. This set has a slightly lighter red filter
and a slightly lighter green filter. Their filter set gives a smoother coverage of the spectrum when used with monochrome film and
fills in the spectral gaps where there is appreciable astronomical information. They also recommend an exposure ratio of 1:1.5:2.
This seems to me to be a wise choice of filters and exposures.
We must realize that the spectral response of the CCD chip is very different from that of photographic film. CCD chips of the types
used in popular amateur imagers are highly sensitive to red and infra red and very insensitive to blue light. This means that
images taken with the traditional filter sets used for photography will require very long exposures for the blue filter compared to
the red filter in order to obtain good color balance. Additionally, the infra red portion of the spectrum must be suppressed or it will
alter the color balance of the composite image. Note that newer blue sensitive chips are being worked on but are currently
expensive and not generally available in amateur imagers. This situation has changed dramatically in the past year with the
production of the Kodak "E" series of chips which have much better blue sensitivity. Also, it should be noted that many if not most
filter sets now have built in infrared rejection. The latest information on these filters can be found on the SBIG web site.
Exposure ratios for the RGB of 1:2:4 or even 1:3:6 are not uncommon. It is generally required, for accurate color rendition, to use
an infra-red reject filter since leakage of infra-red through the color filters will spoil the color balance of the image. Infra-red
rejection filters are of two types. The "hot mirror" or multi-layer type has very sharp rejection characteristics and is generally
considered the best. Another type is simply heat absorbing glass which works fairly well but has a very slow cut-off and not
complete absorption of the infra-red wavelengths.
There is still considerable difference among professionals about the exact filter sets to use. Clearly it depends on the exact spectral
sensitivity of the CCD chip and the spectral accuracy required by the particular application. All manufacturers of filters sets have
shown images which are very beautiful. The color balance is controlled by exposure times and the color balancing attained in the
reproduction of the images as much as in the original choice of filter set.
I have a set of filters from Optec, a company well known for its excellence in photometry, which has a light red filter, a normal
green filter and a very dark blue filter when compared visually to the normal tricolor or separation filter sets. It appears that this
filter set requires a very long blue exposure both because of the density of the blue filter and the lack of sensitivity of the typical
CCD imager. I have seen a filter set consisting of 25, 58 and 38A recommended. This seems to me to be a very good filter set for
CCD imaging since it has a slightly lighter blue filter than the 47 recommended for photographic film. Because the CCD chip is so
insensitive to blue, the exposure for the blue image will not have to be unreasonably out of proportion to the red and green
exposures with the lighter blue filter.
On the other hand Hoot has recommended a filter set consisting of a 23A, 56 and 80A. The 23A is a light red filter, the 56 is a
light green filter and the 80A is a very light blue filter. In fact the 80A is not really a band pass filter at all but a color correction
filter used to change 3200 K tungsten light to daylight balance of 5500 K. It passes 80% blue and 20% red and is used to correct
daylight color film for use with 3200 K (tungsten) illumination. This does not mean that this filter set is necessarily bad. This set
has an exposure ratio of 1:1:1. Such a ratio is useful since it is conserves some time to take a blue exposure which is shorter than
that usually recommended for blue filters. The total time required for the blue image so often
nearly half of the total time. It was also stated that this filter set gives nice looking images rather than color accurate images.
Since the red and green filters are also lighter than usually recommended, the whole set gives overall shorter exposure times.
Since the blue filter is very light it gives an image which is heavy in blue but with considerable green and red. The very lightness
of the blue filter makes up for the lack of sensitivity of the CCD chip in the blue region of the spectrum.
If this set of filters gives nice looking color images, that is fine. But, we should have no illusions that it is accurate color. For
astronomical color imaging a great latitude is allowable since no side by side comparison can be made as it can with photographic
imaging of earthly objects. We are happy to see images that are not just gray. A photometrically accurate set of color filters
combined with the lack of blue sensitivity of the CCD chip requires an almost excessively long blue exposure. We may have to wait
for the general availability of blue sensitive CCD chips to solve this problem.
Addendum (August 1998): Additional discussions about three color imaging has come up with still another set of filters that might
be considered. The idea was to retain reasonable passband integrity but lighten the blue filter so that the total exposure time
would be shortened. This set is 23A light red, 57A light green and 38A light blue. These filters have fairly broad passbands but still
have stopbands which should give reasonable color separation.
Then again what is accurate color? Is it the image in one book or another, the color slides I saw a year ago, the color I see on the
computer screen, the color I saw at the Winter Star Party or some other color balance? It's hard to tell. One might now say,
"What about the color that color film shows?" Is that accurate? Not so simple to say. Color balance is a strong function of
exposure time and reciprocity failure often dominates. Color film can give quite unbalanced color images as well. Professional films
are made for short exposures of under 1/10 second and for exposures of several seconds to insure controlled color balance. It is
probably best to judge a color image on the basis of its detail its range of colors and its general beauty. These are subjective
judgments indeed.
Filter thickness is an important issue when taking three color images. The filters must be exactly the same thickness or there will
be an image focus shift at the imager. Filters vary in thickness considerably. I have always made a practice of keeping a record of
filter thickness for my photographic work.
For a reference, here are some values for a few 2" filters I use.
Lumicon Deep Sky Filter 2.55 mm
Lumicon UHC Filter 3.30 mm
Lumicon H alpha pass 2.18 mm
Lumicon Minus Violet 1.90 mm
Clear Filter 2.60
Clear filter 3.30
I keep these to match the Lumicon filters above when used in the Optec 2" filter slider.
Optec Hot Mirror (minus IR) 4.25 mm
This filter is used with all color filters to get rid of IR
Optec separation filters Red, Green and Blue 2.00 mm
This set of color filters are each exactly the same thickness which is essential for color imaging.
Hoya
Hoya
Hoya
Hoya
ND Filter 2.5 mm
Orange Filter 1.94 mm
Red Filter 1.98 mm
Blue Filter 2.18 mm
The color filter thicknesses in the SBIG color wheel are the same and those in the Meade color wheel are not according to the
information I have seen.
I hope that this information will be of some value to those considering color imaging. (11 September 1997 )
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Solar Light and Solar Filters
Go back to Solar Filters
Goto Solar Filter Addendum
The issue of use of filters to view the sun came up recently on this site and there followed a discussion of how these special solar
filters worked and why they enabled viewing of solar phenomena is such beautiful detail. In order to understand how the filters do
their job, it is necessary to have a basic understanding of the nature of the light emitted by the sun. I have attempted to meet the
challenge posted by some MAPUGGERs. This has been a difficult assignment. I hope this post will be useful.
The details of how the sun works are obviously too complex to describe here. Even the brief but excellent descriptions given in the
Encyclopedia Britannica take many pages. So what follows is a super brief description of the basic processes by which photons are
emitted and absorbed and what we see as a result.
The body of the sun is a mass of hot gasses under great gravitational force because of its mass and with great internal pressure
because of the nuclear reactions that take place deep within the sun. The internal pressure and temperature are large enough to
cause nuclear fusion of hydrogen atoms into helium atoms. (Some 15,000,000 K) This process generates photons which migrate
outward toward the surface of the sun. The photon pressure from the inside is balanced by the gravitational forces. The rather well
defined ball we see as the sun is cooler at the surface. It radiates from its surface at a temperature of about 5800 K. The surface
we see is called the photosphere. The radiation from the photosphere is characteristic of that of a black body radiating at 5800 K
according to Planck•s Law. This is the familiar yellowish color we see . We may compare this to earthly temperatures, a common
incandescent bulb radiates at about 2800 K. Thus the light bulb also has a continuous spectrum but is much more yellow/red in
color compared to the sun. The important thing to realize about the sun is that its basic emission is a continuous spectrum
because it is due to thermal excitation.
Immediately above the photosphere is a layer called the chromosphere. The chromosphere has an emission spectrum of spectral
lines which correspond to those of the gases of which it is constituted. In this case, the emission is in discrete lines because the
gas is relatively rarefied.
Still further out is a region called the corona which consists of very rarefied gasses. These gasses eventually become the solar
wind. During a total eclipse, the corona can be seen as a whitish halo extending several degrees out from the occulted solar disc.
Basic to the understanding of the observed solar spectrums from the various parts of the layers about the sun is the concept of
absorption and radiation of photons from atomic gasses. At low temperatures atoms are in their rest states and do not radiate
significantly. But when atoms in a gas are heated thermally they absorb energy, electrons move to higher (excited) states and
they can then re-radiate this energy in the form of photons. Each atom with its array of electrons radiates photons with
characteristic energies. The line spectra emitted can be used to identify the elements involved. Atoms can also be excited by other
photons, go to excited states and re-radiate their characteristic photons. We see these photon emissions in many lights such as
sodium vapor and mercury vapor lights as well as neon signs and the like. These emissions are not continuous spectra but are line
spectra. Helium was discovered first on the sun because of its characteristic radiation spectrum.
One of the effects we see in sunlight is that the continuous Planck Law radiation is interrupted with dark lines. This is caused by
cooler gases at the outer surface of the sun absorbing photons of certain energies, thus diminishing the spectrum in what are
called absorption lines. These dark lines tell use which elements are doing the absorption in the cooler gases.
The surface of the sun is in a constant state of agitation like a pot of boiling water. Thus there are hotter and cooler regions on the
surface. Because of the strong emission of radiation from hotter regions and absorption of radiation in the cooler regions, the
surface of the sun has a granular or reticulated look. Very large boiling regions are seen as sun spots. Sun spots are so large that
they can be seen with a very simple filter that does no more than attenuate the total light from the sun by a factor of 100,00 or
so.
Very fine filters for observing sun spots are available from several sources. One of the best is Thousand Oaks Optical (800 996
9111). I personally use their full aperture Type 2 plus filters on my LX200s.
In addition to the constant boiling of the solar surface there are much greater spurts of material that are ejected from the surface
of the sun which return in gigantic arcs. These eruptions are called prominences. The are visible during a solar eclipse as arcs of
flame projecting well beyond the dark disc of the occulting moon.
The granularity, the chromosphere and the solar prominences are so dim compared to the photosphere that they cannot be
observed except during a solar eclipse or with very special telescopes and filters. While these features absorb and emit photons in
many parts of the spectrum, one special line is of particular interest. It is the Hydrogen alpha line at 6562.8 Angstroms. This line
is emitted strongly by the features we want to observe. The scheme required to form images is to use this spectral line while
suppressing all other light from the sun. This is done with special interference filters. These filters are very complex, being made
up of numerous thin interference layers and many reflecting optical elements. They are adjusted to cancel all wavelengths except
that of the Hydrogen alpha line. These filters can be made to have passbands that are so narrow that they only pass from a few
down to a fraction of an Angstrom. Such filters are often combined with other special optical attachments which are designed to
obscure the image of the disc of the sun and so make prominences even more visible.
Filters for Observing the Sun
While specialized filters and optical attachments such as those required to observe solar granularity and prominences are, because
of their price, usually out of reach of the amateur astronomer they never the less are enticing. The observation of stars should
surely include our very own star. Ellery Hale was, for example, fascinated by the sun all of his life and made a significant
reputation through his studies of the sun.
Solar filters and attachments are described in some detail in "Solar Astronomy Handbook" by Beck et al (Willmann-Bell) The
design and construction of these devices is for very serious amateurs only. Excellent hydrogen alpha filters are available from a
number of sources. A discussion of the daystar filters is available at <http://www.company7.com/>. These filters are very
expensive and require additional pre-filtering to eliminate the massive heat load from the sun. These pre filters generally have a
small stop to reduce the light gathering power of larger aperture telescopes. The stop is usually set to about f30. This feature has
advantages and disadvantages. The small stop reduces the resolution of the telescope but the narrow cone of light improves the
performance of the final narrow band filter which is at the eyepiece. The filters are very complex multilayer structures which are
designed to reduce the bandwidth to Angstrom or usually sub Angstrom width as described below. Because the filters have such a
narrow bandwidth they need to be tuned during observation. The best of the filters are tuned by controlling the temperature of the
filter by means of a small oven surrounding it. Additional special optical elements are used with the narrowest of these filters to
insure that the filter is in a part of the optical path where the imaging rays are parallel. Less expensive filters are tuned by tilting
the filter with respect to the entering cone of light. This technique work fairly well and the design greatly reduces the cost of the
filters. The cost is also determined by the narrowness of the filter. The width of the filter needs to be chosen to match the purpose
of the observer.
An elegant description of the view seen with filters of various bandwidths is given in "The Manual of Advanced Celestial
Photography" by Wallis and Provin (Cambridge University Press 1988). I quote:
"With a 1 Angstrom bandwidth the prominences lining the edge of the disk can be easily seen and solar flares can be viewed but
the finer details over the sun•s disc are not well displayed. With a 0.8 Angstrom bandwidth, the prominences are seen very well,
with better contrast than is provided by a 1 Angstrom filter, and the details on the disc of the sun are readily seen in detail. With a
0.6 Angstrom bandwidth , the details on the disk are seen with excellent contrast and clarity however, the prominences are now
difficult to see."
Thus it is important to choose exactly the right filter for the visual effect desired. The inexpensive filters which are tuned by tilting
do not give a uniform appearance over the entire field of view. As with so many things, the expensive professional filters are the
best. The wider, 1.5 Angstron filters will be a disappointment to many viewers.
The Wallis and Provin book is, I believe, the most useful book on celestial photography I have found. I recommend it highly. I
have appended to this note a list of other books on astronomical photography which may be of interest, with my notation about
their suitability for various levels of expertise.
______________________________________________________________________________________
Photography References - With particular attention to the Sun and Solar System
Personal comments about a few select books from my library which I have studied in some detail - Doc G (14 September 1997)
A Manual of Advanced Celestial Photography; Brad D. Wallis & Robert W. Provin; Cambridge University Press 1988
The first among good books on astronomical photography. The book concentrates on photographic techniques rather than
equipment. This is an extraordinary book and a must for anyone doing or wanting to do astronomical photography. Not for the
novice but a required book after digesting one or two of the following books.
High Resolution Astrophotography; Jean Dragesco; Cambridge University Press 1995 An excellent book with emphasis on
photography of the Sun, Moon and planets. Equipment is described in detail and numerous examples are shown. Not for the
novice.
Solar Astronomy Handbook; Beck, Hilbrecht, Reinsch, Volker; SONNE (1982 German) (first English Edition 1995) Willmann-Bell
Great detail about equipment and many examples of photograph results. Lengthily discussion of observations and recording of all
solar phenomena. Definitely for those with a deep interest in the Sun. Definitely not for the novice.
Astrophotography - An Introduction; H.J.P. Arnold; Sky and Telescope 1995 A nice, and simple, introduction mainly to
photography of the Sun, Moon and Planets. A fine book for the novice and the serious amateur.
Astrophotography II - Featuring the Techniques of the European Amateur; Patrick Martinez; (1983 French) (English Edition 1987)
Willmann-Bell, Inc.; A fine small book jammed with important information that everyone should know about setting up telescopes
for photography. An excellent book for the serious amateur.
Astrophotography for The Amateur (revised 1991); Michael Covington; Cambridge University Press A nice little book that covers
the basics very clearly. The book tries to cover a bit too much ground for such a short book. Strictly for the novice.
The Cambridge Eclipse Photography Guide; Jay M. Pasachoff and Michael A. Covington; Cambridge University Press 1993
A very nice book about eclipses with detailed tables covering eclipses through 1999. Excellent discussions of observing and
photographing these events.
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Filters for Solar Photography (an addendum)
This is a summary of Mapug-Astronomy posts with most of the information provided by Chris Frye and John Hooper. However, any
errors can be attributed to me.
Recent posts on Mapug-Astronomy have mentioned a method for photographing the sun in the light of the Calcium H and K
emission lines. It seems that Martinez in his book Astrophotography II suggests that a filter which passes these two lines in the
390 to 400 nm region can be made using a Wratten 18A and a Wratten 2B filter together. Together the two band edges of these
filters have an overlap such as to give a passband that has a peak at 396 nm. This passes the Calcium H line at 393 nm and the K
line at 396 nm The idea is that with the hydrogen emission cut out the "faculae" on the sun become visible.
Inspecting the filter passbands actually reveals that the 18A filter has two windows of transmission. One peaks at 360 nm and the
other at 750 nm. One is in the near ultraviolet and the other in the near infrared. The combination of ultraviolet and infrared
passed may give this filter an apparent very dark green tinge. But it is essentially opaque in the visible region of the spectrum. It
is the ultraviolet passband that is combined with the 2B filter which passes light longer than 390 nm. Thus the 2B filter takes out
the ultraviolet below 390 nm and the 18A filter takes out wavelengths above 400 nm. This leaves a passband at about 396 nm.
However, the 2B filter passes all of the infrared and the 18A has a transmission window in the infrared. Thus an infrared rejection
filter has to be used to remove the infrared which leaks through the basic filter combination. The 18A filter comes in glass
according to the Kodak reference information. Glass passes all wavelengths longer than 320 nm. The infrared filter required would
be a so called "hot mirror" such as is used for any filtered imaging with a CCD imager. If film is used, the film must be insensitive
to infrared or be used with the "hot mirror" as well. Tech pan for example is quite infrared sensitive.
The above filters are of course used with a regular solar filter as well to cut down the intense light from the sun. The pass band
characteristics of this "pre-filter" must also be taken into account. Some give an orange-yellow image and some give a blue image.
It is probable that the lighter series 3 filter from Thousand Oaks would be satisfactory since the filter combination shows very
great attenuation even in the "pass band." Note that I have not seen any publication of the actual bandpass curves for either the
glass (orange image) or the mylar film type (blue image) solar filters. If either of these rejects the 400 nm region, it would not be
suitable for the Calcium lines of course. It must be noted that an energy rejection filter supplied with the H alpha type filters is not
suitable since they generally pass just a range of wavelengths between 640 and 740 nm.
It has also been pointed out that there are two filters in the Special Filters section of the Schneider Optics (Kreuznach, Germany)
catalog that are of interest. One is the #420 which starts to pass wavelengths longer that 387 nm and the # 403 which passes
wavelengths below 407 nm. These in combination also give a window at the wavelengths of the Calcium lines. The combined
transmission is about 2.5% at 397 nm.
I have not personally experimented with these filter sets. But it looks like an interesting area for experimentation.
While discussing solar filters, it should be mentioned that there are also very narrow band filters (of the order of 2 to 0.4 angstrom
widths) for the Hydrogen alpha wavelength of 6563 angstroms and also for the Calcium line at 3934 angstroms. Thousand Oaks
and Day-Star make such filters. These filters are described elsewhere on this web site.
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TPoint and Its Use With the LX200 Telescope
History
I have been interested in the use of Software Bisque's TPoint program on applied to the LX200 telescope for some time. I
purchased the program in August of 1997. It is not an inexpensive accessory costing some $249. I expected it to magically turn
my 12" LX200 into a telescope that would at least meet its specifications of GoTo accuracy of a few arc seconds. It did not do this
for reasons that are complicated and not the fault of the TPoint program. I still have the program and plan to use it eventually
when I get a telescope that will respond better to its application.
In October of 1997, there was a series of posts regarding this issue. Again in April of 1998 there was a brief series of posts which
discussed the origins of the TPoint concept which was developed in England by Patrick Wallace for use on professional telescopes
and the application of the program Software Bisque in the Windows environment. More recently, November 1998, there has been
a very much larger series of posts which have been much more detailed but have reached no new conclusions that I can detect.
The total number of posts has exceeded 50. I have in addition had several conversations with Bisque about the program and
several posts from Patrick Wallace.
In Fall 0f 1997, I tested the program rather extensively with my LX200 12" scope which is polar mounted on a permanent pier and
inside a roll off building. I found that the program worked to the extent that the telescope performed better with it than without it.
I did not consider the performance adequate to establish control that could be relied on for a totally remote operation. Thus I have
not used the program for about a year but plan to use it again in a new observatory setup which will be ready in Spring of 1999
and which will have a much more mechanically stable mount.
The following is a summary of how TPoint operates and why it was not completely satisfactory in my application to the LX200.
Basics of Operation and Trial Run of the TPoint Program
The basic concept of TPoint is fairly simple. This is not to imply that either designing the algorithms nor carrying it out in practice
are simple matters. I feel that Bisque has done a very fine job of implementing the original program and of integrating it with their
fine SKY planetarium program. The idea of the program is to measure the coordinates to which the telescope points when it is told
to point to specific coordinates, to remember this information and make a map of the sky so that when the telescope is told to
point to specific coordinates it will actually do so. This is done by making a small pointing correction each time a coordinate set is
requested.
It works this way. The program is trained by telling the telescope to point to a set of stars, generally 40 to 50 are selected, then
manually centering the stars and allowing the program to determine and remember how much position correction is required. This
is done over the entire visible sky. The program then can be made to point the telescope in such a way as to take account of the
fact that the telescope has to be slightly mispointed to have it go to the correct position.
In theory and fact this concept works. The major problem that one finds is that a mechanically unstable telescope or one that is
very sensitive to loading will have a different mapping correction for each mechanical condition. The program can take care of this
situation by remembering a different mapping for each major change in loading of the telescope. One can for example have a map
for visual work, another for loading with a camera and guider telescope and still another for a guider and heavy piggy back camera
and so forth. Problems arise when the mechanics of the telescope are weak or marginal in some way. That is, if the telescope does
not repeat its GoTo position with repeatability. This is a very fundamental limitation with the particular LX200 telescope on which
I tried to apply TPoint This telescope did not point reliably because it had significant flexibility and looseness in the bearings that
did not appear to be repeatable or that changed from use to use. It was not determined if this was due to temperature changes,
shifting of parts, inadequate bearings or whatever. It was clear that basic pointing accuracy was no better than 5 to 10 arc
minutes at best and often as bad as 20 arc minutes from the desired location. The manufacturer claims 5 arc minutes with normal
GOTO pointing operations. This accuracy was never approached with the original telescope. It has since been modified so that
better results have been obtained. This issue is discussed later.
It was hoped that the pointing accuracy might be improved by as much as five to ten times. This was not the case. For a few
limited trials, with light loads and visual equipment only, the pointing became as good as 2 to 5 arc minutes. This is quite
adequate for visual work but falls short of what is necessary for imaging and remote control. It was determined only that the
telescope was weak enough mechanically that it could not be improved as much as desired using TPoint This was not felt to be the
fault of the TPoint program but a fundamental mechanical limitation of the telescope structure. When the telescope was heavily
loaded with a separate guide scope and a heavy camera, the pointing results were much poorer.
In order to stabilize the mechanical performance, I was careful to keep the clutches locked so the gears were used over the same
ranges and the load and balance were also kept as constant as possible. Cables were carefully routed to reduce strain on the OTA.
The mirror was also locked in position. These measures helped but did not produce fully satisfactory results.
Discussion of These Results
After these experiments of Fall 1997, which went on for some weeks, brief discussions were had with the Bisques about this
application of TPoint and some posts were exchanged with Mr. Wallace who is one of the principles in the development of the basic
concepts behind TPoint They both seemed to agree that the program cannot make perfect a telescope that changes mechanically
from time to time with little apparent reason such as mirror flop and that it cannot account for lash in the drives that is nonlinear.
One of the major problems with the LX telescope that I used was the poor quality of the declination drive, lash in the drive and
stickiness of the drives and bearings.
In the meantime, my 12" LX200 has been modified considerably. The declination bearings have been replaced and the drives have
been tightened to remove most of the lash. The TPoint program will be tried again in the near future to see if the improved
telescope mechanics will improve the performance of the system enough to make TPoint more effective.
Summery of Reports From Others Who Have Used TPoint on the LX200
This summary of reports from the posts on MAPUG-Astronomy essentially support the experiments and conclusions drawn above.
But there have been some better and some worse results.
A series of no less that six posts on November 12, 1998 ran from one that said TPoint simply could not model an LX200 to one
that said that TPoint worked fairly well. The words used were that the telescope pointed better with it than without it. Faint praise
indeed, but at least an optimistic report. One report was that the telescope had fixed mechanical misalignment problems which
might be the problem. Of course, it is just these problems that TPoint is supposed to and does fix.
On the next day, a nicely detailed report on a TPoint application came in. The report indicated that before applying TPoint a GOTO
placed the desired object on a 5 by 8 arc minute chip only about 20 to 30% of the time. After applying TPoint this score improved
to 40 to 50%. And, on another night the score improved to 68%. This is interesting data. But the mystery is that the 77 correction
points fit in a circle of 1.7 arc minutes. Still only 70 % of the objects fell on the chip of size 5 by 8 arc minutes. It seems that with
such a good PDF the objects should hit the chip every time.
There was some discussion of using TPoint with the LXD 650 and 750 mounts. It was not clear in these posts if TPoint helped
much but pointing accuracy of about 10 arc minutes was mentioned for the mounts.
By the 16 November there was a flurry of posts about the LXD 650 and 750 mounts and several others. There was indication the
repeatability was very good with these mounts though accuracy was still only about 10 arc minutes. If the repeatability was very
good, then TPoint would be expected to correct the absolute pointing accuracy by a good amount. These reports, while short on
data, are quite encouraging. There was some discussion of the lack of perfect orthogonality of the RA and DEC axes and flexing. Of
course TPoint is designed to compensate for just exactly these mechanical defects. Again, if they are repeatable.
There was also some discussion of HPP which all LX200s have. In a way, HPP is a local mapping correction. It works by resynchronizing the telescope for a small region of the sky. It is in no way a replacement for a full sky mapping such as that done by
TPoint nor is it a mapping algorithm.
Several mounts with noted excellent stability were mentioned. These are:
●
●
●
●
●
●
●
Byers $15,000 The best portable mount. Period. But by far the most expensive, the term portable verges on misleading,
>100# photo load.
AP 1200 $7000 GOTO Specs look good. 1 Year wait.
AP 900 $??? GOTO Specs look good. 1 Year wait.
Losmandy GM200 $6500 The best value. It will carry a 100# photo load, 8000 tics/rev encoders, and good periodicity.
Losmandy G-11 $3000 Decent lightweight mount, fine for a small SCT but becomes shaky under heavier loads, 40# photo
load.
Takahashi EM500 $8500 fantastic tracking, will carry 90# photo load, expensive, pier alone weights 140#. GOTO available
in a year or so.
Takahashi NJP $5500 fantastic tracking, will carry 60# photo load, expensive, GOTO in beta testing, lightest of all but the G11, rock steady, extremely high precision encoders available.
It is interesting to note that most all of these are more than an entire 12" LX200. (which is now about $4400)
Summary
In summary, it is apparent that TPoint works but it cannot make a mechanically weak telescope into a perfect one. This was the
conclusion in October 1997, April 1998 and again in November 1998. When the telescope is strong and mechanically sound,
TPoint can do wonders to improve pointing accuracy.
I am looking forward to a more detailed set of comments from Patrick Wallace who has indicated that he will post to MAPUGAstronomy in the near future. In the meantime here are his preliminary remarks from a November 13, 1998 post to the MAPUGAstronomy group.
Continuing Topics and Advice
From Patrick Wallace:
I'm following the correspondence on TPoint and hope to be able to contribute something useful over the next couple of days. Here
are a few preliminary remarks:
« As one subscriber said, you can't make a silk purse out of a sow's ear. The results can't be any better than the encoders and any
hysteresis effects (for example mirror slop). What TPoint will do is get the best out of whatever telescope you've got.
« Poor results are often because something untoward happened during the test run - something shifted, or it was necessary to
resync for example. The best you can do with such a test is to chop out the good bit and just use that.
« You should only use enough terms to match the quality of the data and the size of the run. Including more and more terms,
especially powerful harmonics and polynomials, will reduce the RMS but make the model worse. If there's trouble, cut right back
to the 6 geometric terms (IH ID NP CH ME MA for an equatorial) and look at the residuals for signs of two populations etc.
« When you've changed your setup, fix those terms which ought not to have changed and only fit the ones that are likely to be
different. For example, if you've got a fixed mounting and no big changes of weight, fit only IH ID and CH. All the other terms in
the model, such as the polar axis terms ME and MA and any flexure or runout terms, should be included but fixed at the values
you got from a previous full-scale test. That way, the start of night procedure needs only to be 3-5 stars typically, to absorb the
resynced zero points and any collimation changes.
« If your model is sensible (not too many terms) and you don't seem to get as good a result in practice as the PSD figure claims,
suspect the implementation of the model in the telescope control system. There's an easy way to test this - do a "dummy pointing
run". Do a normal TPoint run, but leave the telescope where it is each time: don't recenter the image. When you reduce the run,
you should get a small PSD (as limited by the encoder resolution) and all the coefficients should come out to whatever you told
your control system to use.
More in due course.
Patrick Wallace
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This material provided by Frank Loch. I thank him for allowing me to publish this fine article.
THE DEC BEARING MODIFICATION TO THE 12" LX200 Classic
-- or The Best Change I Ever Made To The 12" LX200
To Improve Its Performance!
Also My Experiences with TPoint Software
Early on after the purchase and initial use of the 12" LX200 Classic, I began to realize that there was substantial friction in the DEC
axis bearings or a problem with axis alignment. Several times I measured that about 3-3.5 lbs. of force applied to the corrector
end of the OTA at a right angle was required to initiate motion even though the OTA was balanced (?). I question the "balance"
here because the friction was so high that it was very difficult to determine that the OTA was really balanced. Also, initially, even
with the DEC backlash set at the max (99), there was still very significant backlash . I never tried to measure it, but it was a lot -sometimes 5-8 seconds would elapse before motion was detected in DEC axis.
As a result of the high friction, I was pretty well convinced that the GoTo performance was severely compromised. I struggled with
this lack of pointing accuracy for several years. At one time I loosened one of the forks in an attempt to find a better position for it
to improve the alignment and thereby reduce the DEC axis friction. This action was not very helpful.
In November of 1998 I purchased TPoint from Software Bisque. I was hoping that TPoint would improve the pointing accuracy (i.e.
make a silk purse out of a sow's ear!) without having to make any other changes. No such luck!
TPoint is a fine program designed to improve pointing. After working with TPoint for several weeks, the data TPoint produced very
clearly showed me that the most significant pointing errors were DEC related and a very high scattering of data points could
clearly be seen in the DEC axis direction compared with the RA axis scattering. Studying the data as it was accumulating, it was
obvious that "inconsistent" and "unrepeatable " errors were killing the pointing accuracy in the DEC direction. TPoint needs
"repeatable" and rather consistent errors to perform its magic correctly (and magic it has, believe me). I estimated that pointing
accuracy even with TPoint was un-predictably erratic and at least +/- 15 arcminutes. Thus, the high DEC "Stiction" was killing both
the LX200 pointing accuracy and "GOTO" performance.
AN IMPORTANT DECISION
At that point I made the decision to perform Michael Hart's DEC bearing replacement as nicely described on Doc G's website
Decfixmajor. I was initially very "scared" of undertaking this procedure (as it turns out a needless fear). Even though I consider
that I have above average mechanical skills and a Professional Mechanical Engineering background, the very thought of tearing
apart a $5000 scope and getting it back together again with satisfactory alignment was very daunting.
I followed Michael's and Doc G's instructions very closely. I will only add now a bit here and there to report my own additions,
findings, or possible suggestions to an already fine procedure (many thanks Michael & Doc).
First, I obtained the the two needle bearings (as Torrington BH1616 - $5.49 each) originally specified SCH1616INA -1" shaft ID x
1.313 OD x 1" long from the Brown Bearing Co, (a well known bearing supply house here in S.E. PA). I also obtained 6 (not
knowing exactly how many I would need - actually I only used one) Torrington TRA1625 Thrust washers ($0.64 each), and a 3/4
oz supply (smallest available --$12.00) of Locktite #242 (I actually only used a few drops).
Secondly, I prepared a sturdy table in our family room to receive the scope and then with the assistance of my son Kermit,
loosened (at the pier) the front mounting bolt (scope to super wedge) and removed the two rear (lower) mounting bolts, and
finally sliding the scope off the super wedge, we carried it into the family room and placed it on the previously prepared table in
the Alt/AZ orientation.
Next, using an Exacto knife, I scored the fork to base locations on both forks as in Michael Heart's Figure 4. Then I removed the
DEC drive cover and released the DEC drive electrical cable from its lower fork socket. Finally I removed the DEC drive motor and
control assembly, checked it carefully for any unusual bearing slop (found none) and placed it in a plastic baggie to keep it clean.
I marked the worm gear for future re-orientation and then removed it. Using an old tooth brush and a little alcohol, I cleaned off
all the old "black moly" grease and any dirt particles from the worm gear and placed it in a plastic baggie to keep it clean.
Now the "fun" part (read really scary part here, because if this fails, there is no way to proceed further) begins. This is the action
of removing the two allen screws from the inner clutch plate. Check out your Propane torch first and make sure you can get a
"hot" 2" flame with a 1" super blue "very hot" inner cone showing. If you can't get a really hot flame, get another torch that can
produce a really "hot" sky blue inner cone. Get the proper size allen wrench ready (a long one with at least a 3 1/2 " torque arm
available).
Heat and remove the allen screws one at a time. Place the point of the hot blue torch cone right at the head of the socket screw
(maybe a 1/8 to 1/4 inch space from the tip of the hot blue torch cone to the screw head). Heat for 20 seconds (Michael Hart says
5-10 seconds). Try to remove the screw. If it feels too tight yet (this is a judgment call - you don't want to strip the socket in the
screw head), apply heat for 30 more seconds and try to remove the screw again. Thirty seconds did not work for me -- neither did
40 seconds -- I'm getting very worried now! Try 60 seconds! That worked for me for both screws -- whew! Sit down and pour a
sherry! The rest is much easier and less stressful! Now remove the Clutch plate (after it cools a bit so you can touch it).
Gently lay the scope over on its side so the bulk of the OTA is laying down on the table. Remove the four long allen screws from
the left fork and remove the fork carefully from the OTA. Remove its old plastic sleeve bearing and set it aside. Now, very carefully
"jiggle" the OTA off the other (Right) fork and base assembly. Remove the plastic bearing from the right fork. Now using Duct tape
or masking tape, tape up all the cracks and openings in the LX200 computer base assembly to keep it clean and "chip free" during
the "boring out" procedure.
Researching in the phone book, I located a machine shop only a few miles from my location and after a short phone call I was
pretty sure that they could do the "boring" out successfully. I took the separate fork and taped up fork assembly to them and
within a day the job was finished. They even "pressed" the new bearings in place for me saving me from having to do that. Total
charge $60.00.
I re-assembled the scope following Michael Hart's instructions. This was pretty straight forward and not very difficult. I lubricated
the new needle bearings with red Bosch bearing grease. I used a few drops of "locktite" to lock the DEC clutch plate Allen screws
again. No "primer" was used.
WOW!
WoW! Zero DEC bearing friction! I even think that's an under statement! Better hold on to the OTA as now it really "books" if you
let it go "unbalanced". A virtually "free floating" OTA. I had no idea it could be like this! Can't wait to power up and test! But, must
finish this assembly and alignment first.
While performing the OTA alignment, the longest focal length eye piece I had available was 26 mm, so the resulting light cone
spot at the RA bearing retaining screw (see MH's procedure) was about as big as a nickel. So I had to "eyeball" the centering of
this spot around the screw head (not too precise, but OK). At an auto parts supply store, I found the "black moly" grease for the
DEC worm gear. Re-installing the DEC drive motor assembly was un-eventful. I used a few drops of locktite on the DEC setting
circle spacer to lock the setting circle in place.
OK! Time to power up and test. First, static balance was set with the Meade sliding weights. Now apply the power and boot up!
With the DEC drive cover off it's time to check out the DEC drive visually to see what's going on. Hmm! I can hardly hear the DEC
drive motor running now when I push the keypad N/S buttons. Nice! No more DEC grinding sound! Lets look at the the DEC worm
action. Before the bearing modification, the DEC worm would flip out of position and the spring supported plate would dip down
about 1/8th inch or so at the onset of each new command. Hmm! Difficult to see it moving at all now. Substantial improvement
here. Never thought I would see the worm so still! (later when mounted on the pier, I was able to set the backlash at 75 and have
no (yes zero) backlash.
Did all this improve the pointing accuracy?? You better believe it. Now with Tpoint its better than +/- 2 arcminutes, and it puts the
target inside the ST7 frame at f/10 100% of the time (54 successes out of 54 Slews). I can't believe that this modification could
make that much difference, but it shurly does! Caution -- Further extended testing is in progress and the 100% factor may
eventually drop a percentage point or so with this. My latest mapping run on 01/05/99 had another 100% success rate for 47
mapped points. However, I feel current results may be too optimistic until I have additional data to support it.
AFTERTHOUGHTS
First I want to thank Michael Hart for his very complete set of instructions for performing this modification. Secondly I want to
thank Dick Greiner (Doc G) for making Michael's and others instructions available from his renowned website. Before I started this
project, I printed out all of the pertinent articles from Doc G's site and referred to them frequently while working on my scope.
Last, reflecting back on this modification, I can see no reason for my earlier fears. This is a pretty straightforward procedure. I do
believe that anyone with a reasonable amount of mechanical skills can perform this modification and be successful with the
results. If you have a 12" LX 200 (or other similar scope) with a high friction set of DEC bearings, please consider this
modification. You will be very pleased with the results.
TPOINTERS -- MY EXPERIENCES WITH THE TELESCOPE POINTING PROGRAM "TPOINT".
A LITTLE HISTORY (as I know it) OF TPOINT.
TPoint is an excellent telescope pointing program is the work of Patrick Wallace who many consider to be the worlds leading expert
on telescope pointing. It is my understanding that most of the worlds major telescopes like the giants up on Mauna Kea and
elsewhere use TPoint and the services of Patrick Wallace to set them up and map in their pointing software. As I write this Patrick
is now pointing - in the new Gemini 1 (8.3 meter) telescope on top of Mauna Kea (14,000 ft).
Software Bisque has licensed TPoint from PW and adapted it to work in conjunction with TheSky Level 4. That is the version that I
am using and writing about here. At this point I want to extend a very large "Thank You" to Patrick Wallace who has been
extremely helpful to me personally, as I have been learning to use TPoint with my LX200 12" telescope. When I began to learn
about and use TPoint in early November of 1998, I was struggling and had many questions which generated my cries for help on
MAPUG-Astronomy (Meade Advanced Products Users Group). Patrick jumped to my rescue and has been my TPoint mentor
ever since, sometimes on a daily basis.
I want to make a very strong statement right off here. TPOINT WORKS!
Two factors may cause problems using it! First, your LX200 (particularly if it is the 12" model) may have a high
friction DEC bearing problem (and/or sloppy DEC drive) as mine did originally. Secondly, the implementation by
Software Bisque, in particular some parts of the instruction manual, can be confusing and cause much frustration.
As I progress here, I will outline what I hope is an easy to understand (if I do this correctly) procedure for using TPoint with your
LX200 from night to night.
First, however, you must solve (if you have it) the high friction DEC problem with the bearing modification described above. I can't
say this too strongly. If you have a high friction DEC set of bearings, and/or a sloppy DEC drive system with a lot of uncontrollable
backlash , then TPoint will not help much!! TPoint requires repeatable errors (not random errors) in order to function efficiently
and to significantly improve your pointing accuracy!
Look above and see my LX200 set up. Attached to the pier mounted scope is a Van Slyke Versaport II Flip Mirror Box, then the
SBIG AO7 adaptive optics device and finally the ST7 CCD camera. Not shown, but always used is a Kendrick Dew Shield. With the
Dew Shield attached, the Meade balance weights are positioned to get as good a balance as possible. Most importantly, I was (and
still am) operating at f/10.
TPoint requires that you "Map- in" a significant number of stars (say 25 to 50 is the norm here) and this "mapping" requires
slewing to the "targets" and then centering the targets on the CCD imaging chip before pressing the "MAP" button. I do this using
the CCD frame on my computer monitoring screen, upon which I have drawn with a fine point magic marker a small "+" at the
center of the frame on the monitor. I then use the W,A,S, & Z keys as for the normal method of scope movement using CCDOPS
(DOS version 3.78) to center the target star. TheSky level 4 must be used here. TheSky level 4 is a $50 upgrade if you have one
of the lower levels, but you will also obtain and then use the new improved 2CD level 4 version 5.0 ( I like this version a lot).
Some of you may want to do the "Dummy Run" described below first. I point this out now so that you do not overlook it. Link
here
INITIAL RESULTS
When I first started using TPoint, the pointing results were very variable and confusing. The results were showing a success rate
( target slewed to ending up inside the CCD frame) of 54% to 68 %, while statistically TPoint was showing an RMS of 47 to 48 and
PSD of 48 to 54. These values of RMS and PSD are considered pretty good on an LX 200, but this turned out to be a red herring
(see the later remarks on not using SYNC).
While "mapping" I began to notice (visually, in the FMB, trying to find and "rough center" my targets that didn't make it into the
CCD frame), that the errors were sometimes rather consistently and always off about the same amount to the north for a while,
say 3 to 6 or so slews and then again rather consistently off to the south for another batch of slews, while maintaining very good
and consistent East-West locations (very tight grouping E-W).
My basic intuition was telling me then, that perhaps my high friction and sloppy DEC drive was causing a lot of my TPoint
problems. My first corrective action was to replace my Gel Cell battery power supply (with the 12/18 Mead converter which I
always had suspected of being a "soft" power supply compared with the Mead 120/18 volt "stiff" power supply) with the Meade
12/18 volt supply.. Upon doing this, the DEC motor drive sounded much "peppier" and less "wimpy". Simultaneously the TPoint
scatter diagram changed shape dramatically, from a larger spread in RA to a larger spread in DEC. TPoint was reporting RMS
values like 119 to 130 and the PSD to 131 to 139. On my best slew runs the success rate was at times as high as 92% to 98%. I
felt that this was very encouraging. But repeat runs sometimes produced success rates in only the 50% bracket.
Further experimentation convinced me that I should implement the DEC bearing modification above, if there was ever to be any
hope of having a system on which TPoint could show what it can do.
AFTER THE MODIFICATION
Described below are procedures and techniques that work for me, with my particular LX200 12" f/10 set up. Your situation may be
different and require some changes, but if you are looking for a way to start, then this should be helpful.
TheSky Documents -- Understanding "Documents" in TheSky software was interesting and became very useful for me. When you
open TheSky; the "Normal.sky"; is the title that usually opens as the default document. To open a new "document" you must
"save as" the "Normal.sky" under a different name say Sky1.sky. When you do this, you then are able to set up TPoint with a
"clean" empty data sheet to start a mapping run. If you need in the future a "clean" empty TPoint data sheet, for a new set of
circumstances that might require another new mapping run, then you can use this method to set up other documents like sky2.
sky, sky3.sky etc.
Setting Up Tpoint -- Once you have obtained TPoint from Software Bisque, follow their instructions for installing it on page 8 of
their manual, and after installation, launch it once to initialize it.
Now, open up TheSky, and either set up a new sky document or if you have previously done this, then open up your new
document.
In the Edit menu, click on "Insert New Object", then select "TPoint Model" from the list, and check the "Display as Icon" box and
"OK". TPoint is now working in your new "SKY" document and will be seen as the gray Icon on the upper left of your "sky" window.
Now link your LX 200 to the sky. Double click on the TPoint Icon to open up Tpoint, select the "Model" menu and Observing
Parameters. You should find your LX 200 listed there as well as your local site data. Set the "pressure" & "height" to "0". (This is
important!). Click " OK" and in the "File" menu, select "Exit and return to the sky". Now right click on the TPoint icon, and then
select "show as Icon" which already has a check on it to change to the "scatter diagram view" which will show no data until after
you map the first 6 stars.
You are now ready to start mapping in the stars as data points. Select a fairly bright star and slew to it. Use whatever means you
normally use to "center" that star in your CCD target frame on your monitor. Switching back to your monitors view of "TheSky",
click on the "Map" button in the "Object information box".
CAUTION -- NEVER CLICK ON SYNC WHEN YOU ARE USING TPOINT, as "SYNC" screws up the works immediately! I find that if I
map the first 6 stars or so as stars that are rather close to each other, that this helps TPoint to start controlling your scope, and
almost immediately this will improve your pointing accuracy. Then proceed to map in a total of 25 to 50 stars, spread around the
particular portion of the sky that you normally use. Your pointing accuracy should be very good now and you should be getting
about 100% of your targets in your CCD frame even at f/10 if that is where you work.
Your TPoint SKY RMS should be somewhere between 70 and 90 and the PSD in the same range.
If you wish, continue now to slew to your desired targets and do some imaging! TPoint should be working for you just fine. In my
case, at the end of the evenings imaging, I must un-link, turn off the LX200 (after first returning to "0" RA and "0" DEC, and turn
off my computer. That means that to start again the next imaging evening, I must do a "Short Mapping Run"
A NOTE ON NOTE TAKING
During any observing and imaging session, I keep a rather detailed log of what I am doing and what it happening. I found it very
helpful to do the same with my TPoint mapping runs.
My note taking during a mapping run (or subsequent observing/imaging run) with TPoint consists of the following as a minimum:
1. I list each star as mapped.
2. Aside of the star's name, if after I slew to the star, it is in the CCD frame I draw a little rectangle simulating the CCD frame
and very approximately place a "dot" where the star fell into the frame before centering.
3. If the star did not fall into the CCD target frame, instead of the rectangle, I draw an extended "plus" sign (+), and put a dot
in the plus sign approximately at the location of where this star fell in my FMB reticled eyepiece.
4. As I progress through this session, I use the above information to "keep score" of the successes, so at the end I can
compute the % of successes. I can also spot "trends" that provide clues to possible problems.
ONE OF SEVERAL ADDITIONAL BENEFITS FOR USERS OF TPOINT
TPoint will provide you with much information about your Telescope setup. One of these that I have been making use of, is the
term MA or the Polar Axis Azimuth Error. This is reported in arc seconds, which, with a little Trig you can use to compute how
much you need to crank into your "Wedge" to make the required correction. After making the correction, running another TPoint
mapping run will verify your results.
Return To Beginning
STARTING A NEW EVENING WITH A PREVIOUS TPOINT MODEL!
THIS CAN BE TRICKY!
1. Open your previous Sky document that contains the previous TPoint model that you wish to use again.
2. You must then do a "Short Mapping Run" of at least 6 new star data points. You may refer to pages 24 & 25 in the TPoint
manual, which are two of the most confusing pages in the book, while you follow the procedure below. I will try to explain
why you do what you do here.
3. Now link your LX 200 to TheSky. Double click on the TPoint Icon to open up TPoint, select the "Model" menu and Observing
Parameters. You should find your LX 200 listed there as well as your local site data. Set the "pressure" and "height" to "0".
Click " OK" and in the "File" menu, select "Exit and return to the sky".
4. Double click on the TPoint Icon to open TPoint.In the Model menu select "Short Mapping Run". Click on "Fix and Mask" and
OK.
5. This fixes NP, ME, & MA, leaving IH, ID, & CH un fixed and empty, ready to receive the data from the new 6 points you are
going to map. IH, ID, and CH will then develop values as you map the 6 new points, while the coefficients of NP, ME, & MA
will remain fixed. These will be helping TPoint point your scope. Close TPoint by "Exit and Return to the Sky" from the file
menu.
6. Now right click on the TPoint icon, and then select "Show as Icon" which already has a check on it to change to the "scatter
diagram view". Now, as you did previously, map in 6 or so new stars.
7. You now have six or more new points at the bottom of the spread sheet.
8. Double click on the TPoint icon so that the full spread sheet occupies the screen.
9. Click on Model / Fit Data or just the FIT box. This brings up a Fit Data Box and at, the top left, 6 Terms, all ticked. NP, ME,
& MA will appear "fixed" (brackets around the coefficients), while IH, ID, & CH appear as "not fixed" (no brackets around
the coefficients).
10. Now click on the More Terms box and this brings up a Terms box. Select the terms you want to un-fix (NP, ME, and MA) and
remove the check mark from the box "fixed term" for NP, ME, and MA.
11. You will see as you select the various terms, those that are "unfixed" to start with will not have a check mark in the boxes,
while those that are "fixed" to start with, will have the check mark in the boxes.
12. In other words you must leave the Use box ticked for these three terms (NP, ME, and MA) and
un-tick the Fixed Term box.
13. You must now leave only the Use box ticked for the other three terms IH, ID and CH. Got it?
14. The "use box" will now be ticked for all 6 terms. None will be "fixed" at this point. You will be using these 6 terms for the
rest of the evening with whatever coefficients they have developed. IH, ID,
& CH will have the new coefficients from the 6 points mapped, and NP, ME, and MA will have the coefficients from your old
previous mapping run.
15. This assumes you are just using six terms in all which is my normal situation.
16. Now close the Terms box. You then OK the Fit Data box. You collapse the TPoint spread sheet to an icon ("Exit and return to
TheSky") and you then just carry on slewing to whatever you are interested in for the rest of that evening.
At this point, I usually pour a "sherry", relax and slew to my hearts content, hopefully hitting all my targets nearly center on. If
you have reached this point successfully, you are well on your way to mastering TPoint and enjoying its use.
DUMMY TPOINT RUNS
A "dummy run" provides an end-to-end test of the system, proving that TPoint can reconstruct a specified model from a set of
fake star observations.
It is probably a good idea to do such a run early on, to practice and set up benchmarks for comparison if you later make changes
to your scope and accessories.
The "Dummy" TPoint run can be done any time including in daylight if you want to, as actual stars are not required (keep your end
cap on for this). A "dummy" run is performed by using a new .sky document with a freshly installed Tpoint that has a clean
"empty" data sheet.
Before we start, we want to set all 6 terms IH, ID, CH, MP, ME, & MA to zero (or other known values) and "fix" them at those
values.
The routine for doing a "dummy run" is as follows:
If you want to enter model coefficients without any pointing data at any time you can use this method.
1. Start TheSky.
2. Go to Edit, Insert New Object, and select a TPoint Model to get a model into TheSky.
3. Add six "bogus" mapping points to the new model. (This is the work around to allow you to enter the Fit dialog in TPoint.
The easiest way to do this is to connect to the Telescope Simulator, identify an object and press the Map button on the
object identification dialog six times.)
4. Double click the TPoint Icon in TheSky to fully open TPoint.
5. Mask the six bogus points by clicking on each row and choosing Data, Mask (or click the Mask button on the toolbar).
6. Choose Model, Fit Data, More Terms to allow you to fix and add coefficient values (you must fix all terms you use so their
entered value is maintained). Here now enter "0" for all of the 6 terms (or what ever values you want to use) and fix all 6.
7. Choose Close, OK, then File, Exit and Return to TheSky. TheSky will then be applying coefficients just entered.
In Software Bisque's implementation of TPoint, the mapping data and the model are contained in one document -- the TPoint
Model. The model that is inserted into TheSky is the one that is being used (as soon as there are six or more mapped points, by
default the six most common coefficients kick in, which one can alter as needed). You can shut off the model from TheSky by
clearing the checkmark under Telescope, Setup, Enable TheSky Modeling.
1. Now re-select the LX200 as your scope to link to and link your LX 200 to TheSky. Double click on the TPoint Icon to open up
Tpoint, select the "Model" menu and Observing Parameters. You should find your LX200 listed there as well as your local
site data. Set the "pressure" & "height" to "0". Click " OK" and in the "File" menu, select "Exit and return to the sky".
2. Now right click on the TPoint icon, and then select "show as Icon" which already has a check on it to change to the "scatter
diagram view" which will show no data until after you map the first 6 stars.
3. Then, using "TheSky" slew your LX200 to 25 or so "Stars" as if the stars were "out" at night. Map each (without adjusting
anything).
4. Look at your scatter data, and other plots. The RMS & PSD should be around 30 (limited by the LX 200 encoder resolution)
if all is working ok. Now un - fix the coefficients and fit the model to the data.
5. The coefficient values will change slightly because of "noise" but the RMS shouldn't decrease much.
6. That's it for the "Dummy" run. Print and save your results for later comparisons if you make changes to your setup.
At this point in time , now, after about 7 weeks experience working with TPoint, I have completed 23 mapping runs. I would be
very pleased to answer any questions that readers might have. Please write!
Return to Beginning
Comments from Hugh McKerrell, Lochranza, Isle of Arran, Scotland.
<[email protected]>
Frank Loch's experience with TPoint parallels my own in that we both started out with it at the same time (November last) and we
both conclude that it really does work. However, although we both use a 12 inch lx200 and an ST7 camera, our two systems could
hardly be more different.
Frank uses his LX200 at f10 whereas I use mine with the Optec f3.3 focal reducer. My CCD area is thus 25' x 15' whereas Frank's
is 8' x 5'. And although my slewing would seem to be far less critical in fact we both ended up with remarkably similar precisions
of about 1.5 minutes for both the RMS and PSD of TPoint. Frank also switches off his power supply after use while I keep mine on
with an un-interruptible power supply and a Sleeper plugged into the CCD socket. Thus Frank has to do the Short Mapping Run
after starting TPoint whereas, in principle, I do not.
I ran 60 test slews under TPoint control and got every one well within the confines of my CCD chip. Indeed the PSD of 1.5 minutes
is just about right for what I found. About two thirds of the results were within 1.5 minutes of the centre of the chip, a quarter
were within 3.0 minutes of the centre and just one in twenty were further from the centre than that. Moreover, the sixtieth slew
was spot on, within a minute of the centre. Compare the results without TPoint when, with just six slews and no ynchronising, the
star was already off the chip. So, unequivocally, TPoint does work.
But, that said, I can only echo Frank's experience with the Software Bisque manual. This really needs clarification and re-writing
from the section after the short mapping run. The only guidance you are given is that your newly mapped six points are to be
submitted to TPoint.
Fortunately Patrick Wallace was able to advise and clarify. So if you are thinking of using TPoint then read Frank's instructions
carefully for the exact method of linking your short mapping run to your basic TPoint model.
MAPUG is hosted by
Return to Motors and Control Index
Primer on Small DC Motors
(with special reference to those used on telescope drives)
This is a primer on electric motors, especially small DC motors. The information is for those who might want to know more about
how the characteristics of the motor are related to the driving of a telescope and how the behavior of the motor might be affected
by the type of electronic circuit used to drive it. The discussion is rather long and complex but it must be to cover the nuances of
DC motor operation which are so important for some aspects of telescope operation.
Small DC motors have many favorable characteristics. They are very reliable, very strong for their size and very inexpensive.
They can also be made to run on low voltages, have good electrical to mechanical conversion coefficients and are available in a
immense variety of sizes and formats. They are thus used in many applications where the voltages available are only a few volts
to a dozen volts. They have basic characteristics, which I will be describing, that are the result of physical and electrical laws. (No
amount of advertising will change the laws of physics.) (though some will try)
The DC motor can be thought of as transducer which changes voltage and current into speed and torque. That is, electrical power
into mechanical power. In the DC motor the current controls the force (or torque) and the voltage tends to control the speed. This
is because of the basic relations,
FORCE = (current) X (magnetic field)
VOLTAGE = (constant) X (time rate of cutting magnetic flux)
For our purposes the constant and the magnetic field term can be considered constants which depend on the design of the motor.
That is, such factors as the magnetic field strength the number of turns of wire etc. We do not need to know more about the
details of the design to understand the basic behavior of the motors. We will use measured values for specific motors as examples
later in this presentation.
The speed of the motor is quite proportional to the applied voltage. The reason is that the rotation of the motor creates a back
voltage which counteracts the applied voltage. When these two voltages are quite different there is a substantial current through
the motor which causes considerable force (torque) and the motor speeds up until the back voltage is closer to the applied
voltage. When the back voltage gets quite close to the applied voltage, the current becomes smaller and the torque decreases.
The motor runs at a speed where all these factors are in balance. That is, where the electrical power delivered to motor is about
equal to the power absorbed by the load. Because of the resistance of the windings in the motor some power is lost to heat. So
the electrical power exceeds the mechanical power by some amount. In large motors the efficiency can be well over 90%. In
very small DC motors, the efficiency might be as low as 50%.
From this behavior we see that to control the speed of the motor, the voltage applied to the motor should be controlled. When
this is done the stability of the motor shaft speed is stable. If the motor is loaded mechanically it slows down, the back voltage
decreases and the current increases thus increasing the torque to handle the higher load. This mode of operation is generally used
when the load is light and the motor shaft is turning rapidly.
The torque of the motor is proportional, as seen from the basic equations, to the current through it. If a fixed current is applied,
to the motor it tends to yield a fixed torque. If the mechanical torque is greater than that available from the motor, the motor
stalls. If the motor has excess torque it accelerates the load. Eventually the back voltage will become great enough to modify the
current to the motor. But this will not happen rapidly because a current source, i.e. one with high resistance does not like to have
its current change. It will resist the change until the value of the internal voltage is reached.
The point is that the value of the current is critical. Too little current, no motion, the motor stalls. Too much current and the
motor tends to run up to an excess speed. Current and the torque it produces are very important when the motor is operating
and very slow speed somewhere near stalled operation in a mechanical system. This is particularly true for systems with
significant stiction. (STICTION is the tendency of mechanical loads to require starting forces that are larger than running forces.
Most mechanical system have significant stiction.)
The above is actually slightly more complicated because any voltage is applied to the motor through some resistance. The
resistance of the motor itself is a part of the total resistance and the resistance of the source voltage is the other part. The source
resistance is called "the source resistance." (surprise) (The Thevenin resistance to EEs) The motor resistance results in heating of
the motor and is thus kept as small as possible within the design parameters of the motor. The voltage and current supplied to
the motor can be easily measured. The speed and the torque of the motor can also be measured if suitable instruments are
available. (I made a small dynamometer to do this.)
From the above discussion we see that for a typical DC motor, torque is high at zero speed and decreases as the speed increases.
The torque tends toward zero as the speed tends toward some maximum. In order to do a motor load design, a graph of this
curve, the speed torque curve, is made. This curve is repeated for different voltages. The speed torque curve of the mechanical
load can also be plotted on the same graph. Where the two curves cross is a stable operating point. This graphical solution to
problems is a very common design technique. The torque required to drive a mechanical load generally increases as the speed or
often the square of the speed. For most mechanical loads there is a significant amount of stiction. After the mechanical system
comes øunstuckÓ, then inertia and friction take over. Finally at some constant speed the torque must overcome the steady state
losses in driving the load. If the mechanical curve intersects the motor curve where the motor curve is changing steeply, a very
stable speed is obtained. This situation is usually considered good since the final speed is stable.
A DC motor is very often used for loads with stiction because its torque is highest at zero speed and that is where the stiction is
dominant. A DC motor will can be depended on to drive the mechanical load if it can get the load started. In a design, we must
be sure the starting torque of the motor will always be greater that the stiction in the mechanical system. Since the dynamic
mechanical load is smaller than the stiction, the load will be driven to some final stable speed.
If there is a lot of resistance in series with the motor due to its own resistance or due to a resistance placed in series with the
voltage source, the current becomes electrically limited. It is said that the motor is being current driven. If the maximum current
in the current driven mode is not large enough, the motor will not turn at all. It will be stuck on the mechanical stiction of the
mechanism. On the other hand, if the current is large enough to get the motor started the motor will speed up dramatically until
it reaches some speed determined by the electrical torque available and the mechanical torque required. Current drive is not a
good operating mode because the electrical speed torque curves with current drive and the mechanical speed torque curves
intersect at a steep angle and the final speed is not clearly defined. This speed depends very much on the mechanical load which
is not very stable and operation is thus not at a stable speed. For stable speed, the motor should be voltage driven.
This complicated discussion is to demonstrate that it is not a good idea to use a series resistor to control the speed of a motor. It
is a better idea to change the voltage of the source to do this while keeping the series resistance as low as possible. It is a bit
more complicated to change the voltage and keep the resistance in the circuit low than to simply use a series resistor. But with
proper design it can be done. While I am, unfortunately, not privy to the actual circuits used in most telescope and focus drives, I
suspect that the motors which behave erratically have speed controlled by simply inserting a resistor. This is the wrong way to do
it.
Manufacturers do not make it easy to analyze their designs. I can get the circuit for almost any television, amplifier or other piece
of electronic equipment made anywhere in the world. But some manufacturers are simply not cooperative in these matters.
Reverse engineering is required and it is very time consuming. Never-the-less, I will try to discuss and evaluate several actual
electrical and mechanical designs including applications to telescope drives.
Small DC motors, like other motors, are not described in a very uniform way by their manufacturers. In many motor catalogs,
only the rated voltage and a high speed limit are given. In some cases the power consumption is given and in some the current is
given and in some the torque is given. The specifications are a hodge-podge of information much of it useless. So design is a
tricky business. In order to do a design of a telescope drive system we need to know several very specific properties. They are,
more or less in order of importance: the stalled torque, the stalled current, the resistance of the motor windings, the voltage
rating, the running torque at some speed and a few other things of course like the dimensions, shaft configuration and so forth.
We also need to know a few things about the mechanical system that is to be driven. This is especially the speed of the shaft
under normal conditions and the torque required to drive the system. The speed required will usually be the speed of the worm
that drives the main gear for sidereal rate. One complication is that both Dec and RA drives will also often require driving at a
slew rate which is as much as one thousand times sidereal rate. This factor complicates the overall design but does not necessarily
change the conditions required for the steady sidereal rate drive system. (nor the Dec drive for that matter)
The motor shaft and the worm shaft rates usually will be different by factors of a few dozen to several hundred times. In the
LX200 drive the factor is 60. Thus a gear reduction cluster is required. This can be obtained in many ways. Everything from
another worm type reduction gear, planetary gears or sets of simple spur gears have been used. In the LX200 a simple set of
four metal/plastic spur gears is used. These gears are not very high precision and are in my opinion a very weak link in the
mechanical design of the drives.
Some small DC motor characteristics are in the following ranges. Motors with voltage ratings of 12 to 18 volts have typical torque
ratings of 1 to 5 oz.in. per ampere. These motors have typical resistances of 4 to 10 ohms. Thus one might expect stalled
currents of up to 3 to 5 amperes. The lowest values give a power dissipation of 36 watts and the highest 90 watts. These small
motors which have a size of about 1" dia. and 2" length or less cannot sustain such power dissipation for more than a few
seconds. Thus it is very important to design the system so that it does not stall and/or fuse it to protect the motor and power
driver devices. For the brief time that it survives, the DC motor will put out its best effort to break stiction in the mechanical
system. But because of saturation of the iron in the motor it will not put out the torque estimated by linear extrapolation.
The LX200 motor has a torque of 1.55 oz.in. per ampere from measurements made with my mini-dynamometer. (this is for only
one motor sample and may vary somewhat for other samples) It has a resistance of 4.5 ohms. A practical dissipation for a few
seconds is about 10 watts. This dissipation heated the windings enough to increase the resistance to 5 ohms after 5 seconds. I
felt this to be a safe maximum dissipation but that more would damage the motor.
Thus a safe stalled torque is about 2.33 oz.in. These numbers seem to be in the low end of DC motor specifications that I have
found but are still within reason. The motor is only 3/4" dia. by 1" long. It is a very tiny motor. Other motors in this class that I
have looked at have similar characteristics. The motor would probably put out as much as twice this torque for brief periods but I
felt the heating of the motor would be excessive.
The torque is translated by the gear box to the worm shaft through a 60 to 1 gear ratio. Thus the torque at the worm shaft should
be about 140 oz.in. depending how hard the motor is pushed. This assumes no losses in the gear train which is of course very
optimistic. The torque at the worm shaft is fairly high considering the size of the system and it should work satisfactorily. . That
this is the case is clear from the relatively good success the design has provided to many users. The torque is additionally
amplified by the worm to gear interface taken as an inclined plane. I have measured the geometry of this interface and found the
lever ratio to be 15. The force at the approximate 3 inch main gear radius is about 2097 oz.in. This is effectively 2.74 ft.lb. at the
telescope optical tube.
We know that the friction on the Dec drive is somewhere between 0.5 and 1 ft.lb. so there is enough torque left to drive a
significant unbalance on the optical tube. We do know that occasionally there is mechanical binding of the Dec drive. Mechanical
binding at any place in the chain that transfers motor rotation to telescope tube motion takes a large toll on the available torque.
At slow, sidereal, rates we must assume the mechanical system is going in and out of stiction constantly. There is thus a slightly
jerky motion due to the rapid and non-linear changes in the stiction/friction coefficients. This causes rapid fluctuations in the
torque required to move the mechanical system. It may be these fluctuations that are causing the very tiny vibrations that I have
measured with a geophone and that have been reported by several observers.
The stiction in the RA main drive shaft is small because of the ball bearings in the mount. This means that most of the torque
generated by the drive is available to move the fork in RA. The RA drive is well known to work much more reliably than the Dec
drive in these instruments.
This analysis of the LX200 drives is quite approximate. It is rather satisfying to see the reasonable numbers appear in the
analysis. The numbers show that the design is basically sound. It works fairly well as it should. The analysis also points out that
the drives are by no means over designed in terms of motor strength. The mechanical system must be keep tuned up, clean,
balanced and the like. I have never had problems with the drives on the 8" LX200 Problems with the 10" have been minimal and
traced to a breakage in the drive rather than a fundamental design problem. With the 12", the problems with the Dec drive have
been more numerous in my experience. I feel that the size and strength of the drives on the 12", which are identical to those on
the smaller telescopes, are much more marginal.
Some thoughts on the design of a strong precision drive will appear on this WEB site soon.
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A Discussion of Control Systems
used to Point Telescopes
This is a very brief discussion of control systems, especially those used on telescopes. Understand that this is a overview of the
parts of the system and how they interact to control the position of the telescope. This is not a discussion of control theory which
is a major concentration in electrical engineering. My purpose is to clarify some of the misconceptions I so often see in posts to
various web sites and to perhaps clarify the terminology that should be used to describe these control systems.
Control systems can be categorized as open loop or closed loop or sometimes a combination of the two schemes. What these
terms mean needs to be clarified. A control system generally has an input and an output. In the case of a telescope, the input is
a command, usually from a computer, to point in a specified direction. The output is the mechanical motion of the telescope to
reach that location. There are two coordinate numbers used to specify the position. These are usually Declination and Right
Ascension. (call them Dec and RA) Or, they may be altitude and azimuth. Each can be easily converted to the other. The way
the control system work is the two directions is almost symmetrical so only one direction of motion need be considered to
understand the concepts of control.
Clearly a number in a computer has to be processed by a lot of intervening electrical and mechanical transformations to become
mechanical position. These are myriad. We do not need to consider the details at the moment. Just consider that there is an
input, a control system and an output. When we look at the output as related to the input with an open loop control system, we
are usually disappointed in the result. This situation lead to the concept, in the 1930s, of actually measuring the output and
sending a signal back to the controller to tell how close it is to the correct output. This is the concept of feedback control. It has
become vital to all control systems and is even applied to biological and economic systems these days. The concept is very
important and has some subtle implications. If the feedback is in error or too strong, the control system errors can be made
worse or the system can even become unstable. (like the stock market or some biological functions) We find that feedback
control was used in mechanical systems many years ago. Examples are the governors on steam engines or the water flow
through a mill. A modern one is the cruise control in you automobile.
The theory of how these systems operate is called control theory and is studied by electrical engineers, mechanical engineers and
many others. It is rare that an open loop control system, one in which there is no feedback will be adequate for precision
systems. One might consider how manually pointing a telescope is a closed loop control system. This is one in which the
observer is the feedback element. Say you want to go to an object at declination "d" and you have a declination scale on the
telescope. First you look at the scale and move the telescope to Dec "d". Right there is a feedback process. Your arm moves the
telescope, or you push a button to move it, your eye tells you when you are at the right point and you stop the motion. That is
closed loop feedback. With a good scale you can get to within a fraction of a degree of the desired position. Now you look
through the telescope and see the object in the finder. You then move the telescope slowly to center the object. This is also
feedback control. The loop that is closed consists of: the movement of the object in the eyepiece, your eye, your brain, your
finger on the control and the actual motion of the telescope. Even the concept of the loop gain of the system is in this example.
With a low power eyepiece you can point with a certain accuracy. If you go to a high power eyepiece you can point more
accurately. The higher eyepiece power has increased the loop gain of the system and has increased the accuracy of the feedback
element in the system at the same time.
Now all of these concepts can be applied to automated pointing systems. Here is how. If you have what is called an absolute
pointing system, the telescope axis will have a very precise feedback element called an absolute encoder which tells the system
where the telescope is actually pointing in absolute terms. This can be done, but the cost of the system is very high and the final
accuracy limited by the precision of the encoder used. Absolute encoders of reasonable accuracy cost, say $400, are usually only
accurate to a part in 4000. That is about 5 arc minutes. This is not bad for pointing, it generally will put your field of view on the
object. Then you can use the human feedback control system to move the object to the center of the field. This level of pointing
is generally not good enough for CCD imagers which have a small field of view. To make use of this absolute encoder, you need to
have a computer that reads the encoder and compares this value to the desired value and sends a signal to the telescope which
tells it to move until the encoder position matches the desired position. This can and has been done in many systems but the cost
is high.
There is a second way to point the telescope. It is called the differential location system. This is the system used in most
telescope control schemes since it uses relatively inexpensive encoders called differential encoders. The differential scheme
requires you to tell the telescope exactly where it is pointed and how far to move to get to the next object. Most amateur
telescope systems work with these differential encoders. The demands on the computer are slightly greater since it has to be
calibrated and keeps track of exactly where it is pointed but the encoders are much cheaper and can count differential position to
an arc second or so. Computing power is cheap so this system is almost universally used. Unfortunately, each move of the
telescope depends on the accuracy of the previous move and so errors accumulate. The pointing accuracy achieved is usually only
to a few arc minutes in the final complete system.
So it seems that pointing to a few arc minutes is not only possible but a reasonable goal. (Note this is only the initial pointing to a
new position, not the final tracking accuracy which has to be much, much better especially for imaging.)
Several examples of how the closed loop feedback is accomplished will now be described. Most systems use the differential
system so it will be covered in more detail. There are several manifestations of the differential system. One is the closed loop
system which includes the actual pointing position of the telescope as measured by an encoder on the shaft of the OTA. This
pointing system thus includes all of the electrical and mechanical parts of the system. The accuracy of such a system depends
only upon the accuracy of the encoder used to report the position of the OTA. All mechanical and electrical errors are reduced by
the gain of the feedback system. In these systems, the controller moves the telescope until the encoder reports the correct
position, limited only by its own ability to differentiate position. Here again, the accuracy for an encoder connected directly to the
OTA shaft is about a part in 8000 for the best encoders. (2.5 arc minutes) However, the encoder can be geared up by 5 to 10
times to improve the positional sensitivity. The gearing up mechanism must of course be accurate. It can be made accurate
mechanically because the forces in this link are very tiny and high precision gears can by used. One can get differential pointing
accuracy of 10 to 20 arc seconds with these systems. Many commercial and amateur pointing systems use this method.
A second form of differential feedback control placed the encoder on a shaft which is in the mechanical power loop but moves at a
higher speed than the OTA axis. This is the system used in the LX series of telescopes. The advantage of this tactic is that the
higher speed shafts are available anyway and the encoder does not have to be high precision in angular terms. The encoder
differential accuracy in these systems is in the single arc second range. However, the problem with these schemes is that the
encoder does not report the actual OTA position but an inferred position through the gear reducer and the worm drive. The
accumulated error through the gears that form the mechanical power link between the encoder and the OTA position is easily 100
arc seconds or more. Thus while this method is theoretically very accurate, it is in practice not much better than the first method
and is additionally subject to the vagaries of problems with the reducer gearing and worm drive. Loading of the system, balance
and other factors make this scheme somewhat problematic. The problem is that while the high speed shaft is in the feedback
loop, the OTA shaft is considerably removed from the loop.
Clearly, the better and more stable scheme is to have everything within the feedback loop if possible. That is, to have the position
encoder, whether it is differential or absolute, connected directly to the OTA shaft.
So in summary, one finds that the pointing accuracy achievable depends mainly on the encoder used and the directness of the
connection between the encoder and the actual OTA position. Issues of what to do with the signals once they are in electrical form
are relatively easy and can be done to almost any precision desired. It appears that amateur systems, with an affordable price
tag, can be made to point with an accuracy of a few arc minutes. These are adequate for most amateur uses.
There are other differential control system limitations. Since the final position of the telescope is determined by a differential
move from the current location, the current location error will be reflected in the final location error. When slewing by the
differential method between regions of the sky that are many degrees apart, this error plus the basic errors in the mechanical
parts of the mount will usually cause disappointing goto results. One way to alleviate this problem is to use what is sometimes
called precision pointing. This is a misnomer since using the technique does not change the system to a higher precision, but only
seems to. What is called for, is pointing to the general location desired and then re-synchronizing the computer and mechanical
system for that portion of the sky, This is done by locating and re-centering an object in the viewfinder and synchronizing. This
basically means that the current reference object, which has been made accurate by the viewer, is near the desired objects. Thus
the differential moves are small, the mechanical contortions are reduced and the accuracy of the pointing improved for that region
of the sky. This is a perfectly legitimate way to proceed.
A computer controlled version of this technique is manifest in the fine T-point computer program provided by Bisque. This
program essentially maps the locations to which your telescope points when moved over the entire sky to the actual celestial
sphere. The computer remembers this mapping function and applies it to the position request being made. It then points the
telescope to the actual location of the object by correcting the mechanical errors in the telescope mount. This is a very clever
technique which works well for a permanently mounted telescope. The telescope has to be stable in its mechanical behavior of
course since this is an open loop correction which has gone through a training session. If the telescope is mechanically very
differently loaded or re-balanced or otherwise changed mechanically, The program might have to be re-trained as well. A number
of different configurations can be stored and recalled as necessary.
Now for imaging none of the above pointing schemes are adequate. No telescope will track an object for more than a few minutes
to an accuracy of a couple of arc seconds. (except by total accident and luck) For imaging, arc second or even sub arc second
pointing accuracy are useful and sometimes even necessary. Such tracking accuracy, called guiding, is either done manually
through an auxiliary telescope or with an off axis guider or it is done with a photodetector or imager. Usually an auxiliary imager
is used as a guider. This sort of setup is the ultimate closed loop feedback system. In this case the sensor is actually receiving
the image of the object, usually a star, and holding it in place on the image plane by feeding a corrective signal to the telescope
drive. The telescope OTA is essentially locked to the motion of the celestial sphere.
To make such a system work, the system has to be stable, linear, single valued and without either dead zone or hysteresis. Now
that is a mouthful. But these terms refer directly to familiar terms like solid mount, strong drives that move the telescope when
told to do so, freedom from stiction and back lash. A fine mount with a good basic control system can be made to work with the
feedback signal from an imager very nicely. The final problems found are usually with noise caused by mechanical defects in the
telescope mount, jiggling of the mount or ultimately jiggling of the atmosphere. Any of these effects can cause the feedback
system, which requires very high loop gain, to become slightly unstable. There are usually adjustments in the software that
provide additional stabilizing factors to control such marginal behavior. Elements like selecting the exposure time, the star
brightness and more obscure elements like aggressiveness, lash, gain and so forth are used to gain optimal guiding. These may
vary greatly from system to system.
Finally, a comment about temperature control of CCD imagers may be in order. Thermal changes in the interior of the imager
and at the chip can with sensitive temperature detectors be easily held to a fraction of a degree C. (Typically 0.2 C) The main
requirement is that the response of the TEC be fast compared to the thermal time constants of the internal environment. It should
be true, if the design is good, that holding the temperature accurately is not a problem. It is important that the cooling system
have adequate cooling capability to quickly overcome any sudden change is chip temperature. Thus the cooler should probably
not be run over 90% capacity under normal, relatively stable operating conditions. The thermal control system is a full fledged
closed loop feedback control system. The accuracy is largely determined by the ability to measure the chip temperature
accurately. This would best be done by having a temperature sensing junction built into the chip or at least mounted directly on
the cold finger near the chip. Most temperature control systems in modern CCD imagers are excellent both in terms of accuracy
and stability.
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Doc G's Observatory Control Room
This is the control room for the Doc G Observatory. It is located in the club house at the Madison Astronomical Society dark site.
The computer equipment is connected with underground cables to the Observatory building shown through the window. The
observatory is closed in this photo. When the building is rolled back, the telescope is visible from the control position in front of
the computers. At this time, there are two computers assigned to run the telescope. One is used to point the telescope and
guide it and the other is used to operate the imaging system. The philosophy of this design is to ensure that the telescope can be
pointed and guided accurately no matter if it used for CCD imaging with or without focal reducers or magnifiers, piggy back
photography or video imaging. Having the pointing and guiding system entirely independent of the imaging needs help ensure
that it is permanently adjusted for optimum performance.
The computer in the background is a 486DX2 machine with a large memory and three 1/2 G hard drives. It is an aging server
type machine but quite adequate for this application. A number or other large planetarium programs are available on the hard
drives and/or from the CD rom drive. It is usually run under a WIN 95 OS. One of the hard drives holds the entire SKY- IV
software system complete with Bisque's T-Point for precision pointing of the 12". It is devoted entirely to telescope pointing
operations. The 12" LX200 is connected to this computer through the standard 232 port. Once basic pointing has been
accomplished, the computer is tasked over to guiding via a CCD imager located on the Celestron C-5 guiding telescope. This in
effect locks the telescope to a selected sky field for imaging. The telescope is very carefully polar aligned so that there is no
detectable field rotation for long periods of time.
Imaging is done with the second computer shown in the middle ground. It is a fast Pentium machine with large memory and
storage capacity and large capacity removable medium drives (Bernoulli and Syquest). With this arrangement, images can be
processed on the spot or stored and taken to another computer for processing. Imaging is done with a parallel connection to the
ST-7 imager, at a distance of 90 feet. This length cable has worked well on the ST-7. Full automation for imaging is provided
with the Optec filter slider.
Additional control cables are provided for photography with Canon cameras that have remote shutter control and auto winders for
the film. In addition to the computers, a TV monitor and VCR (S-VHS), shown in the foreground (rather dark), are provided to
remotely view and record images of planets or larger objects such as the moon or sun with appropriate lenses and filters. The
optical projection attachment, mainly for planets, is shown at another location on this web site.
Because the telescope is in direct view through the window, it can be monitored visually or even through binoculars if desired. An
intercom is provided so that the operator can communicate with a helper when certain setup maneuvers have to be performed.
There are also indicator lights which are illuminated when the telescope is in use though unattended.
The Observatory is shown below in fine Wisconsin viewing weather with slightly frozen Doc G. We had five weeks of cloudy days
and nights followed by three days of cold drizzle followed by more cloudy nights followed by 6 inches of damp snow and a week of
10 degree weather. Not even remote control helps under these conditions. :-(
Note the Societies newly rebuilt 11" building in
the background.
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More Thoughts About Observatories
These thoughts contain a summary of some of the topics that need to be considered when building a personal observatory. They
represent those things that came to mind while I was considering a new observatory and include some of the ideas and concerns
expressed by other member on the MAPUG-Astronomy list. All conclusions presented here are my own. For some more
detailed discussion of thermal effects see this link (Observatory Thermal Discussion) For conciseness, some of the following
arguments have been shortened. For more detailed discussion of some of these issues see (Observatory Design Discussion
Addenda) There is also a section on Control Room concerns. (Control Rooms)
Note: this discussion is about basic considerations for an observatory and does not include detailed construction topics. Such
topics will be added in addenda to this discussion from time to time. There discussions are at (Observatory Construction
Details) Much detailed information on the design of buildings and piers is also available on the web in the Mapug-Astronomy
Topical Archives at: <www.mapug-astronomy.net> (This is Ed Stewart's fine Mapug-Astronomy site.)
Considerations for the Design of an Observatory
Climate: The type of observatory required is highly dependent upon the climate. This is an obvious fact. However, in any
climate, the purpose of the observatory is to protect the equipment, allow the equipment to be used easily and to provide an
environment which provides for the comfort of the observer as much as possible. If the climate is harsh, the observatory must be
waterproof, snow proof and free of leaks which might allow damage to the astronomical instruments. At the same time, the
structure should be designed in such a way so it can be brought into use in a short time.
Creature Comforts: A major element of the observatory design should be the comfort of the observer. For normal visual work,
it is necessary for the observer to have direct access to the telescope eyepiece and thus the observer will be at the same ambient
temperature as the telescope. However, protection from wind and radiation to the open sky can give the observer some comfort
from the worst of the elements in cold viewing conditions. There are a number of "very local" accessories that can help greatly to
keep the observer more comfortable. Among these are, a heating pad for the seat or placed on one's lap. a heating pad onto
which the eyepieces are placed so they stay a bit warmer and will not be so ready to accumulate dew when they are used. When
instruments are used with the telescope in such a way that an observer does not have to be continually in attendance, it is
desirable to have a two or more part structure. One part can then provide the best conditions for the telescope and its auxiliary
equipment and another part can provide for the comfort of the observers/operators. The observer/control area can easily have
several functions as well.
Types of Telescopes: The observatory must be designed to be large enough so that the telescope or telescopes can be used
easily and so that auxiliary equipment is conveniently located, accessible and protected. Within reason, if the observatory is large
enough, it can accommodate a variety of telescopes. Remember, of course, that refractors on GEMs take a considerably larger
area than fork mounted Cassegrains because of their design and mode of use.
Types of Observing: Direct visual observing is interesting for the novice and should be made possible with the observatory set
up. However, more and more the purpose of the serious amateur is photography, imaging, photometry, spectrometry and other
activities which require instrumentation at the telescope other than the human eye. If one type of use dominates, t is desirable to
optimize the observatory for that use even if it makes a more minor use a bit less convenient.
Automation: Automated, robotic and even remote control and observing are becoming more and more common. Thus the
observatory should be designed so that it can accommodate these function or can be modified easily to accommodate them. The
telescope for remote or robotic use will of course have to have its own computer control. The housing for the telescope will, in the
most extreme case of remote use, have to be designed in such a way that the building can be opened, set up for use and put to
rest at the end of a session without local human intervention. For the intermediate case, of automated use, but with a local
attendant, the building can be much less automated.
Observatory Quality Factors
Space: There might need to be enough space for just one observer and possibly for several observers who take turns at the
eyepiece of the instrument. This is necessary only if the telescope is to be used for visual observing and for demonstration of
viewing to a group. For the more advanced user, the desirability of having multiple person access to the instrument becomes less
important when quality personal observing is the primary concern. In this case, principle attention has to be paid to proper space
for use of the instrument and its accessories by one individual.
Sky View: There must obviously be a satisfactory view of the sky for the instrument. There are two major types of sky view
provided by observatories. One is the view provided by a traditional domed building with a shutter opening for the instrument.
The other is the so called roll open building which is in most cases provided by a roll off roof. The domed building has some
disadvantages of cost and requires some level of automation for either remote or local robotic use. The roll off roof has the
advantage of relatively low cost and full exposure of the instrument to the sky. These factors are discussed n more detail below.
Thermal Considerations: Thermal considerations are very important since they affect the comfort of the observer, the behavior
of the telescope and instruments as well as seeing factors such as dewing and disturbances in the visual air path. There are
competing considerations regarding thermal factors. In the one case, it can be argued that the telescope and its associated
equipment will come to thermal equilibrium with its surroundings more quickly with a fully open roof. Contrarily, the domed
building needs more time to come to thermal equilibrium and there is a greater possibility of air disturbances do to mixing of
ambient outside and internal air at the shutter which might spoil seeing.
A second part of the thermal equilibrium equation is what happens after some time has passed. The totally open structure
exposes the instrument to the sky and by radiation the instrument and everything else in the building cools to a temperature lower
than the ambient air and collects dew. The amount of dewing depends upon the dew point relative to the ambient temperature
upon wind conditions. A still air condition allows dewing to build up to a situation where condensate actually runs and form pools
of water. A significant wind will dissipate light dew but the wind in turn can shake the telescope so as to spoil viewing as well.
In the case of the domed building, the telescope radiates to the dome which is very nearly at ambient temperature. The
temperature of the dome will be at actual ambient temperature or possibly slightly cooler since it too radiates to the sky.
However, the dew in this case forms on the dome and tends to keep the dome at the dew point which is usually not that much
different from ambient when dew is a problem and certainly not near sky temperature. Telescopes, instruments and other items
and surfaces within the dome almost never collect condensation. With domed observatories, it is very useful to have an exhaust
fan near the base of the domed structure to force the temperature of the dome and inside air to be near the ambient temperature
as quickly as possible. This exhaust fan will also remove the heat do to the observers in the building. This heat can raise the
internal temperature of the building enough to cause deterioration of viewing do to heat current eddies through the shutter
opening.
Versatility: The design of the observatory must accommodate the telescope installed but should also allow for installation of a
different or larger instrument and accessories that might be needed in the future. The totally open design is more versatile in that
almost any type of instrument can be set up as long as space and headroom allow. In fact, if there is enough floor space, several
instruments can be operational at the same time. The dome limits the multiple instrument option unless the instruments can be
mounted on the same pointing platform and see through the shutter opening. With the dome, some added thought has to go into
the placement of the pier and the instruments on it.
Conclusions: These conclusions are based on at the design of a building in Wisconsin which is in a cold/hot, dry/wet, calm/windy
location. The telescope is) will consist of several instruments mounted on a single pier/pointing platform. The use will be almost
entirely for photography/imaging and possibly other instrumented use such as variable star measurement or solar studies.
Creature comforts are essential since the principle observer/operator is too old to want to get his eyelashes frozen to the
eyepiece. The telescope/instruments will be automated, but run locally (not run remotely) at this time.
The building will be a two part structure 11 by 22 feet in size with a partition separating the control room from the telescope room.
The dome will be a Pro Dome 10' in diameter with full automation. (I would really like an Ash dome but they are just a bit too
expensive for me) The control room will be both heated and air-conditioned for the comfort of the operators. There will be a single
pier with a large pointing platform that will take a variety of instruments. The most likely candidates at this time are a 14"
Celestron Faststar, Meade 10" CAS, Celestron C-5 and several large APO photographic lenses. (I have all but the 14" which looks
very good to me because of its versatility)
The decision to have two rooms, a control room and observatory room, was not difficult. The control room can be held at a
temperature suited for computer equipment on a long term basis and brought quickly to a comfortable temperature for the
operators. The control room will be well insulated and connected to the observing room by a common insulated wall and thermal
window. An insulated pocket door will connect the two rooms.
The decision to go with a conventional domed structure was the most difficult and deserves some explanation. After reading the
many opinions recently posted on Mapug-Astronomy and after considerable other reading and calculation of various factors, I
came to the conclusion that the conventional dome is the best for the following reasons:
1. Provides the best protection against weather conditions. i.e. can be well sealed against blowing snow and heavy rain as well
as high winds and dust.
2. Provides best protection for the observer from radiation to the night sky. This is particularly important in winter when skies
are often very clear in Wisconsin.
3. Provides the best conditions for eliminating dewing on the telescope and all of the other instruments in the observatory. An
open venue allows for condensate to form on everything in the observatory. Cases, shelves, anything on the shelves,
instruments, telescopes even canvas chairs and tables in an open venue get wet. The dome prevents all of these problems.
4. Has no disadvantage to viewing do to air mixing if the right sort of forced ventilation (exhaust fan) is provided. This
requires the shutter to be opened sometime before use and significant forced ventilation to be employed to bring everything
to ambient and hold it at ambient.
5. Provides the best protection against wind for both the operator and the telescope. Even a slight wind can cause
considerable operator discomfort. And as we well know, wind spoils imaging very easily. An additional wind screen can be
easily installed in the shutter opening. It is even possible to consider adding a Lanphier style shutter.
6. Can be easily fully automated with commercially available equipment, especially from Home Dome.
I welcome any discussion and arguments or counter arguments anyone has to offer. Special thanks to John Menke of Technical
Innovations for discussion and input to this analysis.
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Return to Observatory Design Discussion
Expanded discussion of various design considerations in the main article for those with incredible patience and will power:
(The original article contents are in italics, the additions not.)
These thoughts contain a summary of some of the topics that need to be considered when building a personal observatory. They
represent those things that came to mind while I was considering a new observatory and include some of the ideas and concerns
expressed by other member on the MAPUG-Astronomy list. These ideas were under discussion toward the end of February 1999.
All conclusions presented here are my own. For some more detailed discussion of thermal effects see this link (Observatory
Thermal Discussion) For conciseness, some of the following arguments have been shortened. For more detailed discussion of
some of these issues see (Observatory Design Discussion Addenda) >>>>This is where we are right now.
Note: this discussion is about basic considerations for an observatory and does not include detailed construction topics. Such
topics will be added in addenda to this discussion from time to time. There discussions are at (Observatory Construction
Details) Much detailed information on the design of buildings and piers is also available on the web in the Mapug-Astronomy
Topical Archives.
Considerations for the Design of an Observatory
Climate: The type of observatory required is highly dependent upon the climate. This is an obvious fact. However, in any
climate, the purpose of the observatory is to protect the equipment, allow the equipment to be used easily and to provide an
environment which provides for the comfort of the observer as much as possible. If the climate is harsh, the observatory must be
waterproof, snow proof and free of leaks which might allow damage to the astronomical instruments. At the same time, the
structure should be designed in such a way so it can be brought into use in a short time.
This might be expanded to consider Climate and Location.
It is clear that there are extraordinarily varied climates within the US much less in the entire observing world. In some cases the
climate is dry and mild such as Arizona and New Mexico. Even there snow and cold night exist from time to time. Similarly in
Florida the climate is mild but damp much of the time. In Wisconsin the climate varies from hot and damp in the summer to very
cold and damp in the winter. In all of these climates there are perfect days and nights which call for viewing from an open venue
under the full starry skies. But generally, considerable protection of expensive and delicate equipment is required. This means a
permanent building of some sort is necessary.
The issue of security of the instruments from nature's predators, animal and human, is also not a trivial concern. Appropriate
precautions from local animal pests are very location specific and can't be considered in detail in this short discussion. In
Wisconsin ants, wasps, field mice and the ubiquitous mosquito are the main pests. In other climates, I am sure other animals are
pests of concern. The human pest comes in several forms. The most benign is the looker on who is generally curious and can in
some cases be converted to a sky gazing convert. The less benign would be the thief who is determined to steal valuable
equipment. Even this creature is a minor problem since such specialized equipment as might be in an observatory is hard to
fence. The worst and most dangerous of the humans are the vandals. Incredibly costly damage can be done in an instant. Even
the isolated observer is not totally safe. Equipment can and should be insured so it can be replaced or repaired. The safety of the
observer is largely a matter of local conditions.
The design of the observatory may have an effect on the relative attractiveness of the site to vandalism of various sorts. I have
not been able to determine what factors might be involved other than location with respect to lived in structures and local
conditions. I rather doubt that the construction of the building is a deterrent to the determined vandal. Some discussion of these
issues might be of interest.
January 2000 addendum. I have received a report from John Menkeof Technical Innovations that in over 2000 user years of use
of their domed structures, there have been no reported events of vandalism of any sort. This is most encouraging and we can
hope will be a continuing situation.
Creature Comforts: A major element of the observatory design should be the comfort of the observer. For normal visual work,
it is necessary for the observer to have direct access to the telescope eyepiece and thus the observer will be at the same ambient
temperature as the telescope. However, protection from wind and radiation to the open sky can give the observer some comfort
from the worst of the elements in cold viewing conditions. When instruments are used with the telescope in such a way that an
observer does not have to be continually in attendance, it is desirable to have a two or more part structure. One part can then
provide the best conditions for the telescope and its auxiliary equipment and another part can provide for the comfort of the
observers/operators. The observer/control area can easily have several functions as well.
Observer comfort should not be overlooked. Much of the pleasure of astronomy is actually viewing of objects. A comfortable
observer will enjoy visual observing much more, do a better job of observing and engage this pleasure more frequently. All of
these are desirable outcomes. In many cases, personal observation will be the main or only purpose for having an observatory.
Both viewing position and thermal considerations are important. One can become strained and tired if contorted body position is
required for viewing. One can become ill if chilled or heated to much during extended viewing sessions. Minimal use of heating
pads on the seat or in the lap are great help in avoiding chilling of the body. Appropriate clothing such as layering, hooding,
gloves and good socks and boots are essential. It is generally necessary to have the telescope at ambient temperature to avoid
thermal effects in the surrounding air that reduce viewing acuity. Thus open air or shuttered domes are most common and the
viewer simply has to dress for the occasion.
There is one interesting dome and shutter arrangement called the Lanphier shutter that can be used in a totally heated/cooled
domed building. I have not seen it at work nor do I know the effectiveness of this specialized shutter. It is made by Ash domes
and described briefly in their literature. It consists of an optical panel that moves with the shutter and dome and lies between the
inside and outside air. Interesting design which probably deserves additional consideration. In this case, the entire observatory
and control area including the observers are inside looking out through a window!! Experiments have been done with covering
the shutter opening with a very thin Mylar sheet. This can sustain a significant thermal differential and has been reported to have
no effect on seeing or imaging. This tactic might be well worth additional experimentation.
If the observatory is to be used mainly for instrumented observing with short or no time spent on direct viewing, the problem
becomes much easier. Only the instruments have to be at ambient temperature and have to be designed so that they will work
properly at those temperatures. The observers can be in a separate room which is heated and air conditioned and which can
house much of the instrumentation at more nominal operating temperatures. This is the ideal which most professionals and
serious amateurs strive for.
Types of Telescopes: The observatory must be designed to be large enough so that the telescope or telescopes can be used
easily and so that auxiliary equipment is conveniently located, accessible and protected. Within reason, if the observatory is large
enough, it can accommodate a variety of telescopes. Remember, of course, that refractors on GEMs take a considerably larger
area than fork mounted Cassegrains because of their design and mode of use.
The type of telescope determines to some extent the size of the building required. In the case of the open air building, like the roll
off roof type, the rolling of the roof need only clear the instruments when they are in repose. In the case of the dome, size is a
significant issue. A SCT on a fork is the most compact design for a given aperture and focal length, while a refractor requires full
clearance for the swing of the OTA as well as the added swing required if the OTA is mounted on a GEM. The 63 foot focal length,
40" aperture, Yerkes refractor requires a 90 foot dome. (and a three story moving floor)
Amateur telescopes are often as large as 6 inch f 9 which is a 54 inch focal length. Such a telescope requires a large dome and
even then has an eyepiece position which is very inconvenient for the viewer. Even a Newtonian requires significant dome size and
demands inconvenient eye positions. It is important to design the dome type building so that not only current but future
telescopes can be accommodated.
Types of Observing: Direct visual observing is interesting for the novice and should be made possible with the observatory set
up. However, more and more the purpose of the serious amateur is photography, imaging, photometry, spectrometry and other
activities which require instrumentation at the telescope other than the human eye. If one type of use dominates, it is desirable to
optimize the observatory for that use even if it makes a more minor use a bit less convenient.
----------------------------------------------------------------------Working on the rest of this -- Doc
-----------------------------------------------------------------------Automation: Automated, robotic and even remote control and observing are becoming more and more common. Thus the
observatory should be designed so that it can accommodate these function or can be modified easily to accommodate them. The
telescope for remote or robotic use will of course have to have its own computer control. The housing for the telescope will, in the
most extreme case of remote use, have to be designed in such a way that the building can be opened, set up for use and put to
rest at the end of a session without local human intervention. For the intermediate case, of automated use, but with a local
attendant, the building can be much less automated.
Observatory Quality Factors
Space: There might need to be enough space for just one observer and possibly for several observers who take turns at the
eyepiece of the instrument. This is necessary only if the telescope is tube used for visual observing and for demonstration of
viewing to a group. For the more advanced user, the desirability of having multiple person access to the instrument becomes less
important when quality personal observing is the primary concern. In this case, principle attention has to be paid to proper space
for use of the instrument and its accessories by one individual.
Sky View: There must obviously be a satisfactory view of the sky for the instrument. There are two major types of sky view
provided by observatories. One is the view provided by a traditional domed building with a shutter opening for the instrument.
The other is the so called roll open building which is in most cases provided by a roll off roof. The domed building has some
disadvantages of cost and requires some level of automation for either remote or local robotic use. The roll off roof has the
advantage of relatively low cost and full exposure of the instrument to the sky. These factors are discussed in more detail below.
Thermal Considerations: Thermal considerations are very important since they affect the comfort of the observer, the behavior
of the telescope and instruments as well as seeing factors such as dewing and disturbances in the visual air path. There are
competing considerations regarding thermal factors. In the one case, it can be argued that the telescope and its associated
equipment will come to thermal equilibrium with its surroundings more quickly with a fully open roof. Contrarily, the domed
building needs more time to come to thermal equilibrium and there is a greater possibility of air disturbances do to mixing of
ambient outside and internal air at the shutter which might spoil seeing.
A second part of the thermal equilibrium equation is what happens after some time has passed. The totally open structure
exposes the instrument to the sky and by radiation the instrument and everything else in the building cools to a temperature lower
than the ambient air and collects dew. The amount of dewing depends upon the dew point relative to the ambient temperature
upon wind conditions. A still air condition allows dewing to build up to a situation where condensate actually runs and form pools
of water. A significant wind will dissipate light dew but the wind in turn can shake the telescope so as to spoil viewing as well.
In the case of the domed building, the telescope radiates to the dome which is very nearly at ambient temperature. The
temperature of the dome will be at actual ambient temperature or possibly slightly cooler since it too radiates to the sky.
However, the dew in this case forms on the dome and tends to keep the dome at the dew point which is usually not that much
different from ambient when dew is a problem and certainly not near sky temperature. Telescopes, instruments and other items
and surfaces within the dome almost never collect condensation.
Versatility: The design of the observatory must accommodate the telescope installed but should also allow for installation of a
different or larger instrument and accessories that might be needed in the future. The totally open design is more versatile in that
almost any type of instrument can be set up as long as space and headroom allow. In fact, if there is enough floor space, several
instruments can be operational at the same time. The dome limits the multiple instrument option unless the instruments can be
mounted on the same pointing platform and see through the shutter opening. With the dome, some added thought has to go into
the placement of the pier and the instruments on it.
Conclusions: These conclusions are based on at the design of a building in Wisconsin which is in a cold/hot, dry/wet, calm/windy
location. The telescope (s) will consist of several instruments mounted on a single pier/pointing platform. The use will be almost
entirely for photography/imaging and possibly other instrumented use such as variable star measurement or solar studies.
Creature comforts are essential since the principle observer/operator is too old to want to get his eyelashes frozen to the
eyepiece. The telescope/instruments will be automated, but run locally (not run remotely) at this time.
The following decisions have been made and the observatory will be constructed in April 1999 and instrumented during the
Summer 1999.
The building will be a two part structure 11 by 22 feet in size with a partition separating the control room from the telescope room.
The dome will be a Pro Dome 10' in diameter with full automation. (I would really like an Ash dome but they are just a bit too
expensive for me) The control room will be both heated and air-conditioned for the comfort of the operators. There will be a single
pier with a large pointing platform that will take a variety of instruments. The most likely candidates at this time are a 14"
Celestron Fast star, Meade 10"CAS, Celestron C-5 and several large APO photographic lenses. (I have all but the 14" which looks
very good to me because of its versatility)
The decision to have two rooms, a control room and observatory room, was not difficult. The control room can be held at a
temperature suited for computer equipment on a long term basis and brought quickly to a comfortable temperature for the
operators. The control room will be well insulated and connected to the observing room by a common insulated wall and thermal
window. An insulated pocket door will connect the two rooms.
The decision to go with a conventional domed structure was the most difficult and deserves some explanation. After reading the
many opinions recently posted on Mapug-Astronomy and after considerable other reading and calculation of various factors, I
came to the conclusion that the conventional dome is the best for the following reasons:
0. Provides the best protection against weather conditions. i.e. can be well sealed against blowing snow and heavy rain as well as
high winds and dust.
1. Provides best protection for the observer from radiation to the night sky. This is particularly important in winter when skies are
often very clear in Wisconsin.
2. Provides the best conditions for eliminating dewing on the telescope and all of the other instruments in the observatory. An
open venue allows for condensate to form on everything in the observatory. Cases, shelves, anything on the shelves,
instruments, telescopes even canvas chairs and tables in an open venue get wet. The dome prevents all of these problems.
3. Has no disadvantage to viewing do to air mixing if the right sort of forced ventilation is provided. This requires the shutter to be
opened sometime before use and significant forced ventilation to be employed to bring everything to ambient and hold it at
ambient. (I found a discussion of this matter in a book on observatory design.)
4. Provides the best protection against wind for both the operator and the telescope. Even a slight wind can cause considerable
operator discomfort. And as we well know, wind spoils imaging very easily. An additional wind screen can be easily installed in the
shutter opening. It is even possible to consider adding a Lanphier style shutter.
I am currently looking for information on the design of the Lanphier shutter. It apparently allows for a heated or cooled dome and
provides viewing through a plane glass window.
5. Can be easily fully automated with commercially available equipment, especially from Home Dome
I welcome any discussion and arguments or counter arguments anyone has to offer.
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This page is under development and so will be modified from time to time as the theory and practice relating to thermal problems
becomes clearer. My attempt is to provide herewith a detailed discussion of thermal problems affecting telescopic viewing and
imaging and related problems of dewing of equipment due to climatic temperature and humidity conditions.
Temperature, Thermal Effects, Dewing & Similar Problems
It is usual to hear observers say that the telescope should be kept at ambient air temperature or nearly so. Still because of
dewing problems on some of the optical surfaces or other surfaces, heaters of various sorts are applied. Sometimes heaters are
applied to the optical tube and to the eyepiece as well. So what is going on here? Why does dew form? What other effects does
the application of heaters have on the optical system? What is the effect of wind or artificial air movement? The undesirable
effects of temperature and temperature differences seem to be well known but an understanding of how the physical mechanisms
work seems to be limited to engineers who have understood thermodynamics.
It is best to separate the thermal effects into three parts. One is the effect of non uniform temperature on the optical
characteristics of the telescope itself, another the effect on the optical path of differential air temperatures and the third the
dewing results which are temperature related.
The effect of temperature changes on the optical characteristics of the telescope
For a variety of reasons, it is necessary to force the telescope optical system to go to the ambient air temperature. This is
because the telescope is a large object and exposed to the ambient air and it would be impossible to insulate it from ambient
temperature for any length of time anyway. Normally, the ambient temperature changes over a night of viewing by many
degrees. The actual ranges depend, of course, mainly on the climate at the observing local. One can imagine changes of tens of
degrees from sunset to dawn. Unfortunately, such temperature changes will cause a number of optical effects in the telescope
optical system. With a well designed telescope, the collimation should not change, but it has been shown that the focus does
change. This change is probably detectable for even a degree or so temperature change. Complex folded telescopes seem
particularly sensitive to temperature. There is not much that can be done about this except to watch for focus changes over the
night and re-focus regularly.
The effect of temperature differentials on the external optical path
One of the most obvious optical effects is the visibility of air waves rising from objects that are warmer than the surrounding
ambient air. (similar waves are visible from objects that are cooler than ambient). This is probably the main reason for keeping
the telescope at the same temperature as the ambient. Similarly, when the telescope is in a building and must look over the walls
of the building or through an open shutter in the dome of the observatory, there might be non- uniform temperatures that cause
heat waves in the air. These heat waves, whatever their origin, cause the image formed by the telescope to waver or scintillate.
While the eye can account for some scintillation, in imaging the result is often serious fuzzing of the image. There are modern
digitally controlled means for reducing some of this scintillation. For example, the AO-7 unit available from SBIG has been
reported to reduce scintillation considerably.
But, clearly, avoiding optical path problems by reducing or eliminating the effect in the first place would be a wise design
criterion. It is probably easiest to eliminate heat waves caused by the observatory building by removing the building completely
with a roll off structure. This can be done by rolling the entire building away from the telescope pier or by opening the entire roof
of the building. This factor is probably one of the strongest for using such a roll off design for the building. Additionally, the roll
off structure is relatively inexpensive and simple to construct.
However, it should also be possible to create a dome/shutter design that avoids heat waves in the vicinity of the shutter opening
as well. I believe that this can be done by ensuring that the dome is brought quickly to ambient temperature and kept there with
fans that move ambient air through the building. The air should be drawn in through the shutter and vented at the side of the
dome away from the shutter. It is probable that several fans will be needed to bring the dome and its support structure to
ambient in a convenient length of time. Faster moving air cools the building and telescope quite rapidly.
There are conflicting requirements for reducing the problem of heat waves in the optical path. As seen here, the roll off building
seems to have an advantage. Still there are several other considerations that will demonstrate an advantage for the dome/shutter
configuration.
Dewing, the mechanisms of dewing and ways to reduce dewing
When looking at all of the vagaries of dewing, the results seem to be confusing and the methods for reducing dewing a black art.
Dewing is not, in the simplest situation, a complex phenomenon. Simply stated, moisture condenses on a surface when that
surface is even slightly below the dew point of the air. This temperature is quite well defined and can be measured to a small
fraction of a degree. When the air is moving, dewing becomes more complex. Air motion will directly affect the thin layer of
stagnant air near the surface within which dew formation takes place. Moving air also strongly affects the heat transfer between
the air and the surface.
Generally, the surface temperature on which dew forms must be below that of the air for dewing to take place. In situations
where there is a large amount of moisture in the air, the dew point temperature may be only a few degrees below the ambient air
temperature. I such cases, dew forms on surfaces that are cooler than ambient temperature very easily. In many situations,
when there is considerable moisture in the air during the daytime, the ambient air temperature and the dew point approach each
other as the nigh temperature falls. In the clearest nights, the temperature tends to fall fastest.
Accepting these basic concepts, it is clear that an essential part of the dewing mechanism is the actual temperature of the surface
on which we hope the dew will not form. So it is time to consider the equilibrium temperature that a surface will take when
exposed to ambient air and its surroundings. The equilibrium temperature of a surface will be established when the heat energy
reaching and leaving the surface are the same. There are three mechanisms for heat transfer. Conduction from a warmer or
cooler body, convection of heat to or from the surrounding air and radiation of heat to and from the surface. Let us take these in
order.
Conduction of heat to a surface in the case of a telescope is typically from the attached equipment. This might be the electronics
and/or motors driving the telescope. Or, very typically with amateur telescopes, heat is applied from a so called dew zapper. The
dew zapper is a heating element whose purpose is to raise the temperature of the telescope slightly. Generally it is applied at the
mirror or corrector plate since that surface seems the most subject to dewing. The amount of heat has to be held to the absolute
minimum necessary to stop dewing, and no more, since there are other deleterious effects of heating and raising the temperature
of the telescope above the ambient air temperature. Many amateurs find this equipment not only effective but necessary,
especially when the telescope is used in the open or in a roll off building where it is exposed to the cold night sky.
Convection is the transfer of heat from the air to the surface or vice versa. It is normally necessary to have the telescope very
near the ambient air temperature to prevent convection currents from occurring and causing heat waves near the telescope
objective/corrector plate. In order to increase the heat transfer from the air to the surface, the air must be moved. With good air
motion, the telescope will move rapidly toward the ambient air temperature. This is good for several reasons. One is the
reduction of heat waves just mentioned. Another is the reduction of the possibility of dew formation. If the surface is at the
ambient temperature of the air, dew cannot form. (unless the dew point temperature is actually equal to the air temperature)
Radiation is a much more subtle way to transfer heat. When two bodies are at different temperatures, there will be heat transfer
from the warmer body to the cooler body. The transfer is dependent upon the temperature of the bodies and the area that each
body sees of the other. (It also depends on the nature of the surfaces in a complex way, but this effect will be overlooked in this
discussion.) In the case of a telescope which is in an open setting, the telescope sees the entire sky and radiates heat to the
entire sky. The effective radiation temperature difference between the telescope and the sky is quite large. The sky (atmosphere)
has a temperature of about 200 Kelvin. The telescope, even at say 50 F, has a temperature of 10 Celsius, of 283 Kelvin. Thus
the telescope will radiate more heat to the sky than it receives and it will cool off. If it cools just a few degrees, it might well drop
below the dew point temperature. Then dew forms. All surfaces experience this same effect. We all know that any surfaces on
an open viewing situation, plastic cases, chairs, wooden boxes, telescopes, everything gets wet with dew when dewing conditions
are right. In fact, they get wetter and wetter and water will run and puddle on them.
There is a constant struggle between heat radiation from the surface to the sky and heat transfer from the air to the surface. The
temperature of the surface will and must be below the ambient by some amount. Thus the surface can dew up. A dew shield on
the front of the telescope will greatly reduce the tendency of the corrector plate to dew up because it greatly reduces the solid
angle of the sky that the corrector plate sees. However, eventually the corrector plate will dew up if the heat transfer from the
ambient air cannot keep up with the radiative cooling to the sky.
A simple demonstration of this effect can be made by placing two similar surfaces out in an open field. Over one of them, by a
couple of feet, place a sheet or blanket. The exposed surface will dew up easily and the protected one much later. This is
because the one is protected from seeing the cold night sky.
Now what has this to do with observatory design? One strong reason to use a the domed observatory is to prevent the telescope
from seeing the open sky. Inside the observatory the air and the dome itself will be very close to ambient air temperature if
proper air motion is provided. (forced if necessary) Thus objects inside the observatory including the telescope and other
equipment see a "sky" which is the inside of the dome which is nearly at ambient. (Note that the dome will be radiating to the
open sky and will thus be slightly cooler than ambient.) Thus there is no net radiation from the objects and they will remain at
ambient temperature and will not dew up. The same effect helps keep the observer warmer since the observer is not radiating to
the cold sky but to the dome which is at ambient temperature.
This is a very strong argument for a properly designed dome type observatory. The natural or forced flow of air through the
shutter helps keep the telescope at ambient air temperature as well. Telescopes used in domed observatories rarely have dewing
problems.
The above discussion is somewhat simplified, since the nature of the surfaces comes into play, the amount motion of the ambient
air is important and other non-linear effects take place. However, I believe the first order effects are well taken into account.
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Discussion of Design Factors for Control Rooms
This page is and will forever be under development. It seems that are no perfect solutions for every observatory in every climate.
Much of the design detail is of a totally personal nature.
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Sources for Design & Construction Factors for Small Observatories.
It seems that are no perfect solutions for every observatory in every climate. Much of the design detail is of a totally personal
nature. Entire books have been written on the design of small observatories. There are also many excellent discussions on the
Internet including the Mapug-Astronomy.net home site.
Some of these are:
Unusual Telescopes, Peter Manley, Cambridge University Press 1991
Of interest for wild ideas.
Small Astronomical Observatories, Patrick Moore, Springer 1996
Several authors give good solid building advice.
Telescope Control, Trueblood and Genet, Wilmann-Bell 1997
For sophisticated computer control and the like.
At Home in a Dome, John and Meg Menke, 1993
Specifically for the Home Dome but with many excellent general and practical ideas.
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Observatory Design Index
Preliminary discussion of Winch Design (still under construction)
The first part of the discussion is directed to a winch design that uses a standard pulley type winch with a single pulley. A later
section discusses the two drum winch for two way operation and the final section describes the rather more complex design
required for a continuous two way winch design that would be used for a dome that had to be rotated continuously in either
direction. Finally, there is a brief discussion about pulling and pushing both roll off structures and dome type structures and the
roller arrangements that affect stability of the movement of these structures. Thanks to Chris Vedeler for the very nice Figures.
Part 1. The two-way winch with one driving drum and limited travel
In Figure 1 the problem with a simple basic winch drive is shown. For a winch to work, there must be tension in the output side of
the cable. The winch amplifies this tension, called force 2 (F2), as it rotates by cinching the cable around the winch pulley. The
amount of amplification you get depends in a complicated way on the number of turns of the cable around the winch and the
coefficient of friction between the cable and the winch pulley.
Generally, with two turns around the winch pulley, you can get an amplification of 100 quite easily. The problem with the simple
design is that when you turn the winch CCW, in the case shown, F1 needs to be large to pull the load and this tension in the cable
stretches the cable. enough that the cable on the left side becomes slack and the force F2 goes to zero or nearly so. Then the
winch action no longer works, the winch slips and the force in the right side gets small enough so it can not pull the load. No
amount of tension in the cable will help since the cable will always stretch a bit and the winch will slip. Also, any tension in the left
side of the cable adds to the load that the right side has to pull. Believe me this will not work. In order to minimize cable stretch, it
is essential to use steel cable. I used a plastic covered steel cable for the dome rotation winch on the 12 foot MAS dome which
houses our 16" CAS.
To fix this problem we must add, on each side of the load, a tension loader as shown in Figure 2. The action of this tension loader
is to maintain a tension in the side of the cable that tries to go slack. A detail of the tension loader is shown in Figure 3. The
device, while simple, has several important properties. It consists of a spring which is normally slightly stretched so as to maintain
tension in the cables. But you would find that the spring which is just right to maintain tension in the cable will stretch too much
on the high force side and make the whole cable system again go limp and the winch will slip. Note that a tension loader is
required on each side of the load and must have the following characteristics..
What you have to do is make the tension loader very non linear. This is done by adding a chain across the tension loader spring
which is about 150 to 200% the length of the spring. Then what happens is that the spring on the high force side stretches until
the chain takes over and the right hand side gets taught and pulls the load. At the same time, the spring on the left hand side is
still stretched enough to keep some tension in the left hand side to make the winch action work. In the opposite direction the
other tension loader functions the same way.
Some experimentation and balancing of the spring constants and chain lengths is necessary. But when set up properly, this design
works very well. You have to have a spring that is strong enough to maintain the necessary tension and still has enough extension
to absorb the stretch in the cable. Such springs are available in hardware stores for a few dollars. You also have to keep the chain
short enough so that the spring does not permanently deform.
For the dome drive I used springs with about 60 pounds per inch spring constant and stretched them about 2 inches under no load
conditions. I made the chains so that the maximum stretch was about 3 inches. You will have to play with these variables to
match your load.
I might point out that another solution for this particular drive is that a chain could be used. A chain, captured in a chain sprocket,
will not slip even if the low tension side goes limp. This might be a better solution for the roof drive shown. The only problem is
the chain typically is several dollars per foot. You can get such chain and chain gear drives commercially but you will find them
very costly.
Part 2. The two-way dual drum winch with limited travel
For the case discussed above, where the motion of the moving part of the building, roof or dome, is limited, it is probably easier to
go to a two drum winch. This design is shown in Figure 4. Here each drum has a cable that is the full length required to move the
movable member of the structure as far as necessary. One drum takes up the cable while the other plays it out and vice versa.
This is a very simple design which needs no additional springs and the like. The only problem with it is that it requires a two drum
winch. These are less commonly available and may have to be devised by replacing a single drum with a modified drum
arrangement. It would also be possible to use two winches in opposition. They could be synchronized by placing the motors in
parallel electrically.
Assurance that the moving structure will move smoothly is related to the way the forces are applied to the structure and the
details of the design of the rollers and their placement. These issues are discussed in a following section.
Part 3. The two-way winch design with unlimited travel
The most trick of all the possible designs is the two way winch with unlimited travel. This would be the design desirable for a dome
which is to be rotated round and round without requiring a stopping point and reversal of the motion.
A design for this type of winch is shown in Figure 5. I have just completed this type of winch for a 12 foot dome with excellent
results. Note that in this case, the spring tensioning mechanism must be symmetrically placed on each side of the winch. The
design depends on getting enough cinching effect on the winch to supply amplification of force through the winch and also
sufficient cinching effect on the dome to pull the dome. One problem with turning a dome in this fashion is that the pulling force
necessary to turn the dome is all at one point. This tends to want to pull the dome right off of its rollers. The lateral force must be
taken up by well placed lateral rollers. This issue is discussed in a later section of this article.
This arrangement, which seems complex at first glance, is quite simple to understand. The design is and must be perfectly
symmetrical if it is to run in either direction without limit on the motion. In practice, the cable is run twice around the dome so as
to get enough cinching effect. The cable is run two to three times around the winch pulley also to get enough cinching effect and
thus enough amplification of the in-going tension as compared to the out-going tension.
Since the spring/chain tensioners cannot be in line with the cable, they are but effectively in line using two pulleys. As shown, the
upper pulleys transfer the winch mechanisms forces at right angles to the dome. It takes about 200 pounds force to move the
dome in the particular example I have designed. The winch supplies an amplification of about 10 times. This means the springs
have to have a spring constant something in the order of 50 pounds per inch. I found some replacement springs for a hobby horse
at the local hardware store that were just right when I put two in parallel. The chain allows for about 3 inches of extension of the
springs. Some playing around with the spring strength and extension limit was required to get this just right for the application. I
broke two springs before I got it right. Note that the force in the springs is twice that in the cable.
The whole mechanism works very nicely and is sort of interesting to watch as the dome is juggled back and forth. I will put up a
photograph of this mechanism as soon as I can get one ready. (Figure 6 photo not yet ready)
Part 4. The motion of pulled and pushed structures
The motion of pushed and pulled structures which are on rollers is very interesting. When I build my first roll off structure, it was
the one shown on my home page, I was concerned about the forces required to move it and how well it would stay centered on
the rails. The building rolls at its base and is quite heavy. To keep it moving straight, I decided to use V groove wheels and have
them ride on a single slender rail. Four 8 inch wheels, which are steel sheaves, are used on each side. They are mounted on
hardened steel drill rod and lubricated with heavy pump grease. The rail is an L iron with mounting brackets welded to the sides.
The design is self clearing for any ice that might build up in winter. It takes nearly 100 lb force to get the building rolling from a
dead stop. It takes only about 50 pounds force to keep it rolling once it has started to move. This is due to the static stiction that
the roller system has.
It was my original intention to install a motorized mechanism to move the building. This has not been done since the building is so
easy to move by hand. In fact the design is so stable that one can easily push on one side of the building only and it does not
skew enough to cause any binding. This is largely because the building is very well built. It was done by a professional contractor.
Others, especially with larger structures, like large roll off roofs may not be so fortunate. If the structure tends to flex and
especially to skew, it will likely be necessary to apply the force required to roll the roof in a nice symmetrical fashion. All of these
issues depend greatly on the resistance of the roller system and the tendency of the structure to skew. In the worst case
unacceptable binding will take place when force is applied to one side of the roof. In this case, the lateral integrity of the roof
should be checked to see if cross bracing or diagonal bracing can be used to stabilize the roof to keep it from skewing.
In some cases the application of lateral rollers will help keep the roof symmetrical and relieve binding of the vertical rollers or
wheels. If that does not work or is not possible the point of application of the force will have to be changed to be more
symmetrical. In the worst case, with a rather flimsy roof, it may be necessary to apply the force at two points, one at each side of
the roof and in line with the roller mechanism. Each case is unique. With some roofs and shutters, a rather complex system of
pulleys and cables is necessary to insure smooth movement of the parts. For example, both Ash and Home domes have elegantly
designed mechanisms.
This brings us to the mechanisms for rotating domes and how domes must be stabilized with rollers to insure that they move
smoothly. Domes are almost always on rollers. tiny 3 inch rollers are fine for domes up to 10 feet but may be much larger. The
dome at Yerkes, 90 feet in diameter, runs on flanged railroad car wheels. Rather appropriate since Yerkes was a railroad
transportation mogul.
In the case of non flanged rollers, it is clear that the dome would skitter right off of the rollers if it were not restrained by lateral
rollers. The forces used to rotate the dome are applied in various ways, but all tend to push the dome off the rollers. In the case
described above, the force is applied to one edge of the dome and would pull the dome right off but for the lateral rollers that keep
it centered. Thus, the placement and quality of the lateral rollers should not be under estimated. In the case of the 12 foot dome I
have worked on, the use of 16 vertical and 16 lateral rollers very greatly improved the smoothness of the motion over the
previous setup which used 8 of each. The method of moving this dome causes very asymmetrical force on the dome. Still even
very sophisticated domes, like the Ash dome use only one motor and apply force to the circumference of the dome in one place. In
that case the motor is on the inside and uses a perforated track and cogged wheel to move the dome. In the case of the Home
Dome two motors are used at opposite sides of the dome. This applies a much more symmetrical force to the dome and moves it
more easily. However, the Home Dome drive is slightly less positive since the drives are friction drives instead of a positively
engaging cog wheel drive. In larger domes, there are generally several/many motors that work around the entire rim of the dome
to move it smoothly. The issues are reliability and cost as with most designs.
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Beginning Astrophotography
00. PREFACE & DEDICATION
It is my hope that this series of writings will provide just enough insight toward overcoming the challenges of astrophotography so
that those with limited experience and or resources can obtain good to excellent results from their equipment. This is not intended
to be a highly technical discussion where electronics, mechanics, optics, and theory are debated. Rather, practical experience will
be stressed with "how-to-do-it" steps. Arguably equally important, I will attempt to identify the least complex methods to guide
those with limited experience and modest equipment.
I will say at once that I believe there are many ways to similar results. Others may find slightly or very different methods work
quite well. In fact, I recognize some of the tactics I will propose do not necessarily produce the best possible results. However, I
trust those that are familiar with my earlier writings and the title of these works know I am trying to keep things reasonably
simple for those wishing to avoid complex, (but arguably effective) methods. I am submitting these works to Doc G for his
valuable insight into some of the tactics that will be presented. I believe his added perspective will help round out the material
quite nicely.
It is recognized that many interested in astrophotography simply cannot afford high-end amateur equipment and must make the
best of what they already own or what is affordably available.
In addition, even if one is fortunate enough to own some of the very best equipment, this doesn't guarantee the outcome. I have
seen many examples of rather average images, including my own, taken on very fine instruments. Many of us are quite likely to
experience equipment, light pollution, transparency, and seeing problems not known by those using professional or high-end
equipment at locations with excellent imaging conditions. Those that obtain good results under such conditions can pride
themselves for doing more with less. In addition, many may be using new and unfamiliar equipment. However, those that are
patient, develop their imaging skills, and utilize the full potential of their equipment can obtain satisfactory results, though it may
require a bit more effort.
I have posted an abbreviated gallery of images selected from those taken using low to mid-priced equipment typically owned and
used by many amateurs. Images produced with professional and high-end equipment at ideal locations were not selected because
they are not representative of what is available or affordable to most amateurs. The purpose of these images is to provide a
resource for illustrations and references for various concepts that will be proposed in the forthcoming articles.
It is not intended nor desired that the illustrations or written material promote any particular vendor or even a particular style of
telescope and imaging system. Some of the images are good illustrations of average imaging results we can expect from typical
systems and techniques. Others were taken under rather rare (for most of us) excellent imaging conditions using a bit more
advanced methodology.
I have been fortunate to have access to professional and high-end equipment, however, I recognize many others are not so
fortunate. Many are likely have minimal interest in scientific works, rather, they would like the skills to produce a few nice film or
CCD images to show to friends or hang in their home. They are quite likely overwhelmed by all the seemingly conflicting details
and methodology presented on various forums, perhaps even a bit intimidated. It is to these folks that I dedicate this series of
writings.....
Michael Hart
Husen Observatory
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Section 01. ASTROPHOTOGRAPHY TECHNIQUES
- - TRICOLOR FILTERS FOR IMAGING
BACKGROUND
The selection of good tricolor filters is quite important in achieving predictable results. Current sets available as a package from
several sources are typically compromised a bit for a number of reasons including the exposure times required to obtain a color
balanced image. I believe over the last few years, I have read considerable information and advice about tricolor filters based on
manufacturer's data, transmission curves and actual use by others and myself. Further important information concerning color
filters may found on Doc G's web site under "Color Filters Used for Three Color Imaging"
USING CMY COLOR FILTERS FOR TRICOLOR CCD IMAGING
The promise of excellent tricolor film images with less exposure time using CMY filters is certainly appealing. Since color film uses
the CMY process, it would seem quite possible that CMY tricolor filters should work quite well with black and white film. Without a
doubt, one will obtain color when the individual CMY channels are combined. While the promise for a short cut to images is likely
fulfilled, the question as to how well these filters work at faithfully reproducing the original scene remains in my mind. Perhaps it is
because the color model for this application is unperfected, however, the unexpected colors produced seem to require a
considerable amount of fiddling to compensate for the uneven response of the film or CCD chip and the CMY filters.
I have not directly monitored those using CMY tricolor filters, though I have had inquiries about the process and results. I have
examined images sent me using current CMY color models and have tested CMY filters independently. CMY filters are not new
which begs the question- why haven't they been used before? The answer is, they have. Emulsion base print films use the CMY
process. Camcorders have used CMY in micro-lens overlays for some time (often with a variation of the yellow filter). Often, a
custom DSP (for a given chip) is used to compensate for known chip anomalies. Kodak has chosen RGB overlays with good reason
for many of their consumer cameras. The RGB overlay improves color fidelity using a CCD chip, which has a rather uneven
response. Professional color cameras and camcorders get around much of this by using three separate imaging chips to provide
the very best in color fidelity over a wide range of imaging conditions.
Of course printers use CMYK (K is for black). Mixing all colors using print inks does not produce a very dark black, so printers use
black ink. Using CMY, the combing of wavelengths makes accurate color reproduction of the original scene quite difficult in low
light with a non-linear CCD chip, though achieving color balance is not so difficult. While CMY filters DO pass more photons, all the
light in the blue wavelengths are in the cyan and magenta filters. If we balance CMY colors against a gray card at the effective
temperature of the sun (5770 degrees K., spectral type G2), the cyan and magenta filters pass all the blue wavelength to
accurately represent the desired blue emission present in the original scene.
However, the CCD detector often needs a disproportionate amount of blue exposure (longer) to compensate for it's poor response
in the shorter wavelengths. This is where CMY color models may fall short, because the total signal of actual blue wavelengths is
COMBINED with other frequencies contained in the cyan and magenta filters. The result is the re-mapping of other wavelengths to
blue, which do not accurately represent the original scene. I believe the possible solution for CMY color accuracy is to have a CMY
color model written for specific CCD chips. Even CCD chips that have a better linear response such as the TC series are not likely
to produce more accurate color than straight RGB filters whose transmission curves produce smooth tops, gradual slopes and good
crossovers. Using a light blue 80A filter for the blue channel is also is very efficient because it passes non-blue wavelengths that
the CCD chip can readily record- all the blue light and a lot of red and green light. I describe a process in this article of using IR,
Red and Cyan filters with a resulting throughput exceeding CMY however; this also does not produce accurate visible color, though
the colors (channels) are well balanced.
USING CMY FILTERS FOR GAS HYPERED TECHNICAL PAN 2415 FILM
My film of choice for CMY filters would likely be gas hypered Technical Pan 2415 since this film has arguably the best dynamic
range (contrast) potential (depending on the developer and paper) of any black and white film readily available to the amateur.
Unfortunately, this film has a rather uneven green response. In the CMY color model, this green record is a combined with other
wavelengths in both the cyan and yellow records. To restore the original scene, we must use a very clever algorithm or at the very
least- fiddle with a standard CMY color model to compensate for the amount of green really contributed. What often happens is
that we find our algorithm may produce good results in some objects and very unexpected results in others.
I have probably tried every conceivable combination of filters one can imagine to test the viability of various combinations in an
effort to effectively re-map wavelengths to compensate for non-linear CCD chip and film response. I have also tried various
dichroic filter sets made for CCD imagers. Some use creative ways to improve response at problematic wavelengths. See Doc G's
filter information which adds considerably to this discussion.
I have tried subtractive dichroic sets (CMY) which are quite useful for camcorders (and in color film emulsion) at high light levels
with custom automatic color compensation circuitry built in, but have proved somewhat marginal (for me) with software based
traditional CMY color models. The reason is likely the nature of the subtractive color filter to pass all light minus one of the additive
colors. Thus, C (cyan) is green and blue minus red, M (magenta) is red and blue minus green, and Y (yellow) is red and green
minus blue. As expected, a filter combining two colors would have excellent throughput- arguably excellent for the ST-7/8 with
built in autoguider chip. Unfortunately the nonlinear response of the popular Kodak chips in these cameras and the filters
themselves considerably complicate the process of reproducing the original scene.
Thus far, I have not seen a software based modified CMY color model that addresses the fundamental problems of accurately
restoring the original wavelengths that accounts for the frequency response differences among CCD chips and film operating at low
light levels. I have used photometric filters (BVR) as well. After careful study of transmission curves, I believe there are no
significant shortcuts to good tricolor results in substituting wavelengths for those more readily available and/or more efficiently
recorded for the desirable actual visible wavelengths. The bottom line-- after a considerable amount of tricolor imaging over the
last several years, I have found no filter set I prefer better than using straight RGB filters because the results are predictable and
a better match RGB color models. I must say many of the better known images produced from observatory plates use BVR (blue
visible red) filters. One could argue RGB filters produce more accurate color than BVR filters especially when noting the
descriptions of emission lines represented in observatory images may not appear as the expected color.
This is not to say modest balancing of RGB color filters is not required. Still, I believe re-mapping or shifting visible wavelengths is
best left for scientific endeavors such as displaying the infrared, visible and ultraviolet wavelengths simultaneously. One can
deviate a bit from true RGB filters, but the closer, the better. Knowing the thickness of the filter is important to maintaining focus
between filters. Mixing a 2 and 3 mm filter means the telescope must be refocused between shots, though, using micrometer or
the JMI DRO (digital read out) or equivalent will minimize this inconvenience.
CALIBRATING CCD TRICOLOR FILTER SETS
One example of re-mapping is the use of the pale blue (80A) Wratten to shorten exposures with CCD cameras having poor blue
response. I do not generally prefer the results of using this pale blue (80 A) filter for this application, though John Hoot has
suggested its possible use in what he has termed the "Fun Set". However, John also recommends a "Fidelity Set" which I believe is
much better. The leakage from other wavelengths gives somewhat unpredictable outcome in the "Fun Set" while likely to
producing "colorful" results. Attaching a camera lens adapter to a CCD camera and shooting terrestrial scenes may be a good way
to illustrate why color balance, S/N ratios, kernel filters and more are skewed by these filters. For example, non-linear CCD chip
response in the visible wavelengths often requires long blue exposures to produce an adequate S/N ratio. Using an 80A passes so
much non-blue light that the results are quite unpredictable. One can easily see why this is so with the use of a MacBeth Color
Checker or other color chart while looking through the filter.
John Hoot's "Fun Set" consists of the #23A red, #56 green, and the #80 blue. On a Pictor 216 XT (TC chip) his exposure ratios are
close to 1:1:1. John Hoot's Fidelity Set consists of the #23A red, #58 green, and #38 blue. John feels this set gives the best
results but needs considerably longer exposure times with the green and blue filters. He reports this filter set gives reasonable
color balance and good signal to noise (S/N) ratios.
I am pleased that John uses the terms "Fun Set" and "Fidelity Set" to distinguish color filter sets that re-map colors from those
actually representing specific frequencies. Since color balance is related to color temperature, I would suggest the use of a spectral
class G2 star as the calibration source is quite right, though reasonably bright stars of this particular type are not particularly
plentiful (I can only think of four). I use the light of the full moon well above the horizon and image a neutral gray test card (from
a camera store) noting the light intensities recorded to determine exposure ratios. John reports a similar tactic except he images
the moon without the gray card.
Of course all that non-blue leakage is misrepresented as blue in the RGB color model. Another way to approach a 1-1-1 ratio for
the popular 0400 chip is to use filters at 770 nm, 650 nm and 550 nm. These are likely to produce visibly balanced star colors.
However, when using these filters, and re-mapping their frequencies to a RGB color model, the result is wonderfully blue images,
because all that is really green is remapped to blue. Imagine a terrestrial experiment and the resulting blue grass. We can also
synthesize a third color from two others with mixed results. It is difficult to use programs such as PhotoShop to compensate for
the lack of adequate color information with predictable results. If we don't care about color accuracy, we can use an imageprocessing program to do just about anything with mixed results. However, armed with accurate color information, predictable
results are not overly difficult and arguably worth the effort needed for longer exposures. In the case of RGB, we want signals as
close as possible to accurately representative RGB wavelengths, I think.
CALIBRATING FILM TRICOLOR FILTERS
Since many amateurs do not have convenient access to a densitometer (found in a good photo lab) and related equipment, it is
useful to provide a simplified method to approximate exposure ratios with minimal equipment. Approximate exposure ratios may
be determined by photographing a gray step card in moonlight. The moon should be quite near the meridian to minimize
atmospheric extinction. The exposure should be long enough to be representative of the film's response characteristics during long
exposures. In many cases, a five-minute exposure produces adequate representative results of longer exposures. A good camera
lens may be used to make this part easier, though I have used a telescope for this purpose. The target was set at 50-100 feet.
The use of the imaging scope removes any potential frequency attenuation or aberrations in the camera lens, but is likely a bit
more difficult to accomplish.
Next, ask the lab for prints without any correction. Explaining to the lab what you are doing may be helpful. The resulting prints
will show gray steps that are now likely different shades from each other and the test card. Match up the shades of the prints. If
each shade represents 1/2 stop, one can then extrapolate exposure lengths required to produce the same gray shade for each
filter. Check your results by again photographing the gray step card at the calculated exposure ratios.
CHECKING THE FILTER SET AGAINST THE MACBETH COLOR CHECKER
The use of a MacBeth Color Checker will give you a good idea whether your balanced filters can really reproduce the original
scene. Again, it is a good idea to use exposure times of five minutes or longer to better represent long exposure characteristics. If
you find you cannot reproduce these colors quite accurately, your tricolor images will produce unexpected results as well. If you
are using a monitor for comparing colors, be advised that results are likely to vary considerably from monitor to monitor.
SELECTING RGB COLOR FILTERS FOR TRICOLOR FILM AND CCD IMAGING
Reproducing true color from individual additive colors is best done using a sinusoidal bandpass with overlapping curves. The
weighted center of the curve should be quite close to the actual wavelength represented. Dichroic filters are quite efficient but
have transmission curves with rather steep cutoffs and flat tops. These steep curves and flat tops can enhance less desirable
results when using typical image processing techniques such as background and range adjustments (levels), saturation
enhancement, etc., limiting the ultimate potential of color fidelity. We cannot forget that most CCD cameras are sensitive to the
infrared leakage of most color filters. An infrared (hot mirror) filter should be used to stop undesirable infrared leakage.
Traditional (Wratten) colored filters are absorptive. These transmission curves produce smooth tops, gradual slopes and good
crossovers as compared to dichroic filters which function much as an interference filter, reflecting undesirable wavelengths and
passing the desirable ones. Overlapping transmission curves are important because the overlap (mixing of colors) is what
produces even color representation in the RGB color model.
Fundamentally, I believe traditional colored filters have an edge at producing good results without the aid of dedicated circuitry,
though in practice, a CCD image without adequate S/N ratio in one color arguably produces poorer overall results. If the initial
exposure ratio used is not quite right, we will still get a fairly good color image, however, we will need to adjust the levels to
partially compensate. We cannot compensate for what was not recorded. I must say no exposure ratio or filter set is perfect for all
imaging conditions. I use much longer blue exposures for objects away from the zenith due to atmospheric extinction which
absorbs blue and some green light.
IMPROVING A WEAK TRICOLOR SIGNAL
All current models of adding a monochrome image to a tricolor image are essentially based on a 50 year old television process
used to this day in our color television sets which recognizes the eye's inability to discern detail in color. As details become very
small, all the eye can discern are changes in brightness. Beyond a certain level of detail, color cannot be distinguished, and the
human eye, in effect, becomes colorblind.
The color television signal is composed of the luminance (higher resolution black and white) and chrominance (low-resolution
color). Essentially, substituting a higher resolution luminance image for the one contained in a lower resolution color image
produces what appears as a higher resolution color image. As a result, the essential color information is retained while displaying
the details in the luminance signal. If we increase the luminance signal enough, we can mask over irregularities in color balance,
color filter choices, and the color model used. This method has been used in the MX5-C CCD camera for quite some time.
LRGB and various synonyms to this acronym that essentially combine a monochrome image with a color image are not particularly
new, though the process may be new to some in CCD imaging. Jerry Lodriguss proposed using the PhotoShop CMYK color model 23 years ago for tricolor film imaging. There has even been new terms added such as WRGB (White RGB) MRGB (Monochrome
RGB), LRGB (Luminance RGB), Quadcolor (doesn't really use 4 colors), NRGB (Neutral RGB), and likely more. All are slight
variations to the process described above.
SUMMARY
I believe the RGB color model along with RGB color filters which reject (either through absorption or reflection) undesirable
frequencies produce the best, most predictable, and most accurate CCD color results readily adaptable to a variety of CCD chips
possessing a non-linear color response. The use of the standard color television process designed to conserve bandwidth,
regardless of what it is called, can play a useful role in RGB tricolor imaging but is not a panacea. I believe we should strive for
color accuracy if we are representing images as true color, then if a colorful result is desired, the use of filters that leak other
wavelengths into the represented color channel may be considered as can a tint to a converted monochrome image. Still, we must
keep in mind the results may misrepresent the original emissions as well as mask important details. Selecting the right color filters
for the film or CCD chip used and determining a good exposure ratio for the selected set is an important step to good imaging
results.
-Michael Hart
Husen Observatory
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PRACTICAL METHODS FOR FOCUSING FILM CAMERAS
1.
2.
3.
4.
Background
General Discussion of Focusing Techniques
Popular Focusing Methods in the U.S.
High Precision Focusing Methods - The B and K Astrofocuser
4a. Focusing Devices Applicable to Cameras with Removable Pentaprisms
5. Very High Precision Focusing Methods - Knife Edge Methods
5a. Takahashi FM60 Focusing Microscope
1. BACKGROUND
Achieving correct focus at the film plane can be arduous for even practiced astrophotographers with any camera. This is especially
true under average to rather poor seeing experienced by most amateurs. Focusing techniques that are quite adequate under
excellent seeing and imaging conditions may be virtually unusable for many under their sky conditions. One of the best (though
undesirable) ways to turn reasonably good 3 arc second images into 4-5 arc second images is through poor focus and/or focal plane
movement. Strong focal reduction and fast lenses (low f-ratio) require rather critical focus and should be checked often to assure
focus is maintained when ambient varies by more than a bit. For example, in a compound moving mirror telescope, I have measured
considerable focal plane movement during a 0.6 degrees C. temperature change. This focal plane movement may exceed the limit
for the maximum depth of focus desired for a selected focal ratio. At first this may sound surprising, but not so when one considers
the optical leveraging of these telescopes.
Temperature changes, optical changes, and accidental focus changes all call for focus checks. Frequent focus checks are easier and
encouraged if they are fast, convenient, don't require camera or film removal and don't cause horrible head and body positions. In
this article, I mention a few companies by name and some fine companies are not mentioned at all. This should not be construed as
a specific recommendation, but rather, certain companies are mentioned for purposes of clarity, example and reference.
This article is broken down into four general categories of practical methods for focusing film cameras: General Focusing Techniques,
Popular methods, High Precision Methods, and Very High Precision Methods. The material presented is not intended to be allinclusive. Devices that are not commercially available which require rather significant skills to fabricate are not included. Rather, only
methods that are known to be reliable under actual imaging conditions using typical amateur equipment and experience are
discussed. It is not enough for a device to have the potential to perform a particular function; it must work reasonably well in the
field at the telescope. I have provided rather detailed first-hand experience with each of the devices that I have personally used
under typical imaging conditions. There is one exception- the Doc G Focusing Devices. I included these because they work in a
similar fashion to one that I have used, while providing superior implementation. The reader must decide which method or
combination of methods is adequate for their imaging needs.
2. GENERAL DISCUSSION OF FOCUSING TECHNIQUES
If we have enough contrast, one can focus quite well with rather low power (5X) magnified camera viewfinders. Unfortunately when
contrast is low, the eye accommodates rather significant focus errors quite well. Here, point source focusing is particular useful for
those imaging in light polluted areas where contrast is poor do to the bright sky background. Point source focusing may be improved
with higher magnification. In many cases, perhaps most, the standard 5-6X viewfinder magnification is not adequate.
Even simple framing of an object can be quite difficult through a standard viewfinder due to limited eye relief. Worse, focusing and
framing is often especially difficult for objects at their ideal photographic location near the zenith. In addition, some telescope
designs such as compound telescopes and many Newtonians may not maintain precise focus if moved too far from the object of
interest. Moreover, rather than use a finder scope for object framing, many prefer to use the camera viewfinder for framing the
object of interest.
Many owners of cameras used for astrophotography find standard focus screens are too dim for focussing and framing. They can
replace the standard focus screen with brighter version. Exactly which versions are best for a given camera is debatable. Then, if one
is using one of the discontinued cameras that are popular for astrophotography, the focus screen of choice may not be readily
available. However, the Beattie IntenScreen Plus is readily available for popular astrophotography cameras such as the Olympus OM1, OM-1n, and Nikon F. While it may not be the their first choice, I believe most amateurs can use this focus screen for
astrophotography work.
A brighter focus screen helps with framing, increases contrast and aides rough focus, but does little for critical point source focus by
itself and nothing for comfortable object framing. A typical viewfinder lens and pentaprism such as found in the Olympus provides
about 5X magnification of the focus screen. Others provide around 6X, which are just adequate for high contrast objects but not
adequate for the critical focus of point sources.
It is a good idea to focus on a star 5-6 mm from the center of a 35 mm film frame to average out any field curvature. If one is within
the maximum depth of focus for a particular focal ratio, the use of this technique can produce what appears as a flatter image with
little to no loss of focus in the center. This is so because the smallest star size is determined by the small circles produced by seeing
and the film structure. As long as the stars Airy disc is smaller than the small circles formed, the image recorded will appear to be in
focus, when in fact, it is a bit out of focus in the center.
3. POPULAR FOCUSING METHODS IN THE U.S.
Return to Beginning
The Hartmann Mask
Some amateurs use a Kwik Focus or similar Hartmann Mask style focus tool based on the principle of using two or more holes in a
cover or mask. The mask is placed over the scope aperture. The result is that an out of focus condition produces multiple images or
multiple stars, which converge as focus is reached. In practice this seems to work better for CCD than for visual use. Under average
seeing, it is a bit difficult to determine when the images have fully merged, as the edges are rather soft under these conditions. One
can achieve good focus with this method, but not precise focus.
Review of Low Power Loupe Methods
The method of preference for many U.S. amateurs is focusing directly on the camera screen with a low to medium power loupe or
variation of this. The technique is as follows - one focuses directly through the camera for maximum contrast and/or minimum point
source size, much as focusing a low power eyepiece. Accurate position of the focus screen and camera mirror is important. I
typically prefer and use a knife-edge film plane method for critical focusing which is also useful to verify a particular camera or focus
accessory will accurately focus. It is wise not to assume that the focusing device is working properly. Using an in-camera knife-edge
test will verify other schemes for focusing. It may seem pretty risky to depend on the camera mirror position, mirror stops, mirror
pivots, and camera focus screen position to accurately represent the true focal plane. However in practice, it is possible to do so,
especially with high quality cameras and at larger focal ratios where the depth of focus tolerance is more relaxed.
The Nikon DW-2 Chimney Style Viewfinder
Users of the Nikon F2 camera can remove the pentaprism and replace it with the Nikon DW-2. The DW-2 provides direct viewing of
the focus screen at about 6X magnification; this is not as high as I would like for point source focussing. However the comfortable
right angle use helps to make the most of the low magnification.
The DW-2 will fit the Nikon F by removing a rubber skirt on its bottom side. The Nikon DW-2 is no longer available for the popular
older astrophotography cameras. Those that want the DW-2 must look to the used marketplace. Likely used costs are in the $ 200
range. These types of chimney style finders are available for many other cameras with removable pentaprisms. However, they all
have the disadvantage that they provide less than optimum magnification.
Fabricating a Focus Screen Magnifier
Others (including myself) have fabricated their own magnifier for the focus screen of the Nikon F for even higher magnification of 815X. The general procedure is to remove the pentaprism and fit a relatively high quality loupe to focus directly on the view screen.
The rectangular type designed for 35 mm film work have the correct aspect ratio, though a bit of sanding or cutting may be required
to allow it to rest fully on the focus screen. At these magnifications, one can focus by object contrast as well as focus with a point
source. Those in suburban locations may find focusing by contrast very difficult simply because of the lack of contrast due to the
brighter sky background. Here, point source focusing is useful. With the OM-1, the only real choice for higher magnification has been
to physically cut out the pentaprism and fabricate your own magnifier (I've done this), use the Olympus Varimagni Finder, or
purchase the Astrofocuser (described below).
Using the Olympus Varimagni Finder
The Olympus Varimagni Finder is just adequate for my 20-10 (corrected) vision with point source focusing. It produces a
magnification through the viewfinder of 6X in the 1.2X position and 12.5X in the 2.5X position. Others find the magnification of the
Varimagni Finder inadequate. The Olympus Varimagni Finder is field stopped at 3 & 5 mm at the 1.2X a 2.5X positions respectively
which results in a sharper but dimmer image while making this device much less suitable for contrast based focusing. The Varimagni
also readily falls off the camera, requiring its helical focuser to be re-focused to the camera focus screen. I use a bright star slightly
defocused as a view screen focusing light source or monochromatic light introduced through a guiding port. Like the Nikon DW-2,
you will have to find a used one, as these are no longer in production.
We have described many of the popular methods of focusing film cameras by many U.S. amateurs. However, many amateurs still
lack confidence in the results produced with these devices under their imaging conditions. Let's examine a few methods that promise
greater precision and ease of use:
4. HIGH PRECISION FOCUSING METHODS
Return to Beginning
The B&K AstrofocuserI have tested the B&K Astrofocuser on a standard SCT with a modified JMI NGF-S focuser to provide fine focus adjustments needed
to verify the Astrofocuser accuracy against one of the most, if not the most precise focus method of all - the inside the camera knife
edge test. The unit is shown below attached to an Olympus camera. (For availability and further details on this unit (cost approx.
$240) go to the web site: www.starfinder.com/maxx/images/Web-om-1a.jpg)
Accessories Needed to Use the B&K Astrofocuser
The B&K Astrofocuser was shipped with a black anodized Astrofocuser body which contains the lens, camera bracket and heavy-duty
1/4" thumbscrew. The Astrofocuser bracket is attached to the camera body with the supplied 1/4" thumbscrew. A 1-1/4" star
diagonal is inserted into the bracket holder. The Astrofocuser body assembly (with lens) is inserted into the star diagonal. Finally, the
eyepiece is inserted into the body assembly and focused.
The B&K Astrofocuser requires a user supplied diagonal and eyepiece. Many can use what they already own, so this is not as bad as
it first seems. I chose a chrome plated brass barrel star diagonal without a grooved barrel rather than a machined aluminum barrel.
Barrels with machined undercuts (grooves) sometimes induce optical axis misalignment upon tightening. The smooth chrome was
sufficiently hard enough to resist scratching by the bracket while allowing easy diagonal rotation. Owners of newer 1-1/4 prism
diagonals from Orion and Celestron may want to check collimation. Two Orion and one Celestron sample that I compared with an
older Celestron prism diagonal were badly out of collimation- beyond simple user adjustments. This could unjustly affect first
impressions of the Astrofocuser, which is why I mention this cautionary note.
The complete focuser assembly weighs roughly 12 ounces with a user supplied diagonal, user supplied 15 mm eyepiece, Astrofocuser
bracket, Astrofocuser body, and thumbscrew. This could be enough weight to throw off declination drift alignment on light duty
mounts with the effect of increased field rotation. During declination drift alignment, those with concerns can minimize flexure
induced polar axis misalignment, (which may lead to field rotation) by using weights to simulate the expected Astrofocuser/camera
weight combination.
The Olympus Varimagni weighs roughly 4 ounces. Still, the complete Astrofocuser assembly weighs only 4 ounces more than my
Nikon F with modified loupe for right angle viewing and focusing. The Astrofocuser is mounted by a large 1/4" thumbscrew to the
camera body. A piece of 0.2" X 1.25" key stock securely positions the user supplied 1-1/4" diagonal against the camera viewfinder.
The single 1/4" thumbscrew is more than adequate to secure the entire assembly and makes removal very fast. Those that purge
their cameras with nitrogen and intend to leave their Astrofocuser in place on the camera can drill the thumbscrew to accept a short
piece of brass tubing for attaching a hose.
Focusing with the B&K Astrofocuser
If you are using the Beattie IntenScreen Plus, focus the eyepiece on the side of the screen that faces away from your eye. A red light
shown through an affixes guiding port will reflect enough light to quite easily fine tune focus of the eyepiece to the screen. The red
light will reflect off the focal reducer and back through the camera just like that over bright illuminated reticle guiding eyepiece that
spoils images. The red light will illuminate the concentric Fresnel rings found on many focus screens that are practically invisible to
the unaided eye. If you have difficulty seeing the Fresnel rings, try a 15 mm eyepiece, though focusing on the focusing screen is
pretty close. The light does not need to be red, but indirect lighting from a window or diffused light doesn't illuminate the Fresnel
rings as well as a monochromatic light, I believe.
The ability to focus at the Astrofocuser at the telescope is useful when changing eyepieces from a 15 mm to a 32 mm framing
eyepiece. Done correctly, you will see concentric Fresnel rings on the correct side of the Beattie screen. You can also use a bright
star slightly defocused for the light source as well. I use a similar technique with the Olympus Varimagni Finder, though the
Varimagni requires a brighter star. As the instructions recommend, move the focal point between inside and outside focus several
times to learn where the best focus is. At 20X or so, this is quite easy as the star will flare a little as one just approaches focus, then
become very small (size varies with seeing) and flare again outside focus.
As compared to the Varimagni Finder at 1.2 X setting (about 6X through the viewfinder lens and pentaprism), the Astrofocuser is
much brighter, in part because the Varimagni has a 3 mm field stop. At the 2.5 X position (12.5X), the Varimagni has a 5 mm field
stop. With the Astrofocuser, there are no field stops until the eyepiece, except the 11 X 16 mm camera viewfinder. With the B&K
Astrofocuser, magnifications of the focus screen of 20X and more are quite possible. This higher magnification can considerably
improve results for those that prefer through the camera focusing.
Framing with the B&K Astrofocuser
One of the most important uses of the Astrofocuser is not focusing at all, but for framing the object at various scope positions
including objects near the zenith. Here, a 32 mm Plossl provides almost the entire field of view through the viewfinder and
pentaprism while increasing eye relief. For framing near the zenith, the diagonal is extremely useful. In this case, any spherical
aberrations become more important if you use the Astrofocuser for framing. The visual result is that the center of the field is in focus
while the outside edges are blurred. Spherical aberration was evident with a 32 mm Plossl in my early Astrofocuser (one of the first
made), which did not affect focus results at the center of the field. Models currently shipping have good correction for spherical
aberration.
While B&K promotes the Astrofocuser as a finding and focusing device and not a framing device, reduced spherical aberrations of
current models allow the Astrofocuser to be used for framing as well. I found I could not resist using the Astrofocuser along with a
32 mm Plossl for framing as the bright view and diagonal made precision framing very easy. I consistently switched and focused my
eyepiece to the focus screen in 30-45 seconds at the scope in 20 degree weather. Still, others may find they prefer avoiding
eyepiece changes. The Astrofocuser was tested with several lenses (Plossls, Ultimas, Naglers, Kellners, Orthoscopics, and
Ultrascopics from 32 mm to 4.8 mm). All worked well and reached focus properly including a no name Kellner.
Comparing the Inside Camera Knife-edge to the B&K Astrofocuser
I checked the Astrofocuser with a knife edge tool of my own design precisely calibrated for my OM-1n. I couldn't improve focus with
my camera back knife edge tool. However, others may not obtain similar results if their camera mirrors and/or focus screens are off
spec. The Astrofocuser generally provided excellent focus results with a 15 mm Ultrascopic eyepiece for me, but some may need
eyepieces from 12.5 to 10 mm to achieve my results as I have superb corrected vision (20-10).
4a. The Doc G Focusing Device for Cameras with a Removable Pentaprism
Just recently, Doc G has designed two focusing devices for cameras with removable pentaprisms. They consists of a small high
quality telescope and transfer lens designed to enable focusing directly on the camera view screen at a moderate to high power of
26X and 48X. A nice feature of his devices is that they are positioned at right angles to the camera thus enabling comfortable high
altitude focusing for SCT's and refractors. They are secured to the camera which prevents jiggling of the mount during focusing and
provides hands free use. The following link has a description of two versions of the adapters for attachment to the Canon camera:
The same devices could be designed to work with other cameras with removable pentaprisms. ragreiner/focusdevice.html
5. VERY HIGH PRECISION FOCUSING METHODS
Return to Beginning
Description of the Knife-edge Focus Method
The knife edge method for focusing film cameras is the method against which most others are compared. It is quite simple in
principle, but not particularly intuitive because unlike other methods, it does not bring an image to focus through an eyepiece or
similar lens. If one places a knife-edge at the desired focal plane, the knife-edge will cut the light from a point source (star) when a
focus solution is reached causing an instantaneous flooding or darkening of light. This is easy to say, however, I have found a great
many amateurs are reluctant to admit they really don't know how to do it or are very are uncomfortable with this method. This is
unfortunate, because once mastered, it is powerful, effective and almost never fails. A bit of tutoring (less than 10 minutes) at the
telescope reinforced with personal practice over a few months is usually enough to master this technique, I think.
Below, left, is shown the open back of a camera where the rails are located and where the knife edge is placed. There are two sets of
rails used to guide the film. It is important to set the knife edge in the plane that defines the film emulsion as described in the text.
To the right is shown a Ronchi based focusing tool. The tube with a length of 55 mm is screwed onto the T-thread adapter in place of
the camera for focusing. Care needs to be taken when using this device as described in the text.
For many, it is a good idea to rough focus through the camera viewfinder or perhaps a parfocal eyepiece. The knife-edge tool is
inserted on the inside camera rails- a surrogate open back camera can be used in place of the imaging camera. A real knife-edge or
Ronchi screen of 50-200 lines per inch on thin Plexiglas or glass is taped to the camera rails. The Ronchi screen works well on an
inexpensive amateur mount that might tend to exhibit flexure as well as high end mounts.
One should move the knife-edge fixture away from the inside rails (towards the camera back) a bit. About 0.005" backset is required
for most films & 0.006" for 2415 film to accommodate the difference between the film thickness (about 0.005" for most films and
about 0.004" for 2415 film) and the 0.010" difference (average) between the inner and outer rail heights. This is because the film
curl pushes the film away from the inside rails into the spring loaded pressure plate residing on the inside of the camera back which
helps keep the film flat without touching the image side. The outside rails are used to set the distance between the pressure plate
and the film, thus allowing the emulsion to move freely. I use a cut out piece of discarded film for the 0.005" backset needed for
standard film and voice coil shims for the 0.006" 2415 film backset. As a precaution for absolute precision, one should obtain
precision measuring tools to verify the difference between the inside and outside rail heights.
The Ronchi screen faces towards the front of the camera, away from the eye. One can disassemble the Celestron MFFT 55 and use
it's reticle inside many cameras such as the Olympus OM-1 on the inside film rails (with the addition of 0.005" added to the inside
rails) and between the outside rails. As another alternate, one can use a commercial knife edge focus tool which assume a 55 mm
film plane to T-thread distance. Unfortunately, I have found this distance to vary. My Nikon F camera is around 56 mm. This is of no
surprise to me as the 55 mm "standard" would not be important to a given camera manufacturer using their lenses mated to their
mounts.
How to Focus with the Knife-edge Method
With a discussion of knife-edge methods above, it is appropriate to discuss how to use the knife-edge method because it is arguably
not as intuitive as other methods. However, I believe it is useful to know how to do it to evaluate the suitability of alternative
focusing methods and verify the correct positioning of a camera mirror and focus screen.
Choose a bright star located well above the horizon to avoid seeing and scintillation problems. As seeing worsens, finding exact focus
worsens as well. This is good, because this is an excellent indicator that imaging should be postponed. Center the star about 5-6 mm
from the center of the 35 mm film to average out any field curvature. If one is within the maximum depth of focus for a particular
focal ratio, the use of this technique can produce what appears as a flatter image with little to no loss of focus in the center. Get
rather close to the knife-edge or Ronchi screen (5-10 mm) and view the image. With the Ronchi screen, one typically moves the
telescope. With a real knife-edge, one often moves the knife-edge, which works well on high end amateur mounts with good stability
but may not work as well with some entry level amateur mounts.
When viewed from behind, moving the telescope (or knife-edge) slowly moves a point source into the knife-edge. As the star moves
across the knife-edge, a darkening or lightening from one side will be noticed. It may help speed things up a bit if one notes the
direction of the darkening. If the darkening moves in same direction as the knife-edge, the focus point is too far inside. If the
darkening moves in the opposite direction as the knife-edge, the focus point is too far outside. However, if one starts bit inside of
focus, one can forget about the direction of the curtain and concentrate on continuing in the same direction until the focus solution is
reached. For an SCT where one wants to finish focusing the moving mirror in a given direction to pre-load the focus knob threads,
this method is unequalled. By the way, for those that are wondering, an SCT is inside of focus when turning the focus knob CCW
sharpens the blurred image.
The darkening (or shadow) appearing from one side of a slightly out of focus star is often referred to as a "curtain". As one gets
quite close to focus, the light or darkness engulfs more and more of the entire image, rather than just on one side. When getting
even closer to focus, the lightening or darkening dims or brightens evenly throughout the image. Finally, perfect focus will show
almost instantaneous lightening or darkening of the image everywhere. The knife edge method of focus (based on the Foucault Test)
is arguably the most precise method of all, especially when used at the film plane. It is my personal favorite because of speed and
ease of use. However, there is one drawback- the camera or film must be removed to use it.
Return to Beginning
5a. The Takahashi FM-60 Focusing Microscope
Gene Horr has been kind enough to loan me his personal Takahashi FM-60 focus tool for testing, review, comparisons, and
comments. This tool is shown below being applied to the film rails of a camera. The Allen wrench is used to tighten the position of
the microscope objective after adjustment.
The Takahashi FM-60 is a film plane focus tool, which utilizes a microscope type objective and 60X magnification to focus a film
camera not unlike focusing a high power eyepiece. This makes it rather intuitive to use. As compared to loupes and other lower
power magnifiers used away from the film plane via internal mirrors and/or prisms, this device clearly produced equal or better
results than all other methods- as one would expect with such high power. While the use of a microscope objective at high power
does limit the field of view, it also improves the image considerably. At around $ 200, it is difficult to duplicate similar performance
for less. One of the best ways to improve image results is with accurate and dependable focus techniques. The FM-60 appears to be
quite adequate for this task.
How to Use the Takahashi FM-60 Focusing Microscope
The FM-60 is designed for use without film or in a surrogate camera. Functionally, it works exactly as my custom ground glass and
microscope focus tool, so I am familiar with its use. This is fortunate because Gene reports the instructions are in Japanese. While I
believe the use of this tool is rather straightforward, some new to these devices will benefit from the overview that follows. (For
availability and further information contact Texas Nautical at http://www.lsstnr.com/)
The objective's distance to the ground glass screen is adjustable by a small set screw on the side of the threaded end piece opposite
the rubber covered eyepiece end. Loosen the set screw and adjust the distance needed to reach focus with the eyepiece draw tube.
This is useful for eyeglass users preferring not use their corrective lenses, though the FM-60 works quite well with prescription
glasses. The eyepiece is contained inside an adjustable slider tube, which is secured with a hand nut and split tapered nylon bushing.
A rubber eyecup prevents damage to eyeglass lenses and a nylon ring around the recessed objective prevents scratching of the
screen and a bearing surface allows moving the front of the microscope rather freely. The glass screen should be secured to the
camera over the inside rails (etched side toward the telescope), but between the outside rails. With typical 35 mm cameras, it may
be prudent to add about 0.005" under the glass screen to move the front of the screen toward the camera back where the film
usually resides. This is described in greater detail above. To improve ease of use, it is a good idea to remove the camera back. To do
this, look for a small, spring loaded sliding bolt on the hinge side of the back. Apply pressure toward the opposite side to release the
hinge pin.
The microscope objective must be pre-focused to front side of the ground glass (frosted side). This can be done earlier by simply
providing illumination to the front of the glass. Gene Horr prefers to dot the glass front with a permanent marker, hold the glass up
to sky light or on a light box, then focus on the dot. I prefer to focus similar devices to the front of the glass screen using a
monochromatic light source shown through an affixed port. This readily illuminates the irregular front surface allowing one to focus
the microscope to the front surface of the glass at the film plane quite precisely.
You will likely find pre-focusing the scope or camera lens helps. Next, point the scope to a rich star field. Place the microscope
directly on the ground glass, moving it about looking for a rather dim star for best results. Focus the scope as you would a high
power eyepiece. If the seeing is good, the point of optimal focus will be between the points at which the star just starts to flare.
However, at 60X, optimal focus is easier to see than at 6 to 10X.
Using the Takahashi FM-60 Focusing Microscope on SCT's, Newts and Refractors
I used the focus tool on an SCT, Refractor and Newtonian held in rotating rings by a GE mount. I asked my colleague to use the FM60 focus tool as well. We used it at several focal ratios from f/10 to f/4. We were able to get quite close or dead-on as compared to
my custom knife-edge inside camera tool.
The scopes that were more difficult to focus with the FM-60 were the SCT and the refractor. This was in part because the high 60X
magnification and small field of view (typical of a microscope objective) required a bit of a steady hand to keep the selected star in
the objective field. We needed to hold the tool, find the star on the glass screen at high magnification (yes, a dim star is best), focus
while holding the tool and not shake the mount too badly while our head was twisted to the best viewing and focusing position. The
SCT proved the most difficult as we opted to use the moving mirror focus knob which is best focused in one direction (CCW for most
except the 12" & 16" Meade SCT's). Focusing near the zenith would be all but impossible for the SCT. With the refractor, we had to
lie on the ground for zenith focusing. The Newtonian was the easiest to focus. We positioned the tube to allow a comfortable head
position. We were able to achieve consistent focus every time.
I believe the less than perfect focus on the SCT and refractor were primarily due to the difficulty in holding the tool while turning the
focus knob and viewing simultaneously. For example, when focusing much closer to the horizon, focusing confidence and subsequent
results improved. However, there are many situations and reasons why it is prudent to focus at much higher altitudes. In addition,
the desire to finish focus the SCT in one direction left us with the uncertainty whether moving the knob a bit further would improve
focus. Of course an auxiliary focuser such as the JMI NGF-S, Feather Touch, or several Van Slyke models would alleviate this
problem.
Comparing the Takahashi FM-60 Focusing Microscope to the Inside Camera Knife-edge
We compared the FM-60 to my custom knife edge tool, which by its nature is less intuitive, but doesn't require holding the tool while
focusing and allows a more liberal head position. The knife-edge method allows one to reach a focus solution from one direction
every time. There is no need to move between inside and outside of focus to assure the best possible focus is attained because the
end-point is very clear. Both of us were able to achieve focus in less than one minute on all three scopes with a high degree of
confidence. We were able to approach similar speeds using the FM-60, but not consistently. However, we have many years of prior
knife-edge focusing experience.
Final Thoughts about the Takahashi FM-60 Focusing Microscope
One would like to start imaging when the air is steady and temperature nearly stable. However, in summer, it is quite possible the
rather short period of astronomical darkness will require at least one focus check to allow a bit earlier imaging. This will require
removal of the film to use the FM-60, a surrogate body, or a machined adapter.
I believe those that prefer to focus their camera as they would focus an eyepiece will not be disappointed with the Takahashi FM-60
microscope focus tool. Currently, I believe there is no better commercial focus tool using standard focus methods that will produce
better results for good reasons- it is placed at the film plane and uses high magnification. However, those that are comfortable and
experienced with knife-edge methods will not likely see any improvement in ease of use, speed, or results when using the FM-60
focus tool.
I believe most can obtain improved consistency and results providing they learn to accommodate the manual dexterity and head
position often needed to use this tool with SCT's and refractors. If there was an accessory holder to hold this tool in place, I believe
ease of use with SCT's and refractors would improve.
-Michael Hart
Husen Observatory
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THERMAL EFFECTS IN GUIDING AND FOCUSING
COMPOUND MOVING MIRROR TELESCOPES
BACKGROUND
The Schmidt Cassegrain is Telescope (SCT) is a very popular compound moving mirror telescope in the United States. And why
not? It promises compact design, portability, large aperture, and a plethora of accessories from multiple sources - all at an
affordable price. With the introduction of electronic periodic error correction, those with inexpensive worm and wormholes were
able to get results approaching much larger and more expensive precision drives. Permanent periodic error programming further
increased performance of low-cost mounts. Finally, integrated goto electronics allows even those with minimal knowledge of the
skies to find and center objects in the eyepiece or imaging camera. The larger Schmidt Cassegrain can also provide the image
scale needed to do a nice job of recording the more plentiful smaller objects.
Indeed, the Schmidt Cassegrain is capable of taking some rather nice images. Just visit the web sites of amateur
astrophotographers and one can find very fine images taken with these telescopes. My better SCT images were always taken
under steady skies, stable temperatures and little to no wind. I learned long ago to check focus often. Optimal imaging times
frequently require waiting until the second half of the night and sometimes a couple of hours before dawn. It is quite possible that
during the course of an exposure, the skies will become less steady.
The following image illustrates the effects of seeing deterioration during an exposure. The effect, while subtle, is seen especially in
stars which are slightly bloated and have a red ring around them. The red image was taken last and due to increasing scintillation
the red image is slightly smeared out. This causes the red rings around the stars. Under normal, steady skies, this effect is totally
absent. A "seeing" deficiency is not simply a slight atmospheric bloating of the star images, but rather, is caused by a random
moving about of the normal image at perhaps 30 Hz or so. This scintillation appears as a larger image when integrated over a
second or two.
One can find images taken by amateurs that are not as nice as they would like. One might conclude the better images are a result
of good fortune, good equipment, experience, and imaging talent. One path to good results is to understand the limitations in
oneself, equipment, available seeing, and sky darkness. Let's explore at least one of the aspects we can control now, the telescope
- more specifically, the SCT. For if we fully understand the equipment we are using, we will know how to get the most from it.
CHOOSING A GUIDING METHOD FOR A SCHMIDT CASSEGRAIN TELESCOPE
Guiding is useful in correcting tracking errors resulting from alignment errors and periodic errors. Both new and experienced
astrophotographers will likely find the separate guidescope quite intuitive and much simpler to use than the alternative - the offaxis guider. Separate guidescopes are very nice because one can find a nice bright star in typical suburban sites and start guiding
rather quickly. If an autoguider is employed, fewer and simpler autoguider calibrations are required because the autoguider
orientation remains rather constant. For those getting started in SCT imaging and experienced imagers needing short exposures
such as those used for stacked CCD images, I would recommend a separate guidescope.
Personally, I have not had very dependable results using a guidescope on an SCT where exposures were over 20 minutes. After
that, guiding errors start to appear. When I did get better results, it was during rather specific conditions that are described in the
recommendations section below. The use of the affixes guider is not unusual for larger telescopes over 10 to12 inches in aperture.
There is a good reason for this, any differential tracking between the two instruments result in tracking errors in the image known
to be guided properly. Differential flexure and mirror shift is frequently blamed for poor SCT results. I have long suspected that
there are other factors at work that may be equally and arguably more of a factor than differential flexure and mirror shift.
OPTICAL LEVERAGING IN THE SCHMIDT CASSEGRAIN TELESCOPE
The Schmidt Cassegrain telescope is a really quite versatile. We can attach several accessories to it and reach focus without
remounting the mirrors. The SCT is exhibiting optical leveraging. A small movement of the moving primary mirror results in a very
large movement in the focal plane. The optical leveraging can approach 25 to 1. Small temperature changes that cause expansion
and contraction of various optical components can have rather large effects because they are amplified by the inherent optical
leveraging. To explore effects of temperature changes one might likely experience while imaging, a controlled experiment was
completed using a popular 8" SCT and 90 mm Maksutov employed as a guidescope.
EQUIPMENT TESTED
Here is shown the 8" LX200 with the ETX firmly mounted on a Losmandy rail and ring set which was used in the temperature tests
reported in this article. Note that for the measurement described in this article, the OTA and guider were mounted on a permanent
pier without the LX drive and fork.
I completed testing on an SCT and Maksutov combination that was fixed to a multi-ton isolated structure in a large
environmentally controlled building whose air volume was great enough that ambient temperatures were maintained to about a
quarter of a degree F. over several hours.
The intent was to isolate mirror shift, flexure, seeing, drive errors, and other anomalies to determine which had the greatest
influence effecting imaging results. I had suspected (based on earlier tests of jamming a primary mirror to the baffle tube) that
mirror shift might not be the only cause or even perhaps the primary cause of differential tracking during longer, one hour
exposures.
INITIAL EVALUATION OF THE GUIDESCOPE, PLATES, RINGS, AND EQUIPMENT
The main scope was an 8" LX200 SCT. The guidescope was 90 mm ETX Maksutov. Losmandy plates and rings were used to secure
the ETX to the LX200. The ETX had a rather minimal image shift of about 34 arc-seconds. This was quite good and a bit better
than several other ETX's I have examined. In addition, a small refractor was also tested in place of the ETX.
The guidescope plates and rings were Losmandy's finest plates and rings. The plates are the solid (non milled-out) style; the
strongest and most rigid version of this style sold by Losmandy. The rings were the correct size for the ETX and mounted properly
just inside the front corrector. Note: The ETX front corrector cell should not be used for the rings because the corrector is not
floating in its cell. As a result, placing the rings over the corrector will cause pinching of the optics in the ETX. This LX200 had a
rather minimal image shift of 41 to 54 arc-seconds.
SETTING UP AMBIENT SIMULATION TESTS
The tested combination were moved to a large environmentally controlled building free of air currents (near perfect seeing) such
that the scope focus could be set closer to infinity. The LX200 was secured directly to a multi-ton isolated concrete pier and
allowed to reach thermal equilibrium over a 24 hour period. The LX200 fork or mount was not employed as this might effect
results.
The goal was to eliminate as many variables as possible to determine if this particular setup has potential to take 1 hour guided
exposures. This was a static test- mirror shift, flexure, seeing, drive errors, temperature, humidity and other variables were
virtually removed. Temperatures were controlled to within 0.15 degrees C. (as measured and recorded locally). A target was
placed at the opposite end of the building. Both scopes were adjusted to the same point on the target. A heavily insulated suit, hat
and special gloves were worn to prevent unwanted local temperature rises of 0.2 degrees C. noted within a few minutes of
approaching the equipment.
RESULTS OF AMBIENT SIMULATION TESTS
As little as a 0.15 degree temperature change resulted in a 2 second optical movement in the LX200 and a 1.5 second optical
movement in the ETX. A 0.6 degree C. temperature change resulted in 8 arc-seconds of optical movement in the LX 200 however,
the ETX did not track the LX200 optical movements linearly. Sometimes the two tracked more linearly than other times. I believe
the correlated motion for some periods was coincidental. After about a 2.5 degrees C. change over the course of 4 hours, the
amount of misalignment between the optics approached 24 arc-seconds. Clearly, any temperature change greater than 0.6
degrees C. during the course of a single exposure would result in oblong stars. This amount of temperature change is not unusual
at all in many locations during the earlier part of the night before the temperature has stabilized.
I also replaced the ETX with a small refractor to ascertain if eliminating one compound telescope would improve results. It did, but
it was not much better and certainly not enough to assure dependable results. This is important because even a 75% guiding
success rate may temp one to take two shots for every object, because we don't know which shot in four is the going to be the
bad shot. It is quite likely oblong stars (trailing) will be noticed with this setup if the ambient varies more than 0.6 degrees C.
during the exposure.
I also measured a focal plane change of as much as 0.006" with at 0.6 degrees C. temperature change. Warming moved the focal
plane outside of focus and cooling moved the focal plane inside of focus. At the native focal ratio of f/10, this amount of focal
plane movement is outside the acceptable depth of focus tolerance. At typical focal ratios used with these telescopes, of f/6.3, the
SCT focal plane movement is twice the maximum desired depth of focus tolerance required to maintain the sharpest possible
image. If we consider that it is likely we centered the focal plane during the focus process, it is likely we have exceeded the depth
of focus tolerance by close to a factor of 4. In this situation, the image has drifted out of focus a bit and it shows in the result.
That is too bad because now the image taken under good 2 to 3 arc-second seeing produces a 3 to 4 arc-second result. Thus, it is
prudent to check focus frequently in compound telescopes when the temperature is not quite stable.
Editor's note (Doc G): A focal plane change of 0.006 is 0.15 mm. To realize how much this is, note that the depth of focus of an f
10 telescope, for a 10 micron circle of confusion is only 0.100 mm. Thus the focal shift due to temperature change is significantly
greater that the depth of field. For an f 6.3 telescope the depth of field is only 0.063 mm which makes the situation even worse.
A test was made using 3/8" black foam insulation wrapped around the optical tube to slow cooling or heating. The mirror casting
end was kept open to allow the primary to reach and remain at near thermal equilibrium. The obvious problem with this approach
is the optical tube insulation will also slow down the desirable properties of reaching thermal equilibrium more quickly. An external
fan and filter could be used as needed to overcome this problem. As a result of the tube insulation, the amount of LX200 optical
movement during a 0.6 degree C. change over 1 hour was reduced from 8 arc seconds to 1.5 arc seconds and focal a plane
change from 0.006" to 0.0013". These results were quite encouraging.
RECOMMENDATIONS:
Decide if your imaging site will produce a 0.6 degree C. or less temperature change over an hour. This may require the start of
your image in the second half of the night when temperatures are more stable. If one can live with this restraint, and this is a big
consideration, a separate guidescope might be employed. It may be possible to employ external heating to minimize optical tube
changes. The idea would be to use some sort of precision thermostat or perhaps trial-and-error to keep the optical tube from
cooling during the exposure. The amount of heat added would only be enough to hold the Optical tube slightly above ambient at
the end of the exposure. Corrector dew heaters may be a start. Another option is to add insulating foam over the optical tube to
slow cooling or heating of the optical tube.
Since the focal plane also moves with temperature changes, it may be wise to add 0.003" (at f/6.3) to your focus tool (moving the
focal plane backward) to compensate for falling ambient temperatures (unless external heat is employed). In this way, when we
start the exposure, it is at the fringe of acceptable depth of focus.
CONCLUSION:
I believe I have may have identified and quantified the single biggest factor that spoils SCT images guided with a guidescope or
likely any compound moving mirror telescope for that matter. This is likely why so few have obtained consistent results with the
guidescope on these telescopes. We have also found temperature significantly effects the focal plane location during an exposure.
Those that are looking to get optimal performance out of their SCT will want to watch for relatively small temperature changes
that could spoil their imaging results.
-Michael Hart
Husen Observatory
MAPUG is hosted by
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EFFECTIVE USE OF PEC PROGRAMMING AS RELATED
TO POLAR ALIGNMENT AND AUTOGUIDING
BACKGROUND
Programming an LX200 Smart Drive (PEC) or another drive with permanent periodic error correction (PPEC) should be done on a
night of good seeing, no wind, at high power, and on an excellently polar aligned scope. This is because improper polar alignment
introduces RA drift that effects the PEC programming used in other parts of the sky. One possible variation might be to program
the PEC near the object of interest to compensate a bit for polar alignment errors that could include Dec drive training. I have
used 800-1200X on a 12" LX200 on a star quite near meridian close to the celestial equator. I make 2 to 3 small corrections per
second to avoid overcorrecting at each 2.4 second recording cycle. This is done in RA only. No Dec corrections are ever made.
The measured results are consistently around 4 to 5 arc seconds periodic error peak to peak over the 8 minute worm cycle. Once
done, the PEC corrects for small drive frequency variations and virtually stops RA drift for at least 2 minutes. With a well
programmed PEC and accurately polar aligned scope, one does not need to make frequent guiding corrections at all. In fact, a
human is often better at guiding than an autoguider during poor seeing, because the human can easily determine seeing errors
from alignment/drive errors. Attempting to autoguide out seeing anomalies is a daunting task, especially with a computer
controlled autoguider approaching a limited maximum update rate of 1.45 seconds or greater. A discussion about the operation of
the PEC in the LX200 drive can be found at:
R. A. Greiner -- PEC Correction
DRIFT ALIGN BEFORE PEC PROGRAMMING
If your scope drifts consistently in RA, you may need to tweak polar alignment before PEC programming. Three things happen
when we are NOT polar aligned: The stars appear to drift in RA. The stars appear to drift in declination. The field rotates. Using the
drift method to polar align is recommended highly for high power visual work and precision imaging. As we approach 3000+ mm
focal lengths, alignment that was satisfactory for a camera lens or small refractor is wholly inadequate.
The term "declination drift" in itself implies a polar aligned scope will drift in declination only. In fact, a scope that is not polar
aligned drifts in RA as well, however, we ignore any RA drift during the process and watch "declination drift". As we approach polar
alignment, RA drift slows as well. The reason for RA drift is easy to visualize. If our mount is not quite aligned with the celestial
poles, the amount of misalignment formed causes the RA to appear to drift. The greater the misalignment, the faster the RA drift.
Note: Start by erasing the PEC to remove any programmed RA drift. The idea is to line up our scopes to turn parallel with
the earth's rotation. Those experiencing unusual RA drift should assure polar alignment is quite accurate.
LEVELING THE POLAR MOUNT- IS THIS REALLY NECESSARY?
There is a good reason to level the mount along the altitude and azimuth axis. When drift aligning, a mount out of level will cause
adjustments in one axis to effect the other, increasing the number of iterations that are required to arrive at a polar alignment
solution. For example, a correction that might require an azimuth move will also move a bit in altitude when the mount is not level
in azimuth, the amount depending on the degree of error from level. Once quite level, we are then ready to start the declination
drift alignment process. The method outlined in the Meade manual is based on using a diagonal. Straight through users and south
celestial pole users should reverse the correction directions. It may be a good idea to label the direction moved on the mount
azimuth and altitude knobs to prevent mistakes that would unnecessarily prolong the process.
A SIMPLIFIED METHOD TO DECLINATION DRIFT ALIGNMENT OF A POLAR MOUNT
Now that we know which star to select and have done so, we're not quite ready to drift align. First, we must align our reticule
eyepiece with the RA axis. We can consider this the E-W axis. Everything above the E-W (RA) axis is NORTH, everything below is
SOUTH. If you are uncertain, merely moving the scope with the E-W keys will identify the E-W (RA) axis in the eyepiece. It is
vitally important that we understand that the use of the term north or south as described in drift alignment procedures is not
related to your position at the telescope, rather, the direction the star drifts with respect to the RA axis.
Now, we select a star within 5 degrees or so of the celestial equator and within 30 minutes of the meridian. This provides
maximum declination drift which readily speeds the alignment process. A moderately bright star often provides better results than
a very bright star. Using a 2-3X barlow with extension or star diagonal will produce powers that are quite high, amplifying small
drift movements. If the seeing is so poor that your moderately bright star is too dim and/or moves about, postpone any
subsequent PEC programming for a day of better seeing.
If the star drifts north, use the azimuth control to move the scope east. Keep adjusting and re-centering the star until the
movement virtually stops over a 3-5 minute period. Now, locate a star at about 6 hours RA not much less than 15 degrees of the
horizon (this avoids refractive errors) If the star drifts north, use the altitude control to move the scope down. Keep adjusting recentering until the star does not drift 7-10 minutes. Now return to the paragraph immediately above and repeat, but strive to
improve star drifting from 3-5 minutes to 7-10 minutes.
RETURNING THE TELESCOPE TO A POLAR ALIGNED WEDGE
This leads to the subject of reproducing polar alignment when removing the scope from an adjustable wedge or other device that
maintains the scope parallel to the earth's rotation (aligned to either celestial pole). Once the wedge is aligned, we can move the
scope base up, down, right or left as long as the scope base is not TILTED by a non-flat surface. I have used indexing pins to
position the scope base at the exact point it was aligned. However, are they really needed if you have a flat mounting surface (no
bumps or warpage)? An even better question is, how non-flat (irregular) must the surface be to cause a significant polar alignment
error?
If moving the scope base laterally across the wedge surface (parallel to the earth's rotation) introduces a 12 arc minute error west
(due to a 0.020" bump or warp in the surface), a 60 minute exposure could drift around 3 arc minutes in declination. A 0.020"
deviation in a short span of less than 0.25" (maximum anticipated variance) is clearly visible on the wedge surface and quite easy
to see in the eyepiece during drift alignment. However, the maximum amount of field rotation seen in a typical 60 minute
exposure near the guiding point would be fairly small - around 30 arc seconds. If the amount of field rotation produced is so small,
should most bother with indexing pins? Probably not. Why then, bother at all?
Three reasons come to mind - First, manual guided exposures are much easier with an alignment precision that requires almost no
Dec corrections or RA corrections in two minutes or longer. One can relax a bit. Thus, more precise polar alignment is desirable.
Second, two minute unguided CCD images will move less than 1 pixel, enabling electronic stacking with minimal loss of field of
view and object centering. Third, guiding errors while using an autoguider are narrowed to flexure, vibrations, wind, drive train,
and seeing. We could use precision milling of the wedge surface to 0.002", but simple eyeballing the bolt hole centers or simple
indexing pins as a means to assure returning to precise polar alignment is quite adequate.
RESULTS OF PRECISE POLAR ALIGNMENT AND PEC PROGRAMMING
To see what a well aligned mass produced mount can do, I took two 15 minute shots under 3 arc second seeing, one using 3
second guiding corrections and the other unguided. These images demonstrate the power of excellent polar alignment and PEC
working in concert.
THE AUTOGUIDER ADVANTAGE FOR IMAGING
Since the introduction of affordable and reliable autoguiders, autoguider use has increased considerably. And why not? Guiding can
be a tedious and a monotonous task. With an autoguider, one can set an alarm and wake up hours later refreshed when the
exposure is complete. Still, it is not unusual (though perhaps less so in recent years) to see an amateur using rather inexpensive
equipment, minimal accessories and no autoguider. That amateur may walk away for a minute or so, return, converse and
casually make a small correction. The above illustration demonstrates that constant guiding is not necessary on a well aligned
mount when the PEC is accurately programmed. Contrast this to the amateur with many accessories, a much better drive, more
expensive scope, and an autoguider. Of course, the former amateur will get worse results as compared to the later amateur.
Perhaps not - here's why:
Conventional wisdom used for film imaging with high quality mounts under excellent seeing is not simply transferred to
inexpensive amateur mounts used under typical seeing by many amateurs. I have used inexpensive mounts simultaneously along
side high end amateur mounts and produced similar results, but not using the same techniques. Clearly, it is more of a challenge
to obtain similar results with an inexpensive mount. However, if we modify our techniques a bit to compensate for known
anomalies, a good result is possible on inexpensive mounts with a reasonable level of effort. This is important for those that have
limited resources to invest in astrophotography.
WHY USE PEC WITH AN AUTOGUIDER?
PEC is, of course, needed less with high quality worm and worm wheels at shorter focal lengths. It is easy to dismiss mass
produced mounts as unusable if we are fortunate enough to own or have access to a better mount. However, as we start moving
up in focal length to over 3000 mm coupled with long exposures, PEC again starts to become useful, even for the high end
amateur mount. Fortunately, PEC is available on many of these mounts. If used properly, PEC and an autoguider can have a
synergistic effect. This is especially noticeable with long exposure tricolor CCD imaging at 3000+ mm using a CCD chip where the
CCD chip records small errors much more efficiently than film. Many PECs are capable of averaging corrections over periods of
0.50 Hz (2 seconds) or less (LX200) while a typical autoguider is operating at 3 to 5 second correction rates or greater. If we
increase the autoguider correction rate too much (faster), we may start chasing the seeing resulting in oscillation of the mount,
especially under average to marginal seeing.
THE ROLE OF AUTOGUIDER CORRECTION RATES IN PEC PROGRAMMING
I tested a number of popular amateur autoguiders employed in PEC programming with mixed results. At first these devices would
seem ideal because over long exposures they produce results few (if any) can duplicate with the manual guiding. However, for a
short duration of a few minutes it is possible for the human to exceed autoguider results for PEC programming. In addition, few
autoguiders have sufficiently high correction rate needed. When we speak off correction rate, we should not confuse the exposure
time setting of the autoguider with the correction rate that is the sum of exposure time, integration time, any software/hardware
overhead and drive hysteresis.
Thus, the correction rate is always longer than the autoguider exposure time. This is an important consideration is we try to use
an autoguider to program the PEC. The shortest possible exposure setting might not be adequate to assure correction rates are
less than the desirable sampling rate of 1/2 the record period or about 1Hz (1 second) for the LX200. Then, of course, the
autoguider is fully capable of programming non-periodic seeing related corrections (undesirable) easily ignored by the human
when the seeing isn't superb. Considering the aforementioned limitations in commercial autoguiders, I believe the most
dependable results are obtained by manual PEC programming. Here, if we study the periodic error for a few cycles of the worm,
we can memorize significant deviations, thus anticipating them in near real time. Thus, it is my belief, the best PEC programming
is done manually.
CORRECTION RATE LIMITATIONS IN MOUNTS
We should not assume that because our mount appears to oscillate in response to guiding corrections as a result of seeing, worm
errors, vibrations, alignment errors and/or PEC programming errors that a given mount is not capable of fast guiding rates. For
example, a slightly modified LX200 is capable of responding to simultaneous X-Y guiding corrections at 2 Hz (0.5 seconds). For the
RA axis, this consists of typical slowing and speeding of the RA drive. In the Dec axis, full worm reversal slows drift or changes the
direction of star movement. Of course, obtaining smooth correction rates at 2 Hz requires a thorough understanding of the
relationships of various adjustments in the drive and mount. For most, such fast correction rates are not needed, however, the
reduction in hysteresis required to achieve these rates is desirable.
See the article concerning adjusting a declination drive adjustments without a major rebuild at: Declination Drive Adjustment.
Those who have retrograde motion or suspect they may have retrograde motion can find details to control this anomaly in the
Mapug-Astronomy Topical Archive.
THE PEC ADVANTAGE
For an LX200, if the PEC was calibrated and updated under excellent seeing and polar alignment, not only are the worm errors
smoothed, but residual RA drift is virtually stopped as clock crystal drift and minute alignment errors are corrected. Then,
subsequent updates are used to add or subtract pulses (0.3333 arc second movements) to memory segments which tweak the
drive further allowing the drive correction to track the worm quite close to real time - much closer than is practical for an
autoguider, especially under marginal seeing.
It is important that permanent PEC programming is preceded with good polar alignment and initial PEC erasure. Why good polar
alignment BEFORE PEC programming? Because poor polar alignment produces changes in the RA drift as well. With PEC
programming, in addition to clock frequency errors and worm errors (which are repeatable), we introduce another error (RA drift)
which is corrected by the PEC as well. The next day, we set up and roughly align again and find our RA drive doesn't track as well.
This is likely because we programmed in the RA drift produced by the previous night's poor polar alignment. The previous night's
rough polar alignment is usually more difficult to replicate than accurate polar alignment.
Once the PEC is carefully programmed, we can use the autoguider independently of PEC at whatever correction rate that produces
the best results - perhaps several seconds to even minutes (if desired) for a well aligned mount. Now, autoguiding is truly optional
because manual guiding corrections may result in one correction every few minutes or longer as the PEC has corrected residual RA
drift and the excellent polar alignment checks the Dec drift. Those on a budget can save considerable expense without sacrificing
results using their PEC wisely on a well aligned mount while manually guiding.
CONCLUSION
I believe the value of accurate polar alignment is often underestimated. For casual observing, casual polar alignment is quite
adequate. However, for improved PEC programming and good photography results at focal lengths of 3000+ mm, a well-aligned
mount is essential to excellent results. This is not to say a bit of alignment or tweaking will cause an inexpensive mount to outperform a high end mount, however, clever implementation of PEC such as by the LX200 and others can minimize the differences
in results. Moreover, high end mount users can use well implemented PEC to improve results at longer focal lengths and any
manual guiding endeavors.
_
Michael Hart
Husen Observatory
MAPUG is hosted by
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A Brief, Simple and Approximate Explanation
of Photographic Film and Reciprocity Failure
Photographic film is still probably a commonly used medium for astrophotography. There are many types of black and white film
and color film as well. This discussion is devoted almost entirely to black and white (gray scale) film. Color film behavior is a sort
of sub-set of B&W.
For a detailed discussion of various practical hypering techniques see:
A manual of Advanced Celestial Photography by Wallis and Provin. There is a 22 page chapter on the topic written in their usual
clear and concise style. This is still the best book ever written on astrophotography in my opinion.
Other references of value are:
Astrophotography for the Amateurby Michael Covington and
The Guide to Amateur Astronomy by Jack Newton and PhilipTeece
And a nice reference manual with much information is: class:
Astrophotography II by Patrick Martinez
Black and White (gray scale film)
The great variety of B&W films available narrows down considerably when doing photography of very dim objects. For ordinary
photography, there is generally enough light to form an image with an intensity that enables gathering an image with relatively
short exposures. Normal exposure length is usually in the range where the camera can be hand held. That is from 1/25 of a
second to 1/1000 of a second. Thus films are designed to give predictable results for this range of exposure times. For exposures
that are much shorter, such as with high speed electronic flash, or much longer there is what is called reciprocity failure.
Reciprocity failure is often badly understood as will be explained in the following.
Let us first get an understanding of how a photographic film captures a latent image which can be later developed chemically. The
sensitive elements in the film are crystals of, most often, silver halide which can change their structure when excited by light
(photons). The relationships between these crystals, the details of the use of so called sensitizers and the sensitivity of the crystals
to photons of different energies is very complex. We need only a basic understanding however to understand certain basic
properties (behaviors) of films.
In general less sensitive films (slower films) have finer grains that are closely packed and more sensitive films (faster films) have
courser grains. A film may have a distribution of grain sizes to obtain certain desirable properties. The reason for the sensitivity
relationship to grain size is related directly to how the grains are converted from a stable non-developable state to another stable
state (latent state) from which they can be developed chemically. This happens in something like the following way. When a
photon of light strikes a grain it dissipates its energy in the crystal (grain). This energy may or may not be enough to flip the
crystal into a latent state. Generally it takes a few photons to flip the grain (depending on its size and sensitivity). In the
meantime, thermal energy is jiggling the grain and tending to drop it back into its normal state. If enough photons strike the grain
in a given time, the grain flips to a latent state and sticks there. We then have a grain that can be turned opaque chemically. Thus
the photons build up a latent image that is later developed. The darkness of the image is more or less proportional to the light
striking the film.
It takes about the same number of photons to flip a large grain as a small one. Since the larger grain intercepts more light more of
the larger grains will be flipped and thus less light is required to create a latent image. This later phenomenon makes course
grained films faster (more sensitive). Now let us understand that there a great variety of other factors that control the sensitivity
and graininess of emulsions the case is not as simplistic as stated here. Notice however that there are two controlling phenomena
going on. One is the flux of photons in intensity and time that flips the grains and the other the thermal agitation that serves to
reduce the grain tension that is built up by the photons. The latter effect is continually wiping out the effect of the photons to
some degree.
For example, consider a single grain. Say it takes 3 photons to flip the grain to its latent state. Assume one and then another
photon hits the grain but that the light flux is very small. Before the next photon can hit the grain and flip it, one of the previous
photon actions is undone by thermal energy. The third photo then in not enough to do the job and the grain does not flip. The
concept is that there is a "battle" between the photons accumulating fast enough to flip the grain into its latent state and thermal
energy neutralizing the photon action. If the light intensity is small enough no image at all might form.
If the light intensity is strong enough so that the exposure to it requires a short time, say under a few seconds, the thermal effect
is minimal and the exposure forms and sets the latent image. Once set, the latent image is very stable and will last for years even
at room temperature.
It is now easy to understand "reciprocity failure." Reciprocity is the relationship between the "apparent" sensitivity of the film and
the reciprocal relationship between light intensity and exposure time. What happens is this. When the light is strong enough for a
normal exposure time as described above the, a few seconds or less, the grains are flipped and form an image without significant
intervention of the thermal relaxation phenomenon. But when the light is very weak as is the case for many astronomical sources,
the image is build up very slowly often in many minutes or even hours. Thus all the while the image is forming, the thermal action
within the emulsion is destroying some of the results of the photon flux. The film appears to be less and less sensitive when weak
photon fluxes are present. (i.e. long exposures are required to build up an image)
Note that the problem is not directly the long exposures required but it is the weak photon flux. We can now see how the cold
camera works. The desensitizing effect of thermal agitation is reduced by cooling the film. Liquid nitrogen temperatures or at least
dry ice temperatures are required to reduce the thermal agitation significantly. But film cameras with such cooling are made
mainly for professional use. They are not easy to use because of the problems of insulating them from dew and frost.
We can, by considering one additional phenomenon, understand how "hypering" works. As it turns out in a typical emulsion, it is
free molecular elements and especially water vapor which allows the thermal agitation in the emulsion to couple to the sensitive
grains in it. Thus the coupling of the thermal energy "called phonons" to the grains can be reduced greatly by removing from the
emulsion all possible water vapor and if possible molecules like oxygen and nitrogen. Nominally this is done by placing the film in a
chamber, warming the film and evacuating the chamber. This dries out the film and allows oxygen and nitrogen to migrate out of
the film. But the film will quickly take up the undesired contaminants when taken out of the chamber. A good solution would be to
saturate the emulsion with hydrogen gas. Then it would take a long time for the unwanted agents to penetrate the film and it
would stay "hypered" for a long time. (days to weeks) What is normally done is to use forming gas instead of pure hydrogen
because pure hydrogen is so very dangerous to handle. Forming gas is mainly nitrogen with some hydrogen content, about 8%
Hydrogen, so it can not explode. The forming gas is totally dry so that the worst offender, water vapor, is eliminated. Do not try
pure hydrogen unless you have your insurance paid up.
Hypering film in this manner is commonly done by many amateurs and professionals. The "hypered" film then retains its
sensitivity even though illuminated with very weak light since much of the thermal desensitizing effect is suppressed. The film will
become 3 to 5 time more effective with very low light levels found in astrophotography. Instead of limiting exposures of one hour,
three to five hours will remain effective.
There is still the question of why one does not start with a very fast film like TriX or equivalent instead of the Tech Pan that is so
popular. The reasons are that the fast films because of their grain size are not as effectively "hypered" and have significantly
poorer reciprocity failure in the first place. They are also of low contrast and grainy. A film like Tech Pan, which has a normal ASA
speed of only 25, has inherently good reciprocity characteristics and takes very well to hypering. It has in addition very fine grain,
high contrast and good sensitivity to longer wavelengths, which are important for astrophotography.
The above discussion is a very simplified explanation of the processes but it does describe the results you find with cooling and
hypering film.
Color films show the very same phenomena as B&W films. Color films can also be "hypered" with good results. Because of the
complex structure of color films, being three layers of emulsion with build in filters, they may show significant color shifts because
the three emulsions hyper differently.
From the above discussion, it can be seen that film sensitivity can be improved by cooling and by drying (hypering) it in some
way. The hypering process is not dangerous when done with forming gas but it is complex and tedious, take up to several days
and must be done shortly before exposure to be most effective. Pre-hypered film is available but should I feel be used within days
of hypering. I have not seen data on the long term (many days) stability of hypering. I would assume the film should be sealed in
forming gas until time of use and when exposed in the camera, to air, be used completely and promptly. (in hours)
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Download Excel file
Interesting Objects Listed by Size
Updated March 25, 2005
Here is a list of over 300 interesting extended objects. They are listed by size in order to emphasize the idea that when imaging
extended objects is necessary to try to get a significant sized image on the film or chip so as to make good use of the resolving
power of the film or imager chip. The only way to essentially "fill the chip" with the image is to use the proper focal length
telescope. The proper focal length can be obtained by choosing the correct telescope focal length. The size of the real field in the
sky is generally measured in arc seconds or minutes. This value is dependent only on the size of the imaging chip and the focal
length of the telescope. Thus if the imager wants to encompass a certain range of object sized effectively, it is necessary to choose
a telescope of a given focal length to match the chip. It can be a problem to get focal lengths that are short enough to image
larger objects and also to get enough image size to image planets and some very tiny objects. Often a telescope of excessive ly
long focal length needs to be augmented with a focal reducer to allow getting larger objects on the ship. This can be done with
focal reducers. Unfortunately focal reducers not only reduce the effective focal length of the telescope, but they reduce the circle
of illumination of the telescope. The reduced circle of illumination causes vignetting and often serious degradation of the image
quality near the outer edges of the image. The full list is divided into groups with comments about a suitable focal length to go
with various telescope focal lengths and cameras. I generally like to suggest “filling the chip” so as to make good use of the pixel
array and get the best possible resolution. (At the very end of the list is a short discussion of what I have used as a criterion for
"filling the chip.") Planets are a special case, being significantly less than 0.5 minutes of arc and are discussed separately at the
end of this list.
The first set of objects is very small, up to about 1 by 1.5 arc minutes. With a 3000 mm telescope (12" f10) this yields a largest
image size at the chip of only 0.9 mm. That is, about 90 pixels with typical 10 micron pixels, but a few more with most digital
cameras. This is not a very high resolution situation but probably all that can be expected. The atmospheric resolution, under
good seeing conditions, is only about 1 arc second which is about one pixel with this focal length. There are 28 objects in this
category including 4 M objects. While it is difficult to get great detail in these objects with a telescope of 3000 mm focal length, it
would probably be wise to use a 2X focal extender (Barlow lens) to get a bigger image on the chip. There is another imaging
factor that must be considered. When using a telescope of 3000 mm with a 2X extender the effective focal length of the telescope
is 6000 mm. It requires an exceptional mount to stabilize such a long focal length for imaging. These objects are tiny and quite
faint and thus very difficult to image. It seems amazing that Messier was able to see the four he designated on this list. Note that
all of the planets are in this range or much smaller than the smallest of these objects. Jupiter, the largest is about 40 arc seconds
(0.7 arc minutes), Saturn about 20 to 30, Venus 10 to 60 and the rest very small. To image such small objects, projection
imaging or a 3X to 5X focal extender is usually used to get a larger image. However the planets are quite bright and different
cameras and techniques are usually used to image planets. With very dim objects, such as those listed here, there is not enough
light to use projection imaging. There is no substitute for a large aperture, fast focal ratio and long focal length telescope for
imaging these tiny, dim objects.
NUMBER
UGC 7772
NGC 1535
NGC 6210
NGC 7027
NGC 6826
NGC 7009
NGC 6543
NGC 7662
NGC 4782
NGC 4783
NGC 2392
NGC 3242
NGC 4476
NGC 0040
NGC 4889
NGC 6729
NGC 6994
NGC 7006
NGC 0604
NGC 2346
NGC 4413
Messier RA
M 040
12 22.4
04 14.4
16 44.6
21 07.0
19 44.7
21 04.1
17 58.8
23 25.9
12 54.6
12 54.6
07 29.2
10 24.8
12 30.0
00 12.9
13 00.1
19 01.8
M 073
20 58.9
21 01.4
01 34.5
07 09.3
12 26.5
DEC
58 05
-12 44
23 47
42 14
50 31
-11 22
66 38
42 33
-12 35
-12 34
20 55
-18 38
12 20
72 32
27 59
-36 58
-12 39
16 12
30 46
00 49
12 36
SIZE
0.3x0.3
0.3x0.3
0.3x0.2
0.4x0.4
0.4x0.4
0.4x0.3
0.5x0.5
0.5x0.5
0.5x0.5
0.7x0.7
0.7x0.7
0.7x0.4
1.0x0.7
1.0x0.6
1.0x1.0
1.0x1.0
1.0x1.0
1.0x1.0
1.0x0.9
1.1x0.7
MAG
9.0
9.0
9.7
9.0
8.8
8.0
8.6
8.5
12.9
12.9
8.3
8.9
13.3
10.5
13.2
?
10.0
11.5
?
10.0
13.2
TYPE
Double Star
Planetary Nebula
Planetary Nebula
Planetary Nebula
Planetary Nebula
Planetary Nebula
Planetary Nebula
Planetary Nebula
Elliptical Galaxy
Elliptical Galaxy
Planetary Nebula
Planetary Nebula
Elliptical Galaxy
Planetary Nebula
Elliptical Galaxy
Nebula
Open Cluster
Globular Cluster
Nebula
Planetary Nebula
Barred Spiral a
0
0
4
4
NGC
NGC
NGC
NGC
NGC
NGC
NGC
2438
0278
6720
5367
4435
0650-1
4459
M 057
M 076
07
00
18
13
12
01
12
41.9
52.0
53.6
57.7
27.7
41.9
29.0
-14
47
33
-39
13
51
13
43
34
02
59
04
34
58
1.1x1.1
1.2x1.2
1.3x1.0
1.3x1.0
1.4x0.9
1.5x0.7
1.5x1.0
11.0
11.6
9.0
10.0
11.8
10.0
11.7
Planetary Nebula
Spiral Galaxy c
Planetary Nebula
Nebula
Elliptical Galaxy 4
Planetary Nebula
Spiral Galaxy 0
The next batch of objects goes up to about 3 arc minutes in extent. Even these objects create quite a small image with the 3000
mm telescope. But these images are up to 200 pixels in size. This is a good sized image on an ST-7 size chip which is about 500
by 750 pixels. With modern larger chips especially those with small pixels in the 6 micron range it is possible to image these
objects even though the resolution is not ideal. Pictures taken with this resolution are nice to look at but still do not use up the
chip area available and are nowhere near photographic quality. There are 39 objects in this group. Only 5 are M objects.
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
4479
4105
4478
2782
4106
2732
4387
4458
2261
2419
4374
4402
4425
4461
4473
4621
6302
5195
1999
6638
6642
6652
6934
6569
7335
3379
3389
4567
5908
1068
3587
4038
4039
4697
2207
3077
1275
2748
3513
M 084
M 059
M 105
M 077
M 097
12
12
12
09
12
09
12
12
06
07
12
12
12
12
12
12
17
13
05
18
18
18
20
18
22
10
10
12
15
02
11
12
12
12
06
10
03
09
11
30.3
06.7
30.3
14.1
06.8
13.4
25.7
29.0
39.1
38.2
25.1
26.1
27.2
29.1
29.8
42.0
13.9
30.0
36.5
31.0
31.8
35.8
34.1
13.7
37.3
47.8
48.4
36.5
16.7
42.7
14.9
01.9
01.9
48.6
16.4
03.4
19.8
13.7
03.8
13
-29
12
40
-29
79
12
13
08
38
12
13
12
13
13
11
-37
47
-06
-25
-23
-33
07
-31
34
12
12
11
55
-00
55
-18
-18
-05
-21
68
41
76
-23
35
47
19
07
48
11
49
15
43
53
53
07
44
11
25
39
07
16
43
30
29
00
24
49
27
35
32
15
25
01
02
52
53
48
22
44
31
29
15
1.5x1.5
1.5x1.5
1.0x1.8
1.8x1.6
1.0x1.8
1.8x0.8
1.9x1.1
1.9x1.8
2.0x2.0
2.0x2.0
2.0x1.8
2.0x0.8
2.0x0.5
2.0x1.0
2.0x1.0
2.0x1.5
2.0x1.0
2.0x1.5
2.0x2.0
2.0x2.0
2.0x2.0
2.0x2.0
2.0x2.0
2.0x2.0
1.7x0.8
2.1x2.0
2.2x1.0
2.4x1.6
2.4x0.4
2.5x1.7
2.5x2.5
2.5x2.5
2.5x2.0
2.5x1.3
2.5x1.5
2.6x1.9
2.6x1.0
2.8x1.1
2.9x2.3
12.5
12.0
12.4
12.4
12.0
11.9
12.0
12.0
10.0
11.0
10.5
13.0
12.9
12.2
11.3
11.0
?
11.0
9.0
9.5
8.0
8.5
9.0
10.0
14.7
10.6
12.2
12.0
13.0
10.0
11.0
11.0
12.0
10.5
12.3
11.0
11.6
11.7
11.5
Spiral Galaxy b
Elliptical Galaxy
Elliptical Galaxy
Spiral Galaxy b
Elliptical Galaxy
Spiral Galaxy c
Elliptical Galaxy
Elliptical Galaxy
Nebula
Globular Cluster
Elliptical Galaxy
Spiral Galaxy b
Spiral Galaxy 0
Spiral Galaxy 0
Elliptical Galaxy
Elliptical Galaxy
Nebula
Peculiar Galaxy
Nebula
Globular Cluster
Globular Cluster
Globular Cluster
Globular Cluster
Globular Cluster
Galaxy so
Elliptical Galaxy
Spiral Galaxy c
Elliptical Galaxy
Spiral Galaxy b
Spiral Galaxy b
Planetary Nebula
Peculiar
Peculiar
Elliptical Galaxy
Spiral Galaxy c
Elliptical Galaxy
Syfert Galaxy
Spiral Galaxy c
Spiral Galaxy c
3
1
2
5
0
1
4
3
1
s1
3
2
The next group of objects is up to 3.6 arc minutes in size. This corresponds to an image of almost 400 pixels in size. These
objects really fill the ST-7 chip using the full focal length of the 12" f10 which is again 3000 mm. Even the 10" f10 with a focal
length of 2400 mm gives a reasonable image size and might be considered for imaging these objects. There are 21 objects in the
group but only 8 of them M objects. Again, the digital camera chips of about 15 mm by 23mm will show a small inage, but of
reasonable resolution. A 2400 mm focal length telescope still requires a very good mount to hold it steady.
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
4382
4406
4486
4649
6171
6864
6981
3626
M
M
M
M
M
M
M
085
086
087
060
107
075
072
12
12
12
12
16
20
20
11
25.3
26.2
30.8
43.6
32.5
06.1
53.5
20.1
18
12
12
11
-13
-21
-12
18
11
56
23
33
03
55
33
21
3.0X2.0
3.0X2.8
3.0x3.0
3.0x2.5
3.0x3.0
3.0x3.0
3.0x3.0
3.0x2.0
10.5
10.5
8.6
10.0
8.1
8.0
8.6
10.9
Spiral Galaxy 0
Elliptical Galaxy 3
Elliptical Galaxy 1
Elliptical Galaxy 1
Globular Cluster
Globular Cluster
Globular Cluster
Spiral Galaxy b
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
5824
6441
6624
6712
7339
5248
4298
4710
0185
7479
6229
0221
5694
M 032
15
17
18
18
22
13
12
12
00
23
16
00
14
04.0
50.2
23.7
53.0
37.8
37.6
21.5
49.6
38.5
04.9
47.0
42.7
39.6
-33
-37
-30
-08
23
08
14
15
48
12
47
40
-26
05
03
22
43
47
53
36
10
21
19
32
52
32
3.0x3.0
3.0x3.0
3.0x3.0
3.0x3.0
3.0x0.9
3.2x1.4
3.2x1.9
3.4x0.5
3.5x2.8
3.2x3.5
3.5x3.5
3.6x3.1
3.6x3.6
9.5
8.0
8.5
9.0
12.1
11.0
11.4
12.0
11.8
11.8
8.9
9.5
10.2
Globular Cluster
Globular Cluster
Open Cluster
Globular Cluster
Spiral Galaxy c
Spiral Galaxy c
Spiral Galaxy c
Spiral Galaxy 0
Elliptical galaxy 1
Barred Galaxy b
Globular Cluster
Elliptical galaxy 2
Globular Cluster
The next group of objects goes up in size to 5.5 arc minutes. Now the largest of these images is a full 480 pixels in size and will
require a slightly larger imaging chip like that in the ST-8. These images will of course fit well onto the typical digital camera chip.
With a 12” f 10 telescope, a focal reducer of about 0.63 strength migh be a good attachment to shorten the effective focal length a
bit. The objects also give a nice sized image with the 10" f10. (2400 mm focal length) There are 37 objects in this group with
13 of them being M objects.
NGC 0246
NGC 3115
NGC 3351
NGC 3384
NGC 4438
NGC 4472
NGC 4477
NGC 4579
NGC 6333
NGC 6637
NGC 6681
NGC 0559
NGC 7235
NGC 6603
NGC 4449
NGC 7332
NGC 4111
NGC 4254
NGC 2775
NGC 4302
NGC 5005
IC 2233
NGC 1952
NGC 4388
NGC 4736
NGC 6779
NGC 2129
NGC 5466
NGC 5986
NGC 6520
NGC 0779
NGC 4490
NGC 4321
NGC 5866/79
NGC 4548
NGC 7814
NGC 3511
M 095
M 049
M
M
M
M
058
009
069
070
M 099
M 001
M 094
M 056
M 100
M 102?
M 091
00
10
10
10
12
12
12
12
17
18
18
01
22
18
12
22
12
12
09
12
13
08
05
12
12
19
06
14
15
18
01
12
12
15
12
00
11
47.1
05.3
43.9
48.3
27.8
29.8
30.0
37.6
19.1
31.4
43.3
29.5
12.5
18.4
28.2
37.4
07.1
18.8
10.3
21.7
10.8
14.2
34.5
25.8
51.0
16.5
01.1
05.4
46.1
03.5
59.7
30.7
22.9
06.5
35.5
03.3
03.4
-11
-07
11
12
13
07
13
11
-18
-32
-32
63
57
-18
44
23
43
14
07
14
37
45
22
12
41
30
23
28
-37
-27
-05
41
15
55
43
16
-23
53
43
42
38
00
59
38
48
31
21
18
18
15
26
05
47
04
25
02
36
03
44
01
39
07
10
18
32
46
54
57
38
49
45
09
09
05
4.0x2.5
4.0x1.0
4.0x3.0
4.0x2.0
4.0x1.5
4.0x3.4
4.0x3.5
4.0x3.5
4.0x4.0
4.0x4.0
4.0x4.0
4.0x4.0
4.0x4.0
4.0x4.0
4.2x3.0
4.2x1.3
4.4x0.9
4.5X4.0
4.5x3.0
4.5x0.5
4.7x1.6
4.7x0.6
5.0x3.0
5.0x1.0
5.0x3.5
5.0x5.0
5.0x5.0
5.0x5.0
5.0x5.0
5.0x5.0
3.0x5.0
5.0x2.0
5.2x5.0
5.2x2.3
5.4x4.4
1.5x5.5
5.5x1.0
8.5
10.0
11.0
11.0
11.0
10.1
10.4
10.5
8.0
7.5
8.0
9.5
9.0
11.4
10.5
11.0
10.7
10.4
10.3
12.9
10.8
13.0
9.0
12.0
8.9
8.0
7.0
9.0
8.0
9.0
11.8
10.1
10.4
10.0
10.2
11
11.0
Planetary Nebula
Elliptical Galaxy 7
Barred Spiral c
Elliptical Galaxy 7
Spiral Galaxy a
Elliptical Galaxy 3
Barred Spiral a
Spiral Galaxy b
Globular Cluster
Globular Cluster
Globular Cluster
Open Cluster
Open Cluster
Open Cluster
Irregular Galaxy
Spiral Galaxy c
Spiral Galaxy c
Spiral Galaxy c
Spiral Galaxy b
Spiral Galaxy c
Spiral Galaxy b
Spiral Galaxy c
Nebula
Barred Spiral b
Spiral Galaxy b
Globular Cluster
Open Cluster
Globular Cluster
Globular Cluster
Open Cluster
Spiral Galaxy b
Spiral Galaxy c
Spiral Galaxy c
Elliptical Galaxy
Spiral Galaxy c
Spiral Galaxy b
Spiral Galaxy c
The next group of objects goes up to 9.9 arc minutes in size. This is 870 pixels with the 12" f10. Clearly these objects give
images that no longer fit on the ST-7 chip at all and a focal reducer is required. At this point it is good to consider the ST-8 which
has a chip of twice the size and four times the area. The 10" f10 (2400 mm) now gives the excellent sized images up to 580
pixels. But the 10" f6.3 looks like a good choice as well, giving images up to about 360 pixels. With the 10" f 6.3 scope and the
ST-8 a very comfortable match is had. Again, the digital camera will handle this image size easily and is a good choice for this
group with a 10“ f 6.3 telescope of 1600 mm focal length. This is a very large group of 67 objects with no less than 35 M objects.
This group fives image sizes that start to look very good for a digital camera with a 15 mm by 23 mm chip or one of the larger
SBIG cameras like the 8, 10 or so. There are a wonderful set of objects in this group.
NGC 4303
M 061
12 22.0
04 28
5.7x5.5
10.2
Spiral Galaxy c
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
4501
6934
5377
2362
3368
6266
6273
6402
6626
6715
6838
7099
7793
1300
6819
3432
3992
0147
1982
4569
4594
6093
6613
6913
7089
6885
2506
6723
6997
1904
4826
1055
3079
3556
3623
5746
0205
0581
2068
3034
3627
6254
6341
6853
6946
1365
1502
2627
6866
6939
4192
4216
2683
5897
0628
1097
4590
5055
6694
1857
6649
2818
7788
4762
1560
4517
M 088
M
M
M
M
M
M
M
M
096
062
019
014
028
054
071
030
M 109
M
M
M
M
M
M
M
043
090
104
080
018
029
002
M 079
M 064
M 108
M 065
M
M
M
M
M
M
M
M
110
103
078
082
066
010
092
027
M 098
M 074
M 068
M 063
M 026
12
20
13
07
10
17
17
17
18
18
19
21
23
03
19
10
11
00
05
12
12
16
18
20
21
20
08
18
20
05
12
02
10
11
11
14
00
01
05
09
11
16
17
19
20
03
04
08
20
20
12
12
08
15
01
02
12
13
18
05
18
09
23
12
04
12
32.0
34.2
56.3
18.7
46.8
01.3
02.6
37.6
24.6
55.2
53.7
40.3
57.9
19.8
41.3
52.5
57.6
33.1
35.6
36.8
39.9
17.1
19.9
24.0
33.5
12.0
00.2
59.6
56.5
24.3
56.8
41.8
02.0
11.6
18.9
44.9
40.3
33.2
46.8
56.1
20.2
57.1
17.1
59.6
35.0
33.7
07.4
37.3
03.7
31.4
13.8
15.9
52.7
17.4
36.7
46.4
39.5
15.7
45.2
20.1
33.5
16.0
56.7
52.9
32.8
32.8
14 25
07 24
47 12
-24 57
11 49
-30 07
-26 15
-03 15
-24 52
-30 28
18 47
-23 11
-32 34
-19 24
40 13
36 37
53 22
48 31
05 16
13 09
-11 37
-22 59
-17 08
38 31
-00 50
26 29
-10 47
-36 38
44 39
-24 31
21 41
42 21
55 41
55 41
13 07
01 57
41 41
60 42
00 03
69 42
13 01
-04 07
43 09
22 43
60 08
-36 08
62 19
-29 56
44 10
60 38
14 54
13 09
33 25
02 05
15 47
-30 16
-26 45
42 01
-09 24
39 21
-10 24
-36 37
61 24
11 14
71 53
00 07
5.7x2.5
5.9x5.9
3.0x6.0
6.0x6.0
6.0x4.0
6.0x6.0
6.0x6.0
6.0x6.0
6.0x6.0
6.0x6.0
6.0x6.0
6.0x6.0
6.0x4.0
6.0x3.2
6.0x6.0
6.2x1.7
6.4x3.5
6.5x3.8
7.0x5.0
7.0x2.5
7.0x1.5
7.0x7.0
7.0x7.0
7.0x7.0
7.0x7.0
7.0x7.0
7.0x7.0
7.0x7.0
7.0x7.0
7.5x7.5
7.5x3.5
7.6x3.0
7.6X1.7
7.8x1.4
7.8x1.6
7.9x1.7
8.0x3.0
8.0x8.0
8.0x6.0
8.0x3.0
8.0x2.5
8.0x8.0
8.0x8.0
8.0x6.0
8.0x8.0
8.0x3.5
8.0x8.0
8.0x8.0
8.0x8.0
8.0x8.0
8.2x2.0
8.3x2.2
8.4x2.4
8.5x8.5
9.0x9.0
9.0x5.5
9.0x9.0
9.0x4.0
9.0x9.0
9.0x9.0
9.0x9.0
9.0x9.0
9.0x9.0
9.1x2.2
9.2x1.7
9.9x1.4
10.5
8.9
12.0
4.0
10.2
6.5
7.0
9.0
8.0
9.0
9.0
8.0
9.7
11.3
10.0
10.6
10.9
12.1
8.0
9.0
8.2
8.0
8.0
7.0
6.0
5.7
7.6
6.0
10.0
8.4
8.6
10.6
10.6
10.8
10.3
10.6
10.8
8.0
8.0
9.2
9.7
7.0
6.5
8.0
11.1
11.2
7.0
9.0
8.0
10.0
11.0
10.0
9.7
9.0
9.0
10.6
8.0
9.8
9.5
7.0
9.0
11.0
9.4
10.3
11.5
10.4
Spiral Galaxy b
Globular Cluster
Spiral Nebula a
Open Cluster
Spiral Galaxy b
Globular Cluster
Globular Cluster
Globular Cluster
Globular Cluster
Globular Cluster
Globular Cluster
Globular Cluster
Spiral Galaxy
Barred Spiral b
Open Cluster
Spiral Galaxy c
Barred Spiral b
Elliptical galaxy 4
Nebula
Spiral Galaxy b
Spiral Galaxy a
Globular Cluster
Open Cluster
Open Cluster
Globular Cluster
Open Cluster
Open Cluster
Globular Cluster
Open Cluster
Globular Cluster
Spiral Galaxy a
Spiral Galaxy c
Spiral Galaxy c
Spiral Galaxy c
Spiral Galaxy c
Spiral Galaxy c
Elliptical galaxy 6
Open Cluster
Nebula
Irregular Galaxy
Spiral Galaxy b
Globular Cluster
Globular Cluster
Planetary Nebula
Spiral Galaxy c
Spiral Galaxy b
Open Cluster
Open Cluster
Open Cluster
Open Cluster
Spiral Galaxy b
Spiral Galaxy c
Spiral Galaxy c
Globular Cluster
Spiral Galaxy c
Barred Spiral b
Globular Cluster
Spiral Galaxy b
Open Cluster
Open Cluster
Open Cluster
Open Cluster
Open Cluster
Spiral Galaxy c
Spiral Galaxy c
Spiral Galaxy c
The next group has sizes up to 14 arc minutes. With this group shorter focal lengths are required. A 10” f 6.3 scope of 1600 mm
or even a scope in the 1000 mm range is a good choice. Here is where the ST-8 chip size is a clear advantage. Objects at the end
of this part of the list can now be imaged nicely on the larger SBIG chips or with a typical digital camera chip of 15 mm by 23 mm.
There are 35 objects in this group with 11 of them being M objects.
NGC 0457
NGC 2023
NGC 2323
NGC 2360
NGC 4559
NGC 5024
NGC 5194/5
NGC 5236
NGC 6218
NGC 6531
NGC 7078
NGC 7331
NGC 7635
NGC 0288
NGC 1807
NGC 6530
NGC 6645
NGC 4395
NGC 0663
NGC 2903
NGC 0891
NGC 1960
NGC 3628
NGC 6705
NGC 7654
IC 5146
IC 1613
NGC 1513
NGC 1893
NGC 6709
NGC 5907
NGC 4631
NGC 4244
NGC 5904
NGC 0225
M 050
M
M
M
M
M
M
053
051
083
012
021
015
M 036
M 011
M 052
M 005
01
05
07
07
12
13
13
13
16
18
21
22
23
00
05
18
18
12
01
09
02
05
11
18
23
21
01
04
05
18
15
12
12
15
00
19.0
41.7
02.9
17.7
36.0
12.9
29.9
37.1
47.2
04.8
30.0
37.1
20.7
52.6
10.7
04.7
32.7
25.9
46.0
32.1
22.4
36.2
20.3
51.1
24.2
53.5
04.8
09.9
22.6
51.5
15.9
42.2
17.5
18.5
43.8
58
-02
-08
-15
27
18
47
-29
-01
-22
12
34
61
-26
16
-24
-16
33
61
21
42
34
13
-06
61
47
02
49
33
10
56
32
37
02
61
20
13
20
38
57
10
12
52
57
30
10
26
10
36
32
20
54
32
16
31
41
09
37
16
36
16
07
31
24
21
20
33
48
05
46
10x10
10x10
10x10
10x10
10x3
10x10
10x5
10x8
10x10
10x10
10x10
10x3
10x5
10x10
10x10
10x10
10x10
10x8
11x11
11x5
12x1
12x12
12x2
12x12
12x12
12x12
12x11
12x12
12x12
12x12
12x2
12x1
13x1
13x13
14x14
7.0
?
6.0
9.0
10.5
8.0
8.1
8.0
8.0
7.0
6.5
10.4
11.0
7.2
7.5
6.0
9.0
11.0
7.0
9.7
12.2
6.3
10.3
6.0
7.0
10.0
9.0
9.0
8.0
8.0
10.4
9.7
10.7
6.2
8.0
Open Cluster
Nebula
Open Cluster
Open Cluster
Spiral Galaxy c
Globular Cluster
Spiral Galaxy c
Spiral Galaxy c
Globular Cluster
Open Cluster
Globular Cluster
Spiral Galaxy b
Nebula
Globular Cluster
Open Cluster
Open Cluster
Open Cluster
Spiral Galaxy
Open Cluster
Spiral Galaxy b
Spiral Galaxy b
Open Cluster
Spiral Galaxy b
Open Cluster
Open Cluster
Nebula
Irregular Galaxy
Open Cluster
Open Cluster
Open Cluster
Spiral Galaxy c
Spiral Galaxy c
Spiral Galaxy b
Globular Cluster
Open Cluster
For the next set of objects which goes up to 29 arc minutes, the images are 15 mm with the 12" f10, 10 mm with the 10" f10 and
6.6 mm with the 10" f6.3. These are all good candidates for the ST-8 and larger chips and definitely for the digital cameras. A
1200 mm focal length gives an image for the largest of the objects of 630 pixels. . For these larger objects it is time to consider
one of the excellent refractors in the 800 to 1000 mm range. A Takahashi FSQ 106 is a good choice. Even a good telephoto lens
mounted directly on the CCD imager might be a suitable choice. It becomes more and more necessary to consider but 1000 mm
optics as we move toward the larger objects. There are an astonishing 50 objects in this group with 19 of them being M objects.
NGC 0188
NGC 2682
NGC 4565
NGC 1817
NGC 2281
NGC 6811
NGC 6809
NGC 2403
NGC 7293
NGC 7790
NGC 0247
NGC 2447
NGC 3031
NGC 5272
NGC 6656
NGC 6888
NGC 7023
IC 0342
NGC 6664
NGC 4258
NGC 4656
M 067
M
M
M
M
093
081
003
022
M 106
00
08
12
05
06
19
19
07
22
23
00
07
09
13
18
20
21
03
18
12
12
44.2
51.0
36.4
12.1
49.3
38.2
40.1
36.8
29.7
58.4
47.1
44.5
55.6
42.2
36.4
12.5
02.0
46
36.7
19.0
44.0
85
11
25
16
41
46
-30
65
-20
61
-20
-23
69
28
-23
38
68
68
-08
47
32
19
49
59
42
04
34
56
36
51
13
45
52
04
23
56
25
10
06
14
18
10
15x15
15x15
15x1
15x15
15x15
15x15
15x15
16x10
12x16
17x17
18x5
18x18
18x10
18x18
18x18
18x12
18x18
18x17
18x18
19x6
19x2
10.0
7.0
10.5
8.0
6.0
9.0
7.0
8.8
6.5
13.0
10.7
7.0
8.0
6.0
6.0
?
?
9.2
6.0
9.0
11.0
Open Cluster
Open Cluster
Spiral Galaxy b
Open Cluster
Open Cluster
Open Cluster
Globular Cluster
Spiral Galaxy c
Planetary Nebula
Open Cluster
Spiral Galaxy c
Open Cluster
Spiral Galaxy a
Globular Cluster
Globular Cluster
Nebula
Nebula
Barred Spiral c
Open Cluster
Spiral Galaxy b
Irregular Galaxy
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
NGC
1039
1912
2024
2099
2422
6121
6822
7243
6809
1435
1245
6940
6791
7789
0300
0253
4236
5457
6205
2437
2477
6405
6494
6611
1528
2546
2264
0281
6514
M 034
M 038
M 037
M 047
M 004
M 055
M 101
M 013
M 046
M 006
M 023
M 016
M 020
02
05
05
05
07
16
19
22
19
03
03
20
19
23
00
00
12
14
16
07
07
17
17
18
04
08
06
00
18
42.0
28.7
41.9
52.3
36.6
23.7
44.9
15.2
40.1
46.2
14.6
34.6
20.8
57.0
55.0
47.6
16.7
03.2
41.7
41.9
52.3
40.1
56.9
18.8
15.2
12.4
41.2
53.3
01.9
42
35
-01
32
-14
-26
-14
49
-30
23
47
28
37
56
-37
-25
69
54
36
-14
-38
-32
-19
-13
51
-37
09
56
-23
47
51
50
34
30
31
46
53
56
45
14
18
46
43
42
18
28
21
27
49
33
13
01
47
15
38
53
35
02
20x20
20x20
20x20
20x20
20x20
20x20
20x10
20x20
20x20
15x20
20x20
20x20
20x20
20x20
21x14
22x6
22x5
22x20
23x23
25x25
25x25
25x25
25x25
25x25
25x25
25x25
15x26
23x27
29x27
6.0
6.2
?
6.2
5.0
7.4
10.0
8.0
7.0
6.8
9.0
8.0
11.0
10.0
11.3
7.0
10.7
9.0
5.7
8.0
7.0
6.0
7.0
6.5
6.0
8.0
5.0
8.0
8.5
Open Cluster
Open Cluster
Nebula
Open Cluster
Open Cluster
Globular Cluster
Irregular Galaxy
Open Cluster
Globular Cluster
Nebula
Open Cluster
Open Cluster
Open Cluster
Open Cluster
Spiral Galaxy c
Spiral Galaxy c
Barred Spiral
Spiral Galaxy c
Globular Cluster
Open Cluster
Open Cluster
Open Cluster
Open Cluster
Nebula
Open Cluster
Open Cluster
Open Cluster
Nebula
Nebula
For the next group of objects, up to 60 arc minutes in size, one must use telescopes with shorter focal lengths. There are only 19
objects in this group with 8 being M objects. It is an important group of bright objects of great interest. These objects, because of
their size, have great detail and are the focal point of many photographic and imaging techniques. We might note for reference
that the moon and sun are about 30 arc minutes, right between this group and the one above. Telescopes like the 500 to 800 mm
high quality refractors are the best choice for these objects. Typical SCT scope are too long in focal length and when reduced have
too small a circle of illumination and poor edge of image quality.
NGC 2168
NGC 2287
NGC 7092
IC 0405
NGC 0869
NGC 0884
NGC 6634
NGC 0055
NGC 2244
NGC 2548
NGC 1647
NGC 1977
NGC 2174
NGC 0752
NGC 6618
NGC 1746
Sh2-155
NGC 0598
NGC 6475
M 035
M 041
M 039
M 025
M 048
M 017
M 033
M 007
06
06
21
05
02
02
18
00
06
08
04
05
06
01
18
05
22
01
17
08.8
47.0
32.2
16.2
19.0
22.4
31.7
15
32.4
13.7
46.1
35.5
09.7
57.8
20.9
03.6
56.8
33.9
54.0
24
-20
48
34
57
57
-19
-39
04
-05
19
-04
20
37
-16
23
62
30
-34
20
45
26
16
09
07
15
13
52
47
05
52
30
41
11
48
37
39
49
30x30
30x30
30x30
30x19
35x35
35x35
35x35
25x40
40x40
40x40
40x40
40x25
40x30
45x45
45x35
45x45
50x10
60x60
60x60
5.5
6.0
5.0
6.0
4.4
4.7
6.0
7.8
5.5
5.5
6.5
7.0
?
7.5
6.0
6.0
7.7
5.3
5.0
Open Cluster
Open Cluster
Open Cluster
Nebula
Open Cluster
Open Cluster
Open Cluster
Irregular galaxy
Open Cluster
Open Cluster
Open Cluster
Nebula
Nebula
Open Cluster
Nebula
Open Cluster
Nebula
Spiral Galaxy c
Open Cluster
The final group of objects is definitely candidates for the largest CCD chips and digital cameras. Even then short focal length
telescopes and good telephoto lenses are a best choice. This group holds the really giant objects, those well over 1 degree in size.
To fit M 31 on an ST-10 chip requires a focal length of only 180 to 200 mm. These objects are good size for a digital camera with
a 200 to 400 mm lens. There are only 12 objects in the giant object group but 6 are well known M objects.
NGC
NGC
NGC
NGC
NGC
1976
6960
6992-5
2237-9
2632
M 042
M 044
05
20
20
06
08
35.4
45.7
56.4
32.3
40.4
-05
30
31
05
19
23
43
42
03
41
65x65
70x6
78x8
60x80
80x80
4.0
8.0
8.0
9.0
4.5
Nebula
Nebula
Nebula
Nebula
Open Cluster
NGC 6523
NGC 6603
NGC 7000
NGC 1499
NGC 0224
IC 1396
M 008
M 024
M 045
M 031
18
18
03
20
04
00
21
04.7
18.4
46.9
58.8
3.4
42.7
39.1
-24
-18
24
44
36
41
57
20
26
07
20
25
16
30
80x40
60x90
100x100
100x100
145x40
150x50
140x170
5.0
4.5
1.4
5.0
6.0
4.0
Nebula
Open Cluster
Open Cluster
Nebula
Nebula
Spiral galaxy b
Nebula
As I review this list, I wonder at the great variety and size of the non-stellar objects in the sky. Looking at the distribution of the
Messier objects among them I wonder at how Messier saw some of them at all and how he missed others. It is a wondrous list.
The list also demonstrates the difficulty of imaging all of them with a single telescope or camera. Just on the basis of size, it
becomes clear that one selection of optics or one imager will not capture all of them equally well. In fact the range is so great that
if imaging is a goal, the selection of the base telescope will have to be adjusted to cover with facility the range of objects of
greatest interest.
This review of object sizes should now have shown that any one telescope and any one chip size will not do an optimum job with
all of the interesting objects on even this small list. It would require a telescope of 1600 to 3000 mm to do the smaller group. In
order to extend the range of objects covered by a single telescope, imagers often choose the scope to be rather too long in focal
length and then to shorten it with focal reducers. Unfortunately the focal reducer also constricts the circle of illumination and
introduces severe optical aberrations at the edges of the image. Thus I recommend it best to choose a telescope of shorter focal
length in the first place. It seems that amateurs have aperture fever and think the best scope is the biggest one. This is often not
the case. The best scope is the one that covers you range of needs. If you are interested in wider field images or images of the
groups of larger objects, it may well be a medium to short focal length refractor that is the best choice.
What I am trying to encourage is for imagers to think about the options. The only things that determine you actual field of view
are the focal length of the telescope and the size of your chip. You take the size of the chip in mm and divide it by the focal length
of your telescope in mm. Then multiply the result by 57 to change radians into degrees. You can multiply again by 60 to get the
result in arc minutes. Then you can window your actual field of view, in arc minutes, on the above list and see which objects you
can get on the chip and which will give images that are too tiny and thus have poor pixel resolution.
In an effort to give some examples it turns out that there are just too many cameras and telescopes to exemplify many of them.
Thus cameras will be put roughly into four chip sizes Small chips like the ST-7 and some webcams and guider chips which are
about 4 mm by 7 mm. Medium chips such as those in the ST-8, ST-10 and ST-2000 which are about 10 mm by 15 mm in size.
Large chips which are 15 mm by 22 mm such as are in some of the large format SBIG cameras and in the common Canon digital
cameras. The final category are Giant chips such as are in the largest SBIG cameras and the full format Canon digital cameras
where the chips are about 24 mm by 36 mm.
The number of pixels run from 400,000 in the Small chips to several million in the Medium and Large chips. The Giant chips are
often 6 to 8 or even 16 million pixels. A few of the SBIG cameras have color chips; but all of the Canon digital cameras have color
chips. The costs of these cameras has some correlation with the chips size. The CCD cameras run from $1500 to about $10,000.
The Canon digital cameras run from $800 to $8000. Thus there is a vast range of cameras, chip sized, pixel densities and costs.
The examples here take into account only the issue of what chip sizes go well with what focal length telescopes so that certain
groups of object sizes can be imaged on the chip. More detailed calculations for specific chip sizes and telescope focal lengths can
easily be done using the formula given above.
In the examples the chip is filled but with some room around the main object say for artistic reasons. This is my definition of
“filling the chip.” For an example, take the large group of objects in the list above that are in the 5 to 10 arc second size range.
This is an interesting group which contains about 67 objects, 35 of which are the popular M objects. What would be a good choice
of telescope focal length and chip size for this set of objects. Even with a limited group of object sizes it is apparent that the ideal
focal length depends on the chip size and vice versa. Let us take a CCD camera of the Medium size like the ST-8 or ST-10. These
are quite popular sizes these days. Using a circle of interception for the chip of about 8 mm and a size for the largest objects of 12
arc minutes, we find that a telescope of focal length of about 2200 mm is about right. Note the example allows for some clearance
for getting the object on the chip. But if we move down to another very nice group of objects which are 15 mm to 30 mm in size,
we need a telescope of about 800 mm. Now the problem of filling the chip with a great variety of object sizes becomes obvious. It
will be necessary to choose the shorter focal length telescope to get the larger objects and accept the smaller objects to simply
have smaller images on the chip.
Digital SLR cameras which have chips of about 15 mm by 23 mm when used for the same two groups of objects as used in the
above example would require a telescope focal length of 4000 mm for the smaller sized objects and 1400 mm for the larger sized
objects. One can see that some of the larger telescopes which might be in the range of a 14 inch f 10 (3500 mm) or a 16 inch f 10
(4000 mm) require the larger chips that the digital SLR or the large format CCD cameras have in them. Those who use something
as large as a 20 inch f 8 telescope also find that they are working at 4000 mm focal length and can well use the larger chipped
cameras.
It becomes apparent that those wishing to image the larger sets of objects in the final two groups with even Large CCD cameras
need to go to shorter focal length telescopes. When imaging the last group with a large chip it is possible to use 300 mm or even
150 mm photographic lenses. The object of these exercises is to point out that a first and important issue to consider is the focal
length of the telescope you choose for imaging and its relationship to the chip size of your camera. This may not, usually does not,
dictate getting the largest aperture telescope. In fact for imaging a shorter faster telescope is usually a better choice.
A brief not on very tiny objects is required as well. Planets are very tiny. They subtend under 40 arc seconds but are fortunately
very bright. In this case it is useful to have a focal length of as much as 15,000 mm to 40,000 mm. Wow, that is at the far
opposite end of the focal length spectrum. In the case of planets then a 2000 mm to 3000 mm can be used with a 5 power teleextender like the TeleVue power mate. It is almost impossible to get a mount steady enough for such a long focal length. Also the
focal ratio of the telescope goes to numbers like f 50. But since the planets are bright, it is still possible to image planets
effectively with rapid image capture cameras. Much excellent planetary work has been done with web cams.
Again it can be seen that there is no one telescope that is suitable for all imaging needs. To cover all situations, two or possibly
three telescopes are required. A summary of the situation is to pay special attention to the size of the imaging chip and the focal
length of the telescope chosen for your imaging needs.
Books which discuss many aspects of astrophotography are listed in the Bibliography/Applications section on this web site.
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Specifications for Long Cable for Connecting ST-7 to a Computer
I have tested this 100 foot cable with the ST-7 and in fact extended it to 160 feet by adding three commercial printer cables in
series with it. It works with no attenuation of the speed of downloading from the ST-7. The cable should be made with a length of
Belden cable # 8138. This is a cable consisting of 8 twisted pairs of wires within a single overall shield. The cable is notable
because it has low capacitance. The shield should be connected at both ends to the shell or frames of the plugs as is usual
practice.
The cable must be connected exactly as follows:
Pair
Pair
Pair
Pair
Pair
Pair
Pair
Pair
1
2
3
4
5
6
7
8
Pins
Pins
Pins
Pins
Pins
Pins
Pins
Pins
2 and 3
4 and 5
6 and 7
8 and 19
10 and 11
12 and 13
15 and 20
9 and 18
To
To
To
To
To
To
To
To
pins
pins
pins
pins
pins
pins
pins
pins
2 and 3
4 and 5
6 and 7
8 and 19
10 and 11
12 and 13
15 and 20
9 and 18
The connectors are standard DB-25, one male and one female. I do not know how long this cable can be made. I would think,
from my experience, at least 160 to 200 feet. The cable is approximately $1.00 per foot on a 100 foot spool. It is available in a
500 foot length as well. I got mine from Newark Electronics.
As an added note in constructing this cable, I recommend using metal hoods on the DB connectors and very secure connection of
the shield to a good ground on both ends. Grounding of the entire electronic system is very important to avoid electrical noise
being introduced into the digital transfer. There are other cables which will work, of course. I have been told that Belden 9937
which has totally shielded pairs has been used up to 200 feet. It has many more conductor pairs than needed and so is a very
heavy cable. A lighter weight "pigtail" must run from it to the camera to avoid strain on the camera itself. I have been told that
cables of 100 feet or longer can be obtained as custom made units with the D connectors on each end. I had to pull mine through
an underground conduit and so installed my own connectors.
It should also be possible to use commercial cables and extenders in series if they are of good quality. (IEEE 1284 standard is
best) They should have good grounding especially where the plugs come together.
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ST-7 Used as a Guider
Normal operation of the ST-7 uses its large 765 by 510 pixel chip for imaging and a small 192 by 164 guider chip for guiding the
imager. It is possible to use the large chip in the ST-7 as a guider as well. This might be done in a system which uses a separate
guide telescope. The ST-7 can then be used on the guide telescope to control the pointing of the main telescope. This function is
useful if the main telescope is to be used for film photography or a piggy back camera is to be used. In fact both piggy back and
prime focus photography can be done at the same time using the guider telescope with the ST-7 to control the guiding operation.
One might even have a situation where the ST-7 is used for guiding and a larger imager is used on the main telescope or with a
piggy back mounted lens. Whatever the particular system setup, The ST-7 makes a guider with a large surface, small pixels for
accurate guiding and a large field of view compared to many guider chips in current use.
The ST-7 is shown on the C-5 guide scope in the figure on the left and the par focal, illuminated eyepiece is shown on the right.
Notice the use of a very short 2" tube to connect the ST-7 to the C-5. This insures great rigidity of the guider structure.
Additionally, the C-5 is very firmly mounted to the main telescope tube with a custom mount that allows for several degrees of
adjustment with respect to the main optical tube. The par focal eyepiece is a convenience but almost unnecessary with the large
field of view provided by the ST-7. The large focussing knob has been made to replace the small C-5 focussing knob and is
provided with index marks to facilitate focussing. For final use, the mirror will be locked into place and the focus set permanently
at infinity. (There is provision for a small fine focus adjustment. But guiding does not require the precision focus that imaging does
in any case.)
There are immense advantages to using a large chip in the ST-7 and a medium focal length guider. A principle one is that the field of view is
so large that it is easy to find a good guide star within the large field of view provided. This feature alone is worth the cost when doing remote
controlled setups. The pointing accuracy is still determined by the pixel size, the focal length of the guider telescope and the resolution
accuracy of the program used to run the guider. The ST-7 is not a self guider, but is run by the CCDOPS software through its parallel port.
The following setup procedure is as close to accurate as I can get it. References to the various manuals may vary with the
particular manual you are using. The manual for CCDOPS 3.5 is not quite correct for CCDOPS 3.6 and the operating manual for the
ST-7/8 is not quite correct either. So you have to hunt around a bit to find the pertinent reference. It goes something like the
following.
In order to use the main chip in the ST-7 for guiding, it must be selected in the Camera Menu under ST 7/8 Camera Setup
Command select the imaging CCD. Or in the Track Menu under Calibrate Track Command select the imaging CCD. This will enable
the imaging CCD to be used as the Calibrate CCd as well as the guiding CCD. Follow the instructions in the operations manual to
calibrate your CCD and telescope. (p62 and 63 in either manual) Once the imager and the telescope have been calibrated the
setting should not have to be changed unless you change declination by a very large amount. For example guiding near the pole is
a bit strange. You can fine tune the guiding behavior using the Tracking Parameters Command.
The imager is now ready to guide the telescope. Select the Autoguide Command under the track Menu. Enter the approximate
declination of the object you are guiding on, set the exposure time and follow the directions that appear. After choosing a star for
guiding, the guider will start guiding and report on the screen its progress. Typically there will be a series of small plus and minus
corrections. If these remain stable, the guiding is working properly.
You can then start imaging with any of the optical systems you have mounted such as the main telescope or any piggyback
cameras.
I have tested the accuracy of the ST-7 used as a guider and found the guider servo control loop to be very tight. Guiding accuracy
should easily be to better than 1 pixel. This will depend on your telescope, the brightness of the guide star and the frequency of
the guider correction function. This is the best performing guider of several that I have tested.
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Return to LX200 Mechanical Index
Data on the Flexibility of the Classic LX200 Telescope Mount
(October 1997)
This information is for those interested in telescope motion, vibration and oscillation. This information is about the static deflection
characteristics of a Meade 10 inch LX200 mounted on a Giant Field Tripod with and without a SuperWedge. This information has
be checked for accuracy and some re-interpretation has been included as the result of added experience since March of this year.
The purposes of providing this data are two fold. One is to show that for this telescope and mount there are specific deflection
characteristics related to the position of the applied force and the various bearings which make up the mount. The second is to
provide information about where the weakest points are so one can determine where it pays to spend effort in an attempt to
improve the stability of the mount and thus reduce vibration.
This information is for only one instrument. A different sample of the telescope may provide somewhat different results. On the
other hand, these results are entirely consistent with measurements which I made some time ago on a 12 inch LX200 mounted on
a very strong permanent pier.
This data is for static deflection of the main tube. However, there is good correlation between the static deflection and the
sensitivity of the main tube to vibrations caused by wind, motor drives, bumping, irregular drive forces and the like. So it has
considerable value in judging stability for imaging. I strongly believe that knowing these values really helps to both understand
the telescope and its mechanical mounting and reinforces the ability to improve it. I have been seriously trying to get pointing
stability to be as good as the best seeing conditions. This is in the area of 1 arc second.
The 10 inch telescope was set up in my basement laboratory with a series of weights, strings and pulleys arranged so that the
telescope could be pulled with a known force in any direction. The tube was focused on a target calibrated in arc seconds of
deflection. The setup is very simple so that anyone interested can easily duplicate it to measure their own telescope.
Here are the results for the telescope mounted directly on the giant field tripod using only the center bolt, very firmly tightened.
The legs were not extended and the tripod sitting on a cement floor. The tube was pointed horizontally and the fork set up with its
long dimension parallel to the front of the control panel. The force was applied by a 1 kilogram weight applied directly or via the
string and pulley arrangements. All deflections given are arc seconds per kilogram.
Forces are applied as follows and deflections obtained given. The units are arc seconds per kilogram.
Force applied to the tripod head back and forth and side to side gave a resulting motion of 2 arc-sec.
Force applied to the fork forward and back: motion 20 arc-sec.
Force applied to the fork side to side: motion 15 arc-sec.
Force applied to the fork up and down: motion 1 arc-sec.
Force applied to the tube:
up and down at the eyepiece: motion 60 arc-sec.
up and down at the corrector plate: motion 40 arc-sec.
up and down at the top center: motion 2 arc-sec.
left and right at the eyepiece: motion 50 arc-sec.
left and right at the corrector plate: motion 30 arc-sec.
left and right at the top center: motion 2 arc-sec.
Conclusions: The tripod is very solid and should not be a concern with this mode of mounting. About half of the deflection seems
to be at the fork bearing and about half at the declination bearing. This data is good news and bad news. The good news is that
the tripod is amazingly stable when placed on a solid concrete floor. Adding weight to the tripod or other braces cannot hurt but
will not do much good either. More spongy surfaces might be made more stable by adding weight because the tripod legs are
forced into the surface more.
The bad news is that about half of the motion is at each bearing. The left and right and the up and down motions are difficult if
not impossible to separate. However, the deflections for the fork bearing are about half the total for both fork and declination
bearings. Thus about half is in each and that means that for significant improvements to be effected both have to be improved.
For polar mounting using the Superwedge, the measurements are as follows:
Force applied to the tripod head back and forth and side to side.
Resulting motion 2 arc-sec.
One major conclusion has to be that the telescope behaves quite differently when polar as compared to Alt/Azm mounted.
The tripod and the Superwedge structure are very solid. The tripod and wedge act very much like a solid unit. Flexing of the
Superwedge is insignificant compared to that in other parts of the fork and bearing structure. The side to side stiffness of the fork
is much improved in the polar mount. This fact is verified by the similar motion of the tube to up and down motion but by the
greatly improved stiffness of the tube to side to side motion. This does not necessarily mean that the polar mount will, in practice,
be more stable however as described below.
This result is entirely in agreement with bearing theory. That is, the bearing stiffens greatly when it is loaded. The great loading
upon the fork bearings when the telescope is in a polar mode is clear and the resulting stiffening in the lateral motion is clear as
well. It is for reasons of stability or stiffness that bearings are often "pre-loaded." Thus contrary to expectations for stiffness of
the telescope tube, bearings and mounting system, the entire machine seems to be quite stable in the polar mount mode. At the
same time, the great dominance of one direction of instability, up and down motion at the eyepiece, is very bad news. It means
that the stability of the telescope is dominated by this motion in both modes of mounting and that it is the largest component.
The statements in the preceding paragraph should not be interpreted as saying that the telescope, in actual use, actually wobbles
less in the polar as compared to the Alt/Azm mode. In fact, forces which tend to make the telescope move in either mode
probably tend to be caused gravitationally. This means that the Alt/Azm mounting seems much more stable since the telescope
mount is very stiff in the direction of the gravitational vector.
The worst news is that if the springiness is mainly in the bearings, as it certainly seems to be, there is almost nothing that can be
done about it. Only a major redesign of the mount would enable improvement. Reducing wobbles must then be accomplished by
reducing any irregularities in the drive motion and any external forces such as wind, stomping around the pier or tripod supporting
surface and definitely not bumping or even touching the telescope while imaging.
I have checked out bearing specifications and standards in various mechanical design manuals and these results are quite good for
the bearings used and the way they are mounted. The main problem with the design is that the bearings are very close together
so that the forces generated by the torque in the polar mode are very large. For two bearings to provide excellent stability of a
shaft, they must be separated more. I have come to believe that the bearings used by Meade are quite good and in fact probably
as good as can be expected within the cost parameters of the mechanical as provided. Also, not much can be done to improve the
bearings without massive rebuilding and added cost. Users will simply have to be careful not to disturb the telescope while
imaging. This includes wind protection, stable mounting of the equipment and avoiding touching the equipment while using it.
It is also clear that more complex and costly designs can and have been effected if significantly greater resources (money) are
applied. There are some damping and control devices that can be brought to bear but these are complex to design and use.
One device that can improve damping of vibration is a mechanical damper. I have not seen a design that uses this technique on a
telescope. They are very common on larger motor and engine mounts. Another is active adaptive damping on which I am
working personally. (but at a slow pace) Still another is a method which does not damp the mechanical system, but adjusts the
optical system to stabilize the image directly. This is a promising technique which we will soon see in practice. It is called Active
Optics.
Stabilizing a telescope to one arc second, if that is required for good imaging, is very difficult. But it is a reasonable goal when
imagers and seeing are of that same order.
I hope you find this information useful.
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Go to Details
Issues Regarding Drive Systems
(with special attention to the LX200 drives and including instructions for repair and "tweaking" of them)
In the process of repairing two LX200 declination drives, I have gleaned some information which seems to fit together and make
good engineering sense. I have designed similar systems and feel it is time to try to come to closure on some of the issues
involved in the design of telescope drives. These drives must meet exacting specifications if they are to reliably point a telescope
tube to one arc second. That is dividing a circle into 1.296 million parts. To attain this accuracy of pointing and to generate a
steady motion for guiding the telescope a sophisticated electrical drive system must be used.
Most of the drives used are closed loop control systems including control electronics, the motors and feedback encoders. The
motors are connected to the main drive axes of the telescope tube through a set of gears and a worm. The reduction gears, worm
and main shaft often are not inside the control loop but are extensions, through the gears, of the positions of the drive motor
shafts.
One source of slackness or looseness in these types of drive systems is the lash in the gear train between the motor and the main
drive shaft. Any defects or lash in the gear train is not fixed simply through control of the motor shafts since this part of the
system is outside the feedback loop. However backlash or dead zone in the gear train can be compensated for to some extent
with a backlash setting generally provided by the electronic part of the system. In the LX200 design this feature is only provided
on the declination drive. In the case of the RA drive some of the most dominant periodic errors can be improved by a drive
correction training technique such as is provided in the LX200 design.
There is often no absolute encoder to give the position of the telescope main shafts because of the difficulty of making an encoder
of the required precision. (more about such encoders later) To keep track of the absolute position of the telescope, the number
of turns of the motor shaft must be known and kept track of by a counting mechanism consisting of a computer and encoder on
the motor shaft.. This is accomplished as follows in the case of the LX200. The motor has an optical transducer on the shaft
which sends to the control electronics a series of pulses which the electronics counts and compares to a computer generated
number. The number of counts from the motor shaft tells the computer, indirectly, where the telescope is pointing. When the
motor encoder counts match those required by the computer the motor is stopped. This is an extremely accurate way to make
the motor shaft turn the correct amount but it does not by any means guarantee that the telescope tube has moved correctly
because the of lash and other errors in the reduction gears, worm and main drive gear.
In the case of the LX200 drive, counts are generated by a disk with 90 slots which are measured by two photoelectric pickups.
The reason there are two pickups is so that there is no ambiguity about the number or direction of the count generated by the
rotation of the disk. With only one pickup it is possible to have false pulses generated and the direction of the encoder disk cannot
be determined. With two transducers very slightly offset, the transducers will generate a four state pulse code that gives both the
number of counts and the direction the shaft is rotating. This is a standard encoder technique known as bi-phase coding. (I will
not go into coding schemes here for the sake of brevity.) At the normal RA (sidereal) drive rate the motor shaft turns once per 8
seconds this yields a count of 45 pulses per second. So in order to drive the motor at the sidereal rate, the computer generates
45 pulses per second and the encoder is forced to respond with the same rate. When the motor gets behind the computer applies
more current to the motor and speeds it up. And vice versa.
In a similar way, once the telescope is synchronized with a known position, it moves the telescope to a new position by simply
demanding a calculated number of counts from the encoders on the RA and Dec motor shafts. This is a very simple, inexpensive
and accurate method of positioning a mechanism. One slot movement of the motor shaft encoder corresponds to 1.3333 arc
seconds of motion. There are 11.25 slot moves per second for the RA drive rate. (the encoder delivers 4 pulses per slot move)
This means that the motor shaft moves with a position precision of 0.333 arc seconds referred to the telescope tube. It must be
remembered that this accuracy applies only to the position of the motor shaft. Because of the factors mentioned above, the errors
in the gear drive are not corrected to this accuracy by any means. It is certainly not difficult for the computer to generate the
required differential position distances in the form of RA and Dec motions or for the Alt and Azm motions in the form of a pulse
count. There is a memory and a computer chip of considerable power in the telescope control system.
Now consider the more difficult issue of correcting the backlash in the drives. For the Dec drive there is a backlash correction
number which is entered by the user who selects it manually by observing the motion of the telescope when commands are given.
The computer simply remembers to add or subtract this number to the appropriate move command. Thus the seemingly difficult
backlash correction is taken care of relatively easily by the computer through the same motor drive counting mechanism. This
system only works if the mechanical lash is symmetrical and consistent. We will see that this is not usually the case.
The correction for the irregularities in the worm gear are also taken care of by the user through the worm correction facility
provided in the LX200 computer. This correction is entered by the user in the form of E and W pushes of the direction keys on the
keypad while manually guiding the telescope through the eyepiece. The computer knows the rotational position of the worm gear
by means of a transducer on the worm gear shaft. For an 8 minute period, one turn of the gear, a total of 200 corrections are
entered. This is one correction for each 2.4 seconds. At the nominal RA rate the computer sends 45 pulses per second or 108
pulses in a period of 2.4 seconds to the accumulator which the motor must match. Pushing the E key stops the motor for the
pushed period. This action also must subtract some pulses from the number entered by the computer for that period. Pushing the
W key doubles the speed of the drive. So this action must add to the number of pulses entered by the computer during the period
in question.
The precise algorithm used to add or subtract pulses in the computer for the 200 periods is not known. The number of pulses
added or subtracted during each 2.4 second period is small (probably only 3 or so) it is most likely that pulses are added or
subtracted slowly only for the brief periods that the keys are pressed. The details of the algorithm would be interesting to know,
but it is not an issue of principle concern here. It is apparent that the short pushes of the E and W keys that the operator enters
to keep the telescope on track during the training period are used to adjust the computer output many times during the 8 minute
period of the worm. A sort of incremental averaging takes place which smoothes the motion of the main shaft. It is probably
sufficient, at this time, to know that Meade has provided a very nice scheme for correcting the worm drive rate in 200 increments
per revolution and that the user can train the worm through setting up the smart drive mechanism with considerable accuracy.
Typical rate errors of 50 arc seconds can be reduced by a factor of 10 or more.
Meade states, in their instruction manual, that the Smart Drive can be trained and retrained as often as the operator likes and that
a sort of averaging takes place. The computer chip in the telescope could easily be programmed to do a very nice algorithm that
responds to the normal rate and the key pushes and enters in a file an appropriate correction which is then used to drive the
motor/reduction gear/worm as necessary for smooth RA. motion. We do not know the details of how the worm rotation is
corrected.
The main gears in the LX200 have 180 teeth. One turn of the worm is 2 degrees of motion of the optical tube. There is a 60 to
one reduction in the gears which means that one turn of the motor shaft causes 120 arc seconds of motion of the optical tube.
The optical tube must be aligned to a known star and the computer told the position of the star. This action sets the
synchronization of the optical tube and the computer. From this point onward the telescope moves in a differential mode. For
example, the øgotoÓ command tells the motor to make the required number of turns so that the tube moves from where it is
pointing to where it should point. The accuracy of each successive pointing operation is dependent upon the accuracy of the
previous one. It is suggested in the operating manual that with critical alignment of the telescope, the øgotoÓ commands will be
accurate to 2 arc minutes or better. This is certainly believable.
This type of position control is differential position control as contrasted to absolute position control.. Absolute position control
would require an encoder on the telescope shaft itself. In this case the control system would know the exact pointing direction of
the telescope. The differential encoder measures the amount of motion from one point to the other. That is why it is necessary
to establish a precise pointing direction after the telescope is turned on. This is normally done by one of the techniques described
in the operation manual. From that point on, the telescope keeps track of its pointing direction by counting the pulses from the
encoders. Such a differential system can work very well if the loading on the mechanical system is well balanced and symmetrical
and the drive does not slip or make an error at anytime during operation..
The encoder and electronics can easily count pulses and keep the motor shaft synchronized to the computer commands. The
principle problem with a system of this sort is that the main pointing shaft is not inside the control loop. It would be if the
encoder were directly on the pointing shaft. Unfortunately, an encoder accurate to 1 arc second would have to generate 369 X 60
X 60 = 1,296,000 pulses for one revolution of the declination shaft. This is quite impossible. If the worm intersected a main gear
with 360 teeth, one turn or the worm would have to be divided into 3600 divisions. If one required pointing to 1 arc minute for
the latter system the encoder would only have to have 360 divisions. Either encoder technology is well within modern design
capabilities.
So there are several ways to effect accurate computer controlled pointing. The LX200 system is reasonably good and inexpensive
to implement. But the gear reduction and worm drive must be quite accurate mechanically. Other schemes can be devised but
may be more expensive to effect.
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Tweaking and Rebuilding the Dec Drive
The following are my experiences while rebuilding two declination drives on the two LX200s that I own. One is a 10" and the other
a 12". Both telescopes developed large amounts of lash and showed retrograde motion in the declination drives. This made them
unsuitable for auto-guiding for imaging. The instructions indicate that some lash is to be expected when changing direction;
doing North to South reversals. The manual says that values of 2 to 4 seconds are normal. Since the declination drive speed in
normal guiding mode is 15 arc seconds per second of time, one can correct for the delay by entering a number into the computer
to correct for the lash. Nominally, the number entered is 15 times the number of seconds of delay. This number is entered once
and need not be changed. The number entered clearly corresponds to the number of arc seconds of mechanical lash in the
declination drive. Technically the lash should be entirely in the gear reduction train and should be quite symmetrical. Since the
maximum lash that can be corrected is 99 arc seconds, the actual delay time must be less than 6.6 seconds.
Many users have found this correction does not always work. Often, users have found much larger delays and delays that depend
upon the position of the declination axis, the direction of the reversal, the loading on the telescope and many other elements.
Hysteresis, dead zones, of up to 15 seconds have been reported. I too have found all of the above effects. The delay can vary
from a few seconds to 10 or 15 seconds depending on many factors. If the delay were in the motor/reduction gear train as
expected, it should not vary much since the "winding up" of the gear train is similar in either direction. Loading effects on the
declination axis are not strongly reflected back into the gear train because of the almost unidirectional transfer
of forces through the worm gear. Typically it is not possible, with a low pitch worm gear, which this is, to turn the worm at all with
any amount of torque on the main gear. Breaking of the gear would likely take place first. However, loading of the main gear, as
by unbalance of the optical tube will greatly increase friction between the main gear and the worm. Thus with an unbalanced
optical tube, considerably greater drive force through the reduction gearing is necessary.
A goal of this study of the declination drive is to determine the sources of excess declination drive lash and to eliminate them so
that the drive will come up to the specifications required for the declination lash correction to work properly. The main gear and
its clutch mechanism are impressively well build and quite strong. It is nice to have a 146 mm diameter declination gear since it
should provide good pointing accuracy. It might be noted that the same size drive is used on the 8", 10" and 12" LX200s. So
while the gear is adequate for the two smaller telescopes it is somewhat marginal for the 12". Many users have discovered that
end play in the worm gear mounting contributes to the reversal delay. This is certainly an important effect. With the given
characteristics of the gears, a quick calculation shows that 1 arc second of motion of the telescope tube corresponds to only
0.355E-3 mm of axial motion of the worm. This is a required tolerance that is incredibly tight. Thus end play in the worm drive
must be eliminated as completely as possible. The worm must be "snug" in its bearings and the entire drive platform must be
snug in its pivot mount. Adjustments are provided for in the Meade mount via an end screw on the worm shaft bearing and a
screw on the platform mount bearing that can be adjusted. Both should be tightened enough to eliminate all possible end play.
The end play results directly in rotary motion of the main gear and thus in the pointing accuracy of the telescope. I found the
mechanism in one of the telescopes well adjusted (nice and tight) but the other had significant end play.
There is however another source of play between the worm and the main gear. This is the radial motion of the worm with respect
to the main gear. For some reason, the worm in this design is on a "floating" platform which allows for motion of the worm radial
to the axis of the declination drive. It is hard to understand why this "floating" action is as large as it is. No other, of about a
dozen worm/gear drives I have inspected, has an action that allows for the large motion that this one does. If one carefully
measures the "float" action one finds that the worm can move as much as 0.5 mm radially. The amount of motion depends upon
the direction of reversal and also on the accuracy of balance of the telescope about the declination axis. If the full "float" motion of
the bearing platform is allowed, it results in 0.08 mm motion of the main gear edge which is 220 arc seconds of motion of the
telescope tube. This is a motion ratio for the worm to main gear surface of 7:1 which seems a bit large for this type of drive.
The forces upon the worm that push it partly out of engagement with the main gear are caused by the friction in the declination
bearings plus the forces due to unbalance of the telescope tube. This seems to be one source of the varying delay in reversal
operations. It is also the source of retrograde motion. After evaluating numerous operations of the drive with different unbalance
loads, it became clear that the amount of "float" is large, irregular and not necessarily repeatable. Motion of the bearing platform
was measured with a precision dial indicator and varied from 0.025 mm with the tube well balanced to the full 0.5 mm with a
substantial unbalance. In terms of tube motion this amounts to about 11 arc seconds with the tube balanced to 220 arc seconds
with substantial unbalance. The amount of unbalance used was 0.1 Kg-meter. Again both drives behaved in a similar manner.
While the smaller of these "floats" can be compensated for, the larger cannot since the declination lash correction is 99 arc
seconds maximum. There is also a time factor involved with the resettling of the "floating" platform to its new stable position. In
addition, the platform takes on a different position when the tube is being driven compared to what it takes when it is allowed to
rest. This settling of the platform position after ending a motion cycle causes the tube to drift of the order of 2 to 20 arc seconds.
The drive mechanism seems to sort of relax after being exercised. Both drives did similar things but each to a different, and
unpredictable, degree.
The computer based correction scheme would work with constant mechanical relaxation, it does not work well when the relaxation
is variable and erratic. As well as being dependent upon the reversal direction, the "float" and relaxation motion was much smaller
for a telescope tube that is perfectly balanced about the declination axis because then only the friction forces and acceleration
forces must be overcome. There is a small spring under the floating bearing platform that presses the worm against the main
gear. If this spring is strong enough, it can keep the worm pressed properly against the main gear as long as the unbalance is
small and the forces required to move the telescope tube are small. As the unbalance gets larger, the spring no longer maintains
good contact between the worm and the main gear. The concept that the telescope tube should be kept unbalanced to keep the
drive wound up in one direction is not valid in the case of the worm design. Unbalance only increases friction in the drive and
requires greater drive force. Adding unbalance generally will not help nor work consistently even if the end play and pivot play
have been "tweaked" out.
Unfortunately making the spring much stronger than the original causes the force and thus friction between the worm and the
main gear to become too large and the drive binds. The tiny motor which drives the gear train is not nearly as strong as the
motors used in many drives. This is an unfortunate limitation on any attempt to redesign and/or rebuild the drive, as I have found
out. Replacing the motor with a stronger one would probably require redesign of the drive electronics as well at which point the
entire system would have to be redone. Thus the only thing that can be reasonably done to improve the drive is to limit the
maximum "float" action of the worm platform and the motor/gear reduction parts of the drive. This can be done within the rubric
of "tweaking" the mechanism.
One might wonder about the design of the mechanism in the first place. Why is the worm on a "floating" platform at all. One
reason would be to keep the worm, on its floating platform and via a spring, to be held in optimum contact with the main gear.
Another would be to allow for slight run out of the main gear. The main gear run out should easily be kept to under 0.05 mm on a
gear with a 73 mm radius. In the case of the gear measured the run out was 0.1 mm. This is not a particularly refined tolerance
but it is not very bad either. Several other worm/gear drives investigated had better tolerances and did not use the "floating"
worm arrangement. If main gear tolerances are typically 0.1 mm, there seems to be no reason for a "float" of 0.5 mm. In fact,
there is an adjustable stop on the floating platform that limits the disengagement of the worm to the 0.5 mm observed. It seems
that this adjustment could be tightened up to limit the "float" to be not more than required for the main gear run out. Reduction
of the allowed worm platform motion was tried and does reduce the looseness of the drive linkage and the maximum slack
allowed. To do this, the motion limiting screw needs to be raised toward the bottom of the platform. It was possible to tighten
this tolerance until only 0.03 "float" remained on one drive and 0.05 on the other. This caused significant improvement in the
total slackness within the drive systems. The retrograde motion was reduced but not eliminated.
Looseness in the drives was improved greatly. Only about 80 to 120 arc seconds of slack remained compared to 220 arc seconds
without the adjustments described. Now another strange motion of the declination pointing mechanism was observed. When the
motion was reversed in either direction a small retrograde motion remained. This was finally traced to the mounting between the
bearing platform plate and the gear train housing on which the motor is mounted. Unbelievably, the entire drive
train/motor housing is attached to the worm bearing housing with four small bolts and a thick rubber ring or gasket (actually a
small "O" ring.) Thus the whole reduction gear train housing can move with respect to the worm bearing and when it does it
allows the worm to rotate with it. The amount of motion on one drive was 0.5 degrees rotation of the worm. On the other it was
0.2 degrees. This corresponds to an angular motion of the telescope tube of 10 or 4 arc seconds. Before the motor drive train can
move the worm any amount, the rubber gasket must go from clockwise to counter clockwise compression limits. (or vice versa for
a change in the opposite direction.) This working of the rubber gasket is undoubtedly complex and may cause jerky motion of the
worm often observed during reversals. First retrograde and then correct motion is sometimes observed. It is not at all clear
exactly why this strange phenomenon takes place. It was however, observed to be repeatable over many reversal cycles. It must
be related to the use of a rubber coupling element in the drive chain. It is a weird hysteresis
phenomenon which would not take place in a linear system.
It is very tricky to get at the rubber "O" ring. The entire gear drive assembly has to be dismantled. This operation is full of traps
and should not be attempted unless you are ready to replace a broken motor/gear drive assembly in the case that you ruin it. The
drive is assembled from the inside out and at several points items are glued into place and press fitted. It is exceedingly difficult to
take apart. The gear drive assembly was taken apart however and then tightly bolted to the worm drive platform and the entire
drive reassembled. The second drive was similarly reworked after the first was improved greatly.
Additionally, In both drive trains, it was found that the gear at the end of the worm shaft was not tight. In one case 5 degrees and
in the other 3 degrees of looseness was found. This accounts for most of the remaining looseness and consequent hysteresis in
the gear reduction system. Both gears were removed and found to have play between the plastic gear and the steel worm drive
shaft. The gear, probably nylon or delrin, has a flat "keyway" on one side which simply had become distorted and no longer
locked angular position of the gear to the "keyway" on the shaft. This was fixed by filling the distortion with epoxy and locking the
gear to the shaft with an added lock washer under the retaining bolt. This fix holds the gear very tightly to the shaft.
Operation of the drive mechanisms now took on a considerably different nature. The following motions were observed with no
hysteresis correction entered into the computer. There was now no retrograde motion. Instead, there was no motion at all for
about 3 seconds. This corresponds to 45 arc seconds of drive demand with no telescope motion. When stop action is called for,
the tube now stops immediately as it should and subsequently does not move at all. This is both correct and necessary because it
means there is no overshoot or drift. It does however require another 3 seconds for motion to take place in the
opposite direction. This confirms the symmetry of the 3 seconds of hysteresis in the drive. This amount of delay is similar to that
expected when the drive is operating to specifications stated in the operators manual.
One must conclude that when requesting reversal of declination motion, there is a total windup in the gears, worm and main gear
of 45 arc seconds. This seems like a lot of windup in the gear train but it is only a very modest set of plastic (with some metal)
gears. The system as adjusted is now very tight mechanically, but still very smooth running. Since this wind up is symmetrical
and consistent in amount, it can now be compensated for by the declination lash compensation. The compensation entered into
the computer simply causes the drive motor to windup the required amount in the desired direction so that mechanical lash is
absorbed and the forces applied are just enough to start motion of the declination axis.
Both drives, after many hours of remodeling and "tweaking" are now operating fairly well. They seem to be smooth and reverse
with consistency. Apparently no amount of "tweaking" will make the coupling between the motor shaft upon which the encoder is
mounted and the declination axis absolutely tight. This is to be expected with the very simple gear train used. Only expensive
spring loaded gears as used in precision servomechanisms would be free of mechanical lash. Thus it is fortunate that a very clever
computer fix for this problem has been provided. The mechanical hysteresis problem is probably extant in most drives of this type
but is usually not amended.
The conclusion of this study and experiment is that the floating worm drive design while a bit unusual is probably necessary in a
mass produced drive so as to account for production tolerances. And also that it is possible to adjust the drives to optimum
condition by "tweaking" them on an individual basis. This may take several hours of careful mechanical reworking. In addition to
tightening the looseness in the declination drive, it is useful to reduce forces due to unbalance and acceleration. The first is done
by balancing the telescope tube carefully. The second can be reduced by reducing the slewing speed to less than 8, the default
value. A slew rate of 2 is actually the same as the "find" rate which is still fast enough for most GOTO operations. Additionally the
mechanism is not so noisy as to attract embarrassing comments from fellow viewers. As a compromise, a setting of 4 might be
used. Immediately after the computer boots, I generally set the slew rate to 4.
I am pleased with the improvements I have effected by "tweaking" on the declination drive gearing. The actions are now very
tight and very similar for N to S and S to N direction changes. The rebuilding of the drive that I have effected now brings both
telescopes within normal tolerances of 2 to 4 seconds so they can be appropriately compensated using the declination lash
computer setting. I would also note that "tweaking" has tightened up the drives mechanically a bit so it should be determined that
the motor does not stall with any loads used. A stalled motor will heat up as will the driver circuits with possible dire
consequences. Also note that the motor is least likely to stall when used with its full voltage ratings. Running motors run cool.
Stalled motors get hot. That is why low commercial voltages or "brown out conditions" sometimes burn out motorized equipment.
I hope this study and analysis of the declination drive yields useful information to those who are having problems with it, want to
understand better how it works or want to try to improve it or bring it into required specifications. There are many details to be
observed in the rebuilding of these drives. I am not recommending it be undertaken except by persons with considerable
mechanical skill. Some electrical skill is also an asset. I accept no responsibility whatever for the results of any attempts to
"tweak" the declination drive by experts or klutzes. If you klutz it up, it's your own fault. :-)
Return to Beginning
Disassembly of the LX200 Declination Drive -- Details
In order to remove the drive mechanism, it is first necessary to remove the large, black declination locking knob. Be sure to place
the telescope tube in a rest position first. Screw the declination knob entirely out of its socket. The cover plate can then be
removed by removing the three allen head screws holding it in place.
The drive mechanism is then immediately visible. The first thing to do is to check the action of the drive visually. Insert and
tighten the declination locking knob. Put the telescope in LAND mode and turn it on. Now check the action of the declination drive
in two ways. Set the declination lash to 00. Viewing through the optical tube at an object, time the number of seconds it takes
for the tube to move when a reversal of the tube is commanded from the keypad. This time should be symmetrical when
reversing from N to S and vice versa. If the time is symmetrical and less than 6 seconds, the lash is correctable using the
declination lash entry. Enter a number which is 15 times the time in seconds. If the motion is not symmetrical or the lash is more
than 6 seconds, you have a problem that will need attention.
Next, observe the action of the moveable platform when the drive is commanded to move S and N and so forth. This motion will
depend upon the balance of the optical tube. A perfectly balanced tube should force very little motion of the platform while a large
unbalance will usually force the platform to move against its stop. The stop is beneath the platform in the form of a screw which
protrudes toward the platform. This screw can be adjusted but only with the mechanism removed from the telescope. More on
this later. If the motions you see are large and not symmetrical you have a problem that will need attention. If you move the
platform with your finger, you will see appreciable motion of the optical tube due to the fact that the worm is moving radially with
respect to the main gear.
Unfortunately, the stop, the end play on the platform nor the end play on the worm shaft can be adjusted without removing the
drive from its housing. To remove the drive do the following. Simply unplug the connector which is on the inside of the fork mount
at the location of the declination drive connector plug. There is an outside and an inside plug at that location. The entire drive can
now be removed. It is held in place by two allen head cap screws. These can be loosened with a 9/64 allen wrench. For
convenience I suggest a long- shafted, handled wrench. Carefully slide the mechanism out. Be careful to hold the platform and
its mounting plate together since there is a small, loose spring under the platform which will jump out and hide. There is also an
abundance of very slippery black grease on the worm and the main gear which will get on everything. Retain all of the grease
possible.
Now check the end play in the platform bearing and the end play in the worm screw. Both should be very tight since end play in
either results in direct motion of the main gear. They can be tightened if loose with the obvious adjustments at the end away
from the gear box. If these end plays are very snug, proceed to the following.
What has to be done to the drive to tighten up the action depends upon how worn, damaged and out of specification it is. The
next step is to try an adjustment of the stop on the motion of the drive platform. There is a screw with an allen head that
protrudes from the mounting plate toward the platform. For convenience in adjustment with the drive in place in the housing,
remove this screw and replace it with a cap head screw of ¾ inch length. This screw will protrude far enough to push the platform
closer to the main gear but also protrude enough through the bottom of the mounting plate so that it can be adjusted with a long
nosed pliers while the drive is in place.
It is now worth while testing the drive to see if tightening up the platform motion will fix the problems. Reinstall the drive and
reconnect the electronics and motor. Now do the same tests as above with various settings of the stop screw. I obtained
considerable improvement by reducing the platform motion from the original 0.5 mm to 0.05 mm. Reinstall the drive and check it
as described above. If the delay is less than 6 seconds and symmetrical the drive is within specifications and you have fixed the
problem. Ideally, the lash should be 2 to 4 seconds which is the specification described in the operation manual. In fact 2 to 4
seconds is the lash in the gear train itself and the minimum lash that can be expected.
In the case that the above tactics do not fix the drive, the following instructions for a more complete overhaul and modification of
the drive mechanism can be undertaken. Do not do this part unless you have confidence that you can finish the job. It is quite
tricky.
There are three additional areas where lash can take place. One is lash in the gear set itself. This cannot be corrected but should
(must) fall well within the limit that can be corrected with the declination lash setting. Another is in motion between the gearbox
structure and the worm gear shaft housing which results from the slightly flexible rubber connection at that point. This is a very
small amount and may not need correcting. (I corrected it anyway in the course of fixing the third problem area.) The third area
of lash, which existed in both of my drives was a loose gear on the end of the worm drive shaft. This looseness accounted for 5 to
11 seconds of lash all by itself. I believe one of the gears, that on the 12", wore itself loose through use of the telescope over a
period of about a year. The 10" has only a few hours of use so I assume it was out of specification originally. The connection
between the gear and the worm shaft is under the full torque of the motor increased by the gear ratio which is 60. (The motor
turns 60 times as fast as the worm and thus the torque is 60 times larger.) This is the point of largest torque in the gearbox and
was apparently too large for the connection between the final gear and the worm shaft.
The gear has a flat keyway which engages the flat keyway on the shaft. That is the shaft looks like a "D" and the gear has a "D"
shaped hole. The gear is held on to the shaft with a simple screw. Do to the torque and the relative softness of the gear which is
plastic, the flat keyway in the gear becomes distorted and allows the gear to slip on the shaft. The amount of slippage is only 3 to
5 degrees but that corresponds to a pointing accuracy of 60 to 100 arc seconds. This is clearly a significant amount. I do not
know why the gears in both of my drives became loose but they did and thus had to be tightened.
The following material describes alternate repair schemes. But first determine if your worm shaft gear is tight. Remove the drive
from the telescope. Remove the piece of sticky tape from the top of the gear box. Lock the worm shaft by inserting a thin rod in
the hole in the shaft. Carefully apply turning force to the manual declination knob and observe the motion of the gear and the
worm shaft. There must be no motion at all between the gear and the shaft. This can be determined by watching the gear teeth
closely. In my cases the gear clearly moved a fraction of a tooth. I estimate 5 degrees for one and 3 degrees for the other. This
lash must be eliminated entirely. Here are two ways to do it.
The first way is less intrusive than the second but still very tricky. In order to get at the screw that holds the gear on the shaft
without opening the gear box, a hole must be drilled into the side of the drive unit. This hole must be large enough to allow the
screw to be extracted. Also, the drilling must be done slowly and cleanly so that cuttings do not get into the encoder mechanism.
Unfortunately, the encoder is exposed to the interior of the gear box even though there is a membrane inserted to protect the
encoder. A ¼ inch hole must be drilled directly in line with the worm shaft and a similar hole must be made in the protective
membrane. With this done, the screw holding the gear to the shaft can be removed. A drop of epoxy is then put on the shaft
end, a lock washer put under the screw and the screw inserted and tighten as tight as possible. This fix will lock the gear to the
shaft tightly and forever. The hole in the membrane can then be closed with a piece of transparent tape and the hole in the
housing closed with a sticky pad similar to that used on the top of the drive. When drilling the hold do it very slowly, finish the
hole by hand, keep the encoder part to the top, clean out all chips thoroughly. I fixed on drive using this method.
I later decided I did not like the rubber "0" ring connection between the gear reduction housing and the worm shaft mount. So I
finally did both drives using the following more intrusive method. To get at the rubber mounting requires complete disassembly of
the gear box housing. To do this remove the manual declination knob from the long shaft. Before undoing the four Allen head
screws that hold the side of the housing in place, look carefully at the bottom edge of the joint between the housing and the side
plate. You will see the end of a very thin sheet of material clamped between the side plate and the housing. With a sharp scribe,
mark the end of this sheet. This sheet is the end of a very thin metal mask for the encoder disk. It will fall out when the side
plate is removed and must be replaced EXACTLY in the same position. If this frightens you stop now. However, if you are brave
and adventuresome, remove the side plate and slip it off of the long shaft. You now have the whole innards of the gear reduction
set in sight including the very thin encoder disk. There is a clear membrane that separates the gears from the encoder disk. This
will have to be removed. Use a sharp awl and scrape the cement holding it away so it can be removed. It will have to be cut to
remove it and it will not be reused. This step is not entirely desirable since it may lead to long term problems with contamination
of the encoder disk and mask. But it can not be helped. This is a point of no return. If you get cold feet, you must turn back
before you remove the membrane.
With the membrane out of the way it is now possible to carefully remove the gears on the long shaft from the housing. Be careful
not to damage the encoder disc. It is quite flexible but must not be permanently bent. Once the long shaft is out, it is easy to
remove the gear from the worm shaft. This exposes the four screws that hold the housing. Remove these and the gear housing
will come loose from the worm shaft mounting. Remove the rubber "0" ring and replace it with small washers placed on the
screws to he same clearance. Reassemble the two pieces. Remount the gear on the worm shaft as described above with epoxy
and a lock washer. Then reassemble the gear housing. Be sure to get the encoder mask in exactly the correct place. This can be
facilitated with a very tiny spot of glue. Reassemble and tighten everything and put the whole works back into the telescope.
The entire drive system is now tightened up as much as possible. With the drive in place and everything connected electrically, try
the drive and test it in the way described at the top of this procedure. With a bit of luck, the lash will now be about 2 to 4 seconds
and you can remove its effect with a reasonable setting of the declination lash setting in the computer. I found both drives to
meet specifications and they now work satisfactorily.
The procedure described is not simple to carry out. Considerable mechanical dexterity is required as are a few small tools. Only
try it if you have full confidence in your ability to complete it.
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Rebuilding
Details
Beginning
LX200 Mechanical Index
MAPUG is hosted by
Vibration Measurements on an LX200 Classic
This information is for those interested in telescope motion, vibration and oscillation. This information is about the static deflection
characteristics of a Meade 10 inch LX200 mounted on a Giant Field Tripod with and without a Super Wedge. This information has
be checked for accuracy and some re-interpretation has been included as the result of added experience since March of this year.
The purposes of providing this data are two fold. One is to show that for this telescope and mount there are specific deflection
characteristics related to the position of the applied force and the various bearings which make up the mount. The second is to
provide information about where the weakest points are so one can determine where it pays to spend effort in an attempt to
improve the stability of the mount and thus reduce vibration.
This information is for only one instrument. A different sample of the telescope may provide somewhat different results. On the
other hand, these results are entirely consistent with measurements which I made some time ago on a 12 inch LX200 mounted on
a very strong permanent pier.
This data is for static deflection of the main tube. However, there is good correlation between the static deflection and the
sensitivity of the main tube to vibrations caused by wind, motor drives, bumping, irregular drive forces and the like. So it has
considerable value in judging stability for imaging. I strongly believe that knowing these values really helps to both understand
the telescope and its mechanical mounting and reinforces the ability to improve it. I have been seriously trying to get pointing
stability to be as good as the best seeing conditions. This is in the area of 1 arc second.
The 10 inch telescope was set up in my basement laboratory with a series of weights, strings and pulleys arranged so that the
telescope could be pulled with a known force in any direction. The tube was focused on a target calibrated in arc seconds of
deflection. The setup is very simple so that anyone interested can easily duplicate it to measure their own telescope.
Here are the results for the telescope mounted directly on the giant field tripod using only the center bolt, very firmly tightened.
The legs were not extended and the tripod sitting on a cement floor. The tube was pointed horizontally and the fork set up with its
long dimension parallel to the front of the control panel. The force was applied by a 1 kilogram weight applied directly or via the
string and pulley arrangements. All deflections given are arc seconds per kilogram.
Forces are applied as follows and deflections obtained given. The units are arc seconds per kilogram.
Force
Force
Force
Force
applied
applied
applied
applied
to
to
to
to
the
the
the
the
tripod head back and forth and side to side gave a resulting motion of 2 arc-sec.
fork forward and back: motion 20 arc-sec.
fork side to side: motion 15 arc-sec.
fork up and down: motion 1 arc-sec.
Conclusions: The tripod is very solid and should not be a concern with this mode of mounting. About half of the deflection
seems to be at the fork bearing and about half at the declination bearing.
This data is good news and bad news. The good news is that the tripod is amazingly stable when placed on a solid concrete floor.
Adding weight to the tripod or other braces cannot hurt but will not do much good either. More spongy surfaces might be made
more stable by adding weight because the tripod legs are forced into the surface more.
The bad news is that about half of the motion is at each bearing. The left and right and the up and down motions are difficult if
not impossible to separate. However, the deflections for the fork bearing are about half the total for both fork and declination
bearings. Thus about half is in each and that means that for significant improvements to be effected both have to be improved.
For polar mounting using the Superwedge, the measurements are as follows:
Force applied to the tripod head back and forth and side to side.
Resulting motion 2 arc-sec.
One major conclusion has to be that the telescope behaves quite differently when polar as compared to Alt/Azm mounted.
The tripod and the super wedge structure are very solid. The tripod and wedge act very much like a solid unit. Flexing of the
super wedge is insignificant compared to that in other parts of the fork and bearing structure. The side to side stiffness of the fork
is much improved in the polar mount. This fact is verified by the similar motion of the tube to up and down motion but by the
greatly improved stiffness of the tube to side to side motion. This does not necessarily mean that the polar mount will, in practice,
be more stable however as described below.
This result is entirely in agreement with bearing theory. That is, the bearing stiffens greatly when it is loaded. The great loading
upon the fork bearings when the telescope is in a polar mode is clear and the resulting stiffening in the lateral motion is clear as
well. It is for reasons of stability or stiffness that bearings are often "pre-loaded."
Thus contrary to expectations for stiffness of the telescope tube, bearings and mounting system, the entire machine seems to be
quite stable in the polar mount mode. At the same time, the great dominance of one direction of instability, up and down motion
at the eyepiece, is very bad news. It means that the stability of the telescope is dominated by this motion in both modes of
mounting and that it is the largest component.
The statements in the preceding paragraph should not be interpreted as saying that the telescope, in actual use, actually wobbles
less in the polar as compared to the Alt/Azm mode. In fact, forces which tend to make the telescope move in either mode
probably tend to be caused gravitationally. This means that the Alt/Azm mounting seems much more stable since the telescope
mount is very stiff in the direction of the gravitational vector.
The worst news is that if the springiness is mainly in the bearings, as it certainly seems to be, there is almost nothing that can be
done about it. Only a major redesign of the mount would enable improvement. Reducing wobbles must then be accomplished by
reducing any irregularities in the drive motion and any external forces such as wind, stomping around the pier or tripod supporting
surface and definitely not bumping or even touching the telescope while imaging.
I have checked out bearing specifications and standards in various mechanical design manuals and these results are quite good for
the bearings used and the way they are mounted. The main problem with the design is that the bearings are very close together
so that the forces generated by the torque in the polar mode are very large. For two bearings to provide excellent stability of a
shaft, they must be separated more.
I have come to believe that the bearings used by Meade are quite good and in fact probably as good as can be expected within the
cost parameters of the mechanical as provided. Also, not much can be done to improve the bearings without massive rebuilding
and added cost. Users will simply have to be careful not to disturb the telescope while imaging. This includes wind protection,
stable mounting of the equipment and avoiding touching the equipment while using it.
It is also clear that more complex and costly designs can and have been effected if significantly greater resources (money) are
applied. There are some damping and control devices that can be brought to bear but these are complex to design and use.
One device that can improve damping of vibration is a mechanical damper. I have not seen a design that uses this technique on a
telescope. They are very common on larger motor and engine mounts. Another is active adaptive damping on which I am
working personally. (but at a slow pace) Still another is a method which does not damp the mechanical system, but adjusts the
optical system to stabilize the image directly. This is a promising technique which we will soon see in practice. It is called Active
Optics.
Stabilizing a telescope to one arc second, if that is required for good imaging, is very difficult. But it is a reasonable goal when
imagers and seeing are of that same order.
Return to Beginning
MAPUG is hosted by
Oscillations of a Typical Fork Mounted Telescope.
As well it should be, the issue of vibration or oscillation of a fork mounted telescope is of high interest to users of LX200 type
telescopes. I have investigated this problem in considerable detail and present my data and analysis herewith.. I have taken
extensive data on two telescopes, a 10 inch and a 12 inch Meade LX200. The 12 inch is mounted on a superwedge on a 4.8 ton
concrete pier imbedded in bedrock. The 10 inch is mounted on a superwedge on a giant field tripod sitting on a very large concrete
slab.
Both wedges are set for 43 degrees which is my latitude and all data and conclusions drawn are for this setup. Some conclusions
can be easily extrapolated to other setups but be warned that these instruments are full of surprises. Most of the conclusions can
also be extrapolated with a bit of thought to German mounts since they also exhibit serious oscillations. Because I did an
extensive amount of work on gathering data and working out the results and conclusions, I will take time and care to explain my
techniques and results at some length. Thus be warned that this is a very long memo.
All telescopes and their mounts, consisting of forks, rods, bearings, tubes, shafts, masses and the like can be thought of as
mechanical systems that are basically masses and rods which have compliance connected in some fairly complex configuration.
Notice I use the term compliance. This is the inverse of springiness. A soft spring has high compliance and a stiff spring has lower
compliance. The masses move because of the necessary forces applied by the drive motors which guide the telescope optical
tube. But they move in unwanted ways as well do to forces from wind, shaking of the support, or by irregular drive forces from
the very motors that move the telescope or irregularities caused by the mount and bearings. The unwanted vibrations,
oscillations, jiggles and wobbles must be minimizes to accomplish good viewing and particularly good imaging.
All forces on the telescope and the subsequent motions from whatever source are mostly measurable and predictable within the
accuracy of the model used for the telescope mechanical system. A rather simple, first order model used in this analysis gives
relatively accurate results. The trick needed to make sense of the problem stated is to make a model that is trackable but still one
that gives some useful results for the most important deviant motions of the observing/imaging tube. I have found that the
simplest model, fortunately, accounts for the main oscillation of the tube that effects viewing and imaging. While there are
numerous secondary jiggles and wobbles they are mainly of second order. If the main oscillations can be reduced or damped,
greatly improved imaging will result.
The simplest model consists of three parts. The first is a base consisting of the pier/tripod and the wedge. With the pier/tripod
and super wedge used, this base can in first order analysis be considered stationary. To be sure, there are inadequate piers and
tripods sitting on unsatisfactory concrete pads and unstable ground, but for the setup used for this data, the wedge surface upon
which the telescope base is mounted can be considered rigid and unmoving. It is certain that less than 10 % of all unwanted
motion can be traced to the wedge and its mounting.
The second part of the mechanical structure is the fork, in the case of the LX200, and its bearing structure. Within this part, the
fork is quite rigid (stiff) but the bearing structure is quite compliant. Essentially all of the flexure in the RA/Asmuth drive is in the
bearings. Thus close attention must be paid to the bearing structure as the "weak" point and the largest source of unwanted
motion and oscillation.
The final element is the main telescope tube, the toys attached to it and how it is held by the Declination/Altitude bearings. The
tube and items fastened to it may in the first order be considered a single rigid mass. This mass moves on the declination
bearings. These bearings are so strong and stiff that they do not contribute much to the dynamic oscillations of the structure.
That is not to say that they a free of problems, but they contribute very different problems from the vibration and wobbles which
are of primary interest here. For the first part of the analysis we consider the tube and fork to be a single rigid mass.
The mechanical structure becomes a first order system with a spring of some compliance holding a mass out at an angel of 45
degrees. The large mass consists of the main tube, its attachments and an appropriate part of the fork mass. Fortunately this is
the simplest mechanical vibratory system possible. It will have two important properties. Frequency of oscillation and damping.
The frequency is simple to understand and is measured in Hz. (used to be called cycles per second, cps) The structure is similar
for the Alt/Azm mounted telescope but the forces are arranged very differently. This case will be discussed later in the analysis.
The second factor is a bit harder to understand. I will use the term damping (or its inverse, Q, or quality factor) in this
discussion. It is only necessary to understand that a weakly damped system will oscillate for a long time and a highly damped
system will not oscillate very long at all. The inverse, Q, means that a high Q system oscillates for a long time and a low Q system
does not oscillate very long. Long and not very long are relative terms. In the case of a telescope long might be 10 seconds while
not very long only a second or less. Clearly for the telescope we want high damping and low Q. Such a system will stop
oscillating quickly when it is excited by unwanted forces.
Unwanted forces are wind, bumping, pier/tripod movement of any sort and any irregularity in the forces from the drive motors.
Additionally, any irregularities in the moving bearings or surfaces in sliding contact an be interpreted as irregular forces. Some of
these forces are random and seldom. Others, like that from the drive motors are regular and constant.
My study of the two telescopes in question consists of two parts. The first was to take some data and the second to try to figure
out what in the world the data meant and just what was going on within the structure. I believe I have deciphered some of the
mechanical problems and pinned them down to specific parts of the mechanical design of the telescope. On the final part of this
discussion I make some suggestions to minimize the oscillatory problems.
To take vibration data at low frequencies I used a geophone transducer (low frequency magnetic type). The output went to an HP
spectrum analyzer which could record and print out amplitude and frequency data. The telescope was excited by careful,
measured tapping on it in a variety of directions and at many locations. I did not use the more sophisticated impulse hammer
since the structure is not too complex and I did not need transfer function information for the approximate analysis done.
The transducer measures in only one direction at a time so it was moved to several locations (clamped on) on the fork and mirror
tube. Almost all of the motion of the mirror tube turned out to be in a direction up and down with respect to the fixed base of the
telescope. Lateral jiggling of the fork and tube was relatively much smaller. Thus I concentrated attention on the major mode of
oscillation. Measurements were made on the vibrational spectra to determine amplitude and frequency components. Time
domain measurements on the damped oscillations yielded the damping factor. The results are described below.
Measurements made of velocity of motion at various frequencies. These were converted to amplitude of motion of the mirror tube
and finally into angular motion of the mirror tube. It is this angular motion in arc seconds that is important in seeing and
imaging. The compliance of the fork mount was calculated by measuring the deflection of the fork with a known weight placed on
the fork at the declination axis. Many positions of the transducer and location of excitation (tapping) were tried. The following are
my general conclusions and they apply almost equally to the 10 and 12 inch telescopes.
In all cases the resonant frequencies of the telescopes were in the range of 8 to 18 Hz. Several frequencies occur at the same
time and vary in amplitude as the oscillation damps out. This is because there are several modes of oscillation in the tube and
fork and they interact by exchanging energy since they are not orthogonal. (i.e. the structure, while fairly rigid is of complex
shape.) Damping of the main frequencies was in the order of 8 to 10 seconds. This is bad news since it means that for short
exposures up to a minute or so one must wait after touching or disturbing the telescope for at least 10 seconds for the scope to
settle down. This is longer than generally mentioned in the literature.
The immediate force caused by camera mirror flip must be allowed to damp out before the shutter is released. The forces external
to the camera caused by a shutter releasing are probably not enough to cause problems. (depends on the camera of course)
Lateral motion of the fork/mirror tube, in response to tapping, was very small and damped in less than 1 second. Thus it is
reasonable to conclude that unsharp images are caused mainly by up and down motion of the fork structure no matter what the
pointing direction of the optical tube. This motion will manifest itself at the image in differing directions depending on the
orientation of the tube and camera with respect to horizontal.
Because the fork is very stiff, low compliance, it can be concluded that essentially all of the compliance is in the bearing mount.
To put that simply, the bearings are slightly weaker than they should be. The actual numbers for these two telescopes are given
here. Because they are so similar it seems to me that they are indicative of what other telescopes of the same type would show.
But remember that these figures are specific to these two instruments.
The main mode of oscillation along with cross coupled modes was at frequencies of 8 to 18 Hz. Strong modes were at 8, 10, 12,
14 and 18 Hz. Some smaller but measurable oscillations were at 25 to 40 Hz. With a rather large collection of small modes within
that range. Tapping the mirror end of the tube caused the larger oscillations as did tapping the dew hood. All North, South, East
and West jogging of the scope using the keypad controls caused oscillations which persisted for as long as 10 seconds. The North,
South jogging caused the worst jiggling. Additionally, there was a distinct but small jiggling due to the steady running of RA
motor and the declination motors as described below.
The compliance of the fork was about 1 1/2 X 10E-5 meters/newton. It was about the same for both telescopes. (not surprising
since the bearings are the same for the two telescopes) This value taken with the known mass of the fork/tube structure gives a
calculated oscillation frequency of 9 Hz which is in agreement with the measured value. It was slightly higher for the 10 inch
telescope at about 11 Hz.. The compliance of the fork mount can be attributed mainly to the bearing and/or its mounting in the
base structure. The precise springiness will depend upon the exact construction details of the bearing and their mounting. I doubt
that attempts to reconstruct the bearing and mounting would be practical. It is actually quite good for a mount of this type and
cost.
Interestingly, the running of the RA. drive motor causes a small bit of vibration of the fork/tube structure. This vibration is at a
very low level (less than 0.3 arc sec RMS) for the RA drive in the polar mounted position. This should cause no trouble since it is
so much smaller than atmospheric image jiggle. Recent data showed the declination motor vibrational noise to be about 3 to 5
time as great as the RA motor vibrational noise. This is to be expected since the measurements were made on the optical tube
and it is very close to the declination motor as compared to the RA motor. This factor is about the same for polar or Alt/Azm
mounting of the telescope. However, in the Alt/Azm mode, both motors run all of the time. While in the polar mode the RA motor
runs all the time but the Dec motor just runs occasionally. (depending on the accuracy of polar alignment)
The amplitude of the oscillation of the main fork mode is by no means negligible. A static force caused by a weight of 10 grams
causes a deflection of about 1 arc second. This is a very small force. Larger forces cause proportionally larger angular
fluctuations. With good seeing, which can be 1 to 2-arc seconds (even in Wisconsin), forces caused by slight winds are definitely
deleterious to good images.
The previous section gives a general description of how the vibration problems can be found and a very qualitative description of
their magnitude. At this point, we need to ask if these qualitative measures, like a tap on the side or fork, can be quantified a bit
better. First let me say that getting quantitative measures is very difficult. A force due to just touching the telescope causes
shaking which is very large compared both to the accuracy necessary for good imaging. Touching is a large disturbance compared
to the measure of shake and force applied in my more precise measures of mechanical vibration in a structure as sensitive as a
telescope.
The precision required in imaging while actually moving the imaging tube in a precise way is near the very limit of what can be
done mechanically. (with a non feedback, passive system) I am assuming that the goal of this investigation is to determine the
problems and possibilities of solving them for imaging that lies at the limit of viewing. For convenience I will set this limit at 1 arc
second. We know that on nights of very good seeing we can easily resolve the double double in Lira so that 1 arc second seeing is
not uncommon. Also we know that any reasonably good optics will give us arc second resolution from the instrument itself. In
any case, I have chosen to use 1 arc second as the unit of measure for most of the following specification of units of jiggle and
wobble of the telescope. I have also found that it is very easy to see one arc second of motion of the telescope viewing tube when
it is moving at typical frequencies of 8 to 12 Hz.
We need to quantify the unit of "tapping" used so loosely in the previous sections. A major question is whether to define the "tap"
in terms of energy or momentum transferred to the telescope structure via the "tap." By experimenting with a variety of small
weights dropped from various heights on the telescope structure I have found that a weight of 10 grams dropped from a distance
of 10 cm gives just perceptible visual oscillations in the image. Thus, this unit of energy and momentum is used as the basic unit
for the "tap." This is quite arbitrary but it provides a unit which is of convenient size and can be applied in reasonable fractional
terms or in terms of a few or a few tens of "taps." Thus for this work I hereby establish the standard "tap" as 1400 gram-cm/sec
(momentum) or 196E3 ergs (energy). This unit of energy or momentum can be obtained by dropping a 10 gram weight (like a
bolt or nut) on the telescope from a height of 10 cm (about 2.5 inches.) It is easy to adjust the size of the tap by using a more
massive bolt/nut or by dropping from a greater height. This is a very convenient unit of measure. I will call it 1 (one) standard
"tap" in the following discussion.
Note that both the momentum and the energy increase in proportion to the mass but the momentum goes in proportion to the
height of the drop and the energy goes in proportion to the square of the height. Since the energy of the impact may or may not
be conserved but the momentum is always conserved, I will generally do my measurements and report the results in units of
momentum. Thus the standard MOMENTUM tap will simply be called one "Gtap." (a unit of momentum obtained by dropping a 10
gram weight 10 cm in a Gravity field of 1 g) So what does a unit Gtap do to a typical telescope. In the previous section a
qualitative discussion of the various motions of the fork mounted telescope was discussed. Here are some of the results for Gtap
tapping the 10 and 12 inch Meade telescopes.
The telescope is mounted as stated previously on a fixed super wedge set at 43 degrees, with the tube set to point to the southern
horizon, one (1) Gtap applied to the fork mount at the declination bearings will cause 5 arc second of motion of the tube. If the
Gtaps are applied to the eyepiece, one gets about twice that much motion. That is, about 10 arc seconds. This makes very good
sense since the momentum transfer at the eyepiece is further from the point of flexure. The flexure is mainly at the lower fork
bearing but with some at the declination axis as well.
At this point you can do some measurements of your own by viewing an object, dropping a weight on the telescope structure and
estimating through the eyepiece how much motion in arc seconds results. This is really easy and fun to do. The results are
however not fun to contemplate at all. The motion is large for very tiny forces and resulting oscillation takes 5 to 10 seconds to
damp out.
An interesting question is that of camera operation shaking the telescope. I use Canon F1 mechanical cameras of the second style
for film imaging. (Canon experts will know what that means) I have tried to measure the vibrations caused by operating the
camera without mirror lock. It is so small, I cannot measure it with any accuracy. Thus I believe that mirror flop causes very little
problem in the case of the Canon camera. The momentum kick that the camera gives to the telescope is surely less than 0.2
Gtaps. The shutter function alone causes no mechanical motion problem at all. Other cameras may be much different, I do not
know.
I have found that touching the telescope even with the most gentle and careful touch causes motions of the telescope that are
very large compared to those caused by even a few Gtaps. DO NOT TOUCH THE TELESCOPE WHEN IMAGING. Bumping it when
manually guiding is even a worse disaster since this level of "tap" can even cause permanent mis-adjustment of the mount. I
have found for example that when using the 201XT auto-guider or the 216XT as an auto-guider in stand alone mode that touching
the button on the rear of the guider can cause loss of star lock. This is one reason that it is better to use a computer controlled
guider like the 208 or 216 since the guider does not have to be touched.
We now have both qualitative and quantitative measure of the mechanical problems with a class of fork mounted telescopes. All
telescopes, whether fork or German mounted have the similar problems with rigidity and sensitivity to taping. Do not
underestimate this problem. The discussion below is on what works and what does not work to solve the very real problem of
telescope oscillation, vibration, shake and wobble. I have thought about these problems very hard and long but have not really
solved the problem. At this point I think there are three paths to take. The right and proper one is to consider what can be done
to the fork mounted SCTs to improve them to the point where they are as good as possible without drastic rebuilding or possibly
with only a minor modification or addition of a modest attachment of some sort.
Almost all telescope mounts with a few notable exceptions are of the three basic types: fork mount in polar or Alt/Azm
configurations, variations of the German mount, always polar, and Dobson type mounts, mainly Alt/Azm. For an overview of very
interesting variations see "Unusual Telescopes" by Peter L. Manly, Cambridge Press 1991. Fascinating book!!
It is convenient to have the fork mount and German mount (GEM) in mind at the same time when discussion mounts that will
generally be set in a polar orientation. (The Alt/Azm mounting of the fork is really limited to visual work, unless some day a
suitable de-rotator is developed.) So I will limit this discussion to polar mount and imaging quality. Both the fork mount and the
GEM are really similar. They have a main rotating bearing pointed toward the pole and a structure to hold the viewing tube and
allow it to be set at an appropriate declination. The fork mount holds the tube between its two tines while the GEM holds the tube
on one end of a rod and a big weight on the other to balance out the tube. The fork mount has the problem that the declination
axis has to be extended out along the polar axis so that the tube clears the polar axis bearing structure. The GEM can have the
cross bar mounted very close to the polar axis bearing for great rigidity. The fork mount has a very rigid and strong declination
axis since it has very short coupling shafts to the declination drive motor. The GEM has a very long declination shaft which must
hold the viewing tube which is long and has a great moment of inertia on one end and an equal weight on the opposite side. We
should have no illusions that either a good or bad mount of either type can be made. Each mount has its strong and weak points.
Clearly a good design must resolve the weak points of each approach. The difficulty of designing the mechanical structure can be
appreciated if we recognize that to hold the tube to 1 arc second (which is 5 millionths of a radian) requires the ends of a 12 inch
tube to be held firm to 0.05 thousandths of an inch. This is very hard to do. The compliance in the mount must be exceedingly
small so that slight irregularities in the force on the tube will not cause unwanted motion. In other words, the mount must be very
rigid (strong, low compliance). Additionally it must move freely in both declination and right ascension. The design problems are
to a great extent in the bearings. I will say right here I think it is amazing how good modestly priced mounts are in reality. It is
going to be very difficult to improve them without major modifications and probably major expense. But let us discuss some ideas
for improvement.
There will be disturbances from wind, mount motion and irregularities in the driving forces from the motor drives. These can be
minimized by very high precision gearing and motors that are strong enough to easily move the loads placed upon them.
Generally it is not possible to do much to change the drive motors provided by the manufacturer. Unfortunately almost all of the
motors provided are too small to do the job with any reasonable margin for error. In the fork mount instruments, we already
know that the fork is mechanically very strong, that the declination bearings and mount are intrinsically strong and that the weak
point in the design is in the RAE bearing.
We know from the specifications that the 10 and 12 inchers have 4" and 2 1/2" ball bearings. The 16" has 6" and 4" roller
bearings. This seems like proportionate design except that the 10" is a bit sturdier than the 12" in proportion. The bearing design
is probably adequate for the 10 incher. The bearings are very asymmetrically loaded for the polar mount (in the US it varies from
25 to 50 degrees) and adjustment of the bearings might be a factor in getting the maximum stiffness out of them.
Now in the Alt/Azm mode, the bearings are symmetrically loaded and will probably work more smoothly and be better loaded for
highest stiffness. So we would expect the telescope to be more stable in the Alt/Azm setup. So for viewing, where field rotation
is no problem or for very short exposures such as one can sometimes get with CCD imagers the Alt/Azm mounting should be
considered.
The above arguments show why amateurs are salivating over availability of a modestly priced de-rotator for the smaller SCTs.
(note that there are serious difficulties with de-rotators as well as described in another article on this web site) For straight visual
viewing I believe the Alt/Azm setup is quite attractive with a scope that has full computer guiding of the pointing action. Again, if
any imaging, photography or piggy back photography is to be used, the scope must be in the polar mode. So we still have to
consider what to do to improve the oscillations of the telescope in this mode.
I believe it is difficult to improve the bearings very much, and I suspect that that is the case we have to come up with other ideas.
Three things can be done to reduce the oscillations. They can be made smaller for a given disturbance by making the fork
mechanism more rigid, oscillations can be damped more quickly to reduce their net effect on an image or they can be reduced by
applying a counter force to cancel the original disturbance. The first of these requires adding strengthening members to the fork
mount in some way. Two obvious ways to add to the mount would be either an English type mount like that used on the 100 inch
Hooker at Mt. Wilson or a Palomar type mount like that used on the 200 inch at Mt. Palomar. Unfortunately either of these
modifications requires a large structure added to the top end of the fork and associated bearings. Certainly such modifications
would be only for a fixed mount and would require so much effort that an entirely new mount would be a better choice.
If one is willing to make major modifications of the base of the LX200 it might be possible to go up from the wedge area into the
bottom of the fork shaft and add a third stabilizing bearing below the other two and residing in the area between the wedge sides.
This modification seems to be impossible because of the design of base bearings. There are still several ways to reduce the
oscillations that are less intrusive mechanically. One is a mechanical damper system. This would consist of a rod of rubbery
material to provide energy loss and a mass. The principle this damper is that the rod and mass resonate with the telescope
motion thus exercising the rubbery material and removing energy from the oscillation. This increases the damping and quickens
the rate at which the disturbances die down. This technique is used with many structures from larger motors to buildings.
It is called a tuned damper. Another way to reduce amplitude and increase damping is to mount a motion detector and a force
transducer on the telescope and with appropriate electronics force the telescope to remain still by canceling vibrations. This
technique is one used by Digisonix in Madison, WI to reduce vibration and one I am working on for my own LX200. I hope to have
good results, but it might be a year or more before I have a completely successful system.
I have decided to build a new fork mount with adequate strength as an alternate path to having a precision mount suitable for
accurate imaging. The design concept and details are explained in another article on this web site.
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Mechanical Modification of the LX200 Classic
Declination Bearings
This work the contribution of Michael Hart. (Doc G did editing of the text and the several addenda)
CAUTION! ******************************************************
The following declination drive performance modifications should not be attempted until the reader studies the full text and photographs carefully. Not doing so may
result in damage and/or loss of pointing and tracking accuracy. Some mechanical talent is required to carry out the modifications described here. Never-the-less, it is
a modification that is very worth while if the telescope is to be used for imaging which requires smooth tracking.
Additional Caution: The modification described here should very rarely be needed for an 8" or 10" LX200. These smaller telescopes, providing that they are not very
heavily loaded with accessories, have adequate mechanical systems. Thus this discussion relates mainly to the 12" telescope under heavy load and use conditions.
NOTE: Since publication of the original article a number technical problems with such matters as removing screws, manufacturing tolerances and the like have
come up. These have generally been solved through more detailed descriptions of the procedures and discussions of the physical characteristics of the bearing
mounts. An appendix is attached as of 16 March 1998 and should be read before attempting the replacement. See: APPENDIX 16 March 1998.
Additional comments on doing the modification from Jim Mettler and a few comments from Doc G. (picture of loaded, modified telescope with balance adjusted
using Losmandy weight set. See: APPENDIX 21 March 1998
For information on the successful modification of a 12" LX200 go to April 1999 mod of 12"
See: March 2000 addendum. A recent experience which adds some important information to a successful completion of this upgrade has been added. This should
be read along with all other information about this modification before proceeding.
Update: additional information on clutch plate removal and shaft specs is provided by Tom Mote as of February 2003. See: APPENDIX February 2003
__________________________________________________________________
Before rebuilding of the declination bearings is considered, it is wise to get the existing declination drive adjusted as carefully as possible. This will insure that the
current drive is working as well as possible and it is possible that these adjustments will be adequate to solve some drive problems. The adjustment of the declination
drive motor mechanism is discussed in some detail under: Declination Drive Adjustments
Note on nomenclature: A drive consists of a large gear, called the worm gear, which looks lake a large spur gear and a small gear, which has a helical thread, called
the worm. This is standard nomenclature in the industry and is used in the following discussion.
BACKGROUND
What has been bothersome in the whole Dec drive scenario is the fact that the Dec and the RA drives are virtually identical except for a switch and sensor on the RA
drive to synchronize the PEC mode. Rarely does anyone report RA drive problems including lash and retrograde motion. It has been noted by Doc G that there is a
major difference in the friction between the Dec drive and the RA drive. (described elsewhere on this web site) I observed the same problem when I heavily loaded
my telescope. However, both drives on my well loaded telescope proved adequate for four years. Recently I had to revisit the problem in order to mount some rather
heavy equipment on my 12" LX200.
The answer was clear, change the nylon bearings to roller type bearings similar to those used on the RA drive. Needle bearings were selected because of their low
profile. The fork arms were bored out to accept these bearings and the telescope was reassembled and adjusted to insure that the optical tube and the fork arms
were orthogonal. A side benefit for some users will be an increase in pointing accuracy because of the refined alignment.
The results of the addition of needle bearings was dramatic. The Dec drive now has less friction than the RA drive. This modification may well be a breakthrough for
those with potential Dec problems and for those wanting to do the very best photographic and CCD imaging. The worm now exhibits no radial motion upon reversal
even with a significantly unbalanced telescope. The friction is now so low that the telescope must be balanced more carefully. With the nylon bearings, the stiction
was so great that it held the telescope in place until the unbalance forces were quite large. Now, one dare not release the clutch without holding the optical tube or
the front and rear attachments might become history. The Dec motor now appears to run with less strain that the RA motor, hinting an excellent longevity for that
motor and the related electronics. I spent a total of 1 1/2 hours on what appears to be one of the best modifications I have made to my 12" LX200 to date. Most
owners should be able to upgrade the Dec bearings and drives using the detailed instructions and photographs which follow.
PREPARATION
Order the necessary parts needed ahead of time and prepare a work area about 2 by 3 feet for final re-assembly. Arrange with a good machine shop for boring out
the fork arms. The parts needed for the 12" LX200 are listed below. Very likely the same parts will work on the 10" and possibly the 8" as well since the drives are
very similar. Most cities will have a bearing supply house from which the bearings can be ordered. The yellow pages can be consulted for a machine shop to do the
boring of the fork arms. While this is a fairly simple task, shops with CNC and align-bore capabilities may have more experience at precision boring. Consult with
them and explain the need for precision alignment and centering. A small amount of off centering can be adjusted out in the fork arms but precision centering will
enable quicker restoration of fork arm position. There is no simple adjustment for a miss aligned bore. Make arrangements for the work to be done ahead of time.
Like just before full moon for example. The cost should be small. My machine shop did the work as a New Years' gift.
PARTS LIST
Bearings: 2 required - SCH 1616 INA cage guided needle roller bearings. 1" shaft ID by 1.313 OD by 1" long
Thrust washers: 2 required - TRA-1625 Torrington 0.030 thrust washers ($0.58 each)
Permatex Klean and Prime activator # 24163 for anaerobics (optional)
Loctite medium strength thread locker # 24206 (blue color)
open ends drawn cup ($5 each)
Low temperature grease such as Lubrimatic LMX (red color), Mobile One synthetic (red color) or Super Lube synthetic (clear color)
I use the Lubrimatic brand.
Exxon Lidok CG Moly molybdenum disulfide anti-friction grease for the worm gear and worm
Screws: 2 required - 4-40 Allen head machine screws 1" long
Feeler gauge set for sacrificial shims (optional)
REMOVING DEC CLUTCH AND RA DRIVE WORM GEAR
Position the telescope with its tube down or on its side. Remove the Dec clutch knob and washer and use a small Allen wrench to remove the 3 screws holding the Dec
drive cover. You may also want to remove the Dec drive assembly which is held by 2 Allen machine screws and the wiring harness which is plugged into a socket on
the inside of the fork arm with a modular plug. Then remove the Dec worm gear by wiggling it off the inside clutch plate. The gear should be marked so that, when
reassembling the drive, the gear can be replaced in the same orientation as it was originally. See Fig. 1
IMPORTANT! You will now need to remove the inner clutch plate. Use a torch to heat the 2 flat head stainless steel Allen machine screws for about 5 to 10
seconds. This is important as the screws are secured with primed Loctite that requires fairly high torque for removal. Without the heat to break the bond, you may
damage the Allen head of the screw. See Fig. 2. Remove the inner clutch plate from the shaft. See Fig. 3.
Fig. 1 Worm Gear and Drive
Fig. 2 Removing the Worm Gear
Fig. 3 Removing the Rear Clutch Plate
REMOVING THE DECLINATION SETTING CIRCLE
The declination setting circle on the left fork arm was probably secured with medium strength Loctite. Wrap the smooth setting circle knob with electrical tape and
use a Channel Lock pliers, pipe wrench or strap wrench to loosen it for hand removal. Once removed, you can see the nylon bearing and shaft. The bearing shaft can
be measured at this time to verify that it is 1" diameter.
MARKING THE FORK ARMS AND REMOVING ONE FORK ARM
Use an exacto knife with a standard #11 blade to score the fork arm position on the RA drive. This will help to establish proper fork arm alignment at the reassembly
stage. See Fig. 4. There is no need to remove both fork arms as removal of one will allow the removal of the optical tube and aid in securing initial or original fork
alignment. Remove 4 Allen machine screws located under the left fork as shown in Fig. 5. Leave the right fork where the Dec drive is located attached. When this is
done the left fork arm will slip off of the optical tube and the entire optical tube can be removed from the right fork arm. This step should be done with care since the
entire forkarm/optical tube assembly comes apart at one time.
Fig. 4 Marking the Fork Position
Fig. 5 The 4 Fork Bolts
Fig. 6 Nylon Bushing Exposed
Fig. 7 Installing Thrust Washers
REMOVING THE NYLON BEARINGS AND ADDING A THRUST WASHER
The Nylon bearings are now exposed and can be pushed out by hand. See. Fig. 6. This is a good time to re-grease the factory thrust bearing located on the optical
tube and add one additional 0.030" thrust washer to each of the shafts extending from the optical tube assembly. When done, each shaft will have the factory thrust
washer, factory thrust bearing, a second factory thrust washer and one additional user supplied thrust washer. The nylon bearing which was removed had a 0.040"
thrust dimension. This is replaced with a 0.030" thrust washer to maintain thrust bearing preload. If upon reassembly you find inadequate preload, use a thicker
thrust washer should be used. See Fig. 7.
PREPARING THE FORK ARMS FOR BORING AND INSTALLING BEARINGS
Now is the time to take both fork arms and the RA drive to the machine shop for boring. Pad the base a little and cover with a plastic bag securely taped to prevent
aluminum filings from entering the ports, etc. Take the bearings with you for sizing. If you prefer have the machine shop press them FLUSH to the INSIDE of the fork
arm. I preferred to do this myself, using a cylinder hone to approach the final OD dimension. Then, I used primer and Loctite to allow potential removal with heat
and hand tools if that should ever be necessary. Finally I used a thick washer and a hammer to gently seat the bearing. See Fig. 8.
For additional information on boring the bearing holes use this link.
Fig. 8 Inserting the Needle Bearing
Fig. 9 Leveling the Fork Arms
Fig. 10 Squaring the Fork Arms
Fig. 11 Lining Up Fork and Base
CHECKING AND ADJUSTING THE MOUNT TO BE ORTHOGONAL
Before reassembly, we MUST verify that the fork will be orthogonal upon assembly. Failure to do this will result in poor pointing accuracy and unexplained field
rotation during long exposures because the mount will not properly polar align. Drift alignment will not cure this for full sky but only for the local area where
alignment is executed. This step, well done, will improve pointing accuracy of the mount.
The marks made on the fork prior to disassembly are used as the starting point during reassembly. If everything was aligned at the factory during manufacture and
has not drifted for some reason, reassembly to these marks will be very close to correct. This must be verified now. Tighten the 4 fork arm bolts. Place the RA/fork
assembly on a flat table as shown in Fig. 9. It is important for the table to be flat and with room for the entire telescope assembly to move freely about the RA axis.
The assembly must also be firmly in contact with the table surface. Insert a length of steel rod or rule known to be perfectly straight through the forks and measure
the distance to the flat table surface from both forks. Do this with the forks rotated 180 degrees. The measurements should all be equal if the arms are the same
height and the table and base of the RA mechanism are in good contact. Comparing these measurements will tell you if one of the arms is taller than the other. If
so shim the shorter arm as necessary to get the fork heights equal. Repeat this procedure to check for equal fork dimensions. Measure the diagonal dimensions of
the fork as shown in Fig. 10 and shim as necessary to get equal measurements. This insures the fork arms are square. Finally, looking from above, see that the fork
arms are parallel to the bottom of the fork edges. See Fig. 11. At this point, the RA drive and fork arms should be well aligned and square with each other.
Now mark the correct position of the fork arms, as adjusted with the above procedure. Remove the left fork arm in preparation for reassembly of the optical tube
and bearings.
INSTALLING THE OPTICAL TUBE AND THE DEC CLUTCH
Position the optical tube on its side and insert the 1" shaft along with the thrust bearing assembly into the bearing on the Dec drive fork. Before assembly, DON'T
forget to pack the new needle bearings and thrust bearings with grease by forcing it into the bearings with your fingers. See Fig. 12. Finally install the left fork to
the marks you have made. Insert and tighten the 4 fork bolts. Note the amount of preload that is applied to the thrust bearings. If not enough, add a thicker thrust
washer to each side to remove any lateral play from the bearing assembly.
Install the Dec clutch and worm gear in reverse order of disassembly. Do not fail to use Loctite and primer. The Loctite fills gaps and assures the Dec clutch
assembly doesn't work loose. Finally add the Dec clutch washer and knob. CAUTION! Set the scope upright and check the new bearings with caution. With the Dec
clutch unlocked friction is very, very low and the optical tube may move surprisingly easily.
Fig. 12 Reassembly Step
Fig. 13 Light Cone and RA Bearing Screw
Fig. 14 Adjusting the Tube Mount Plate
FINAL OPTICAL TUBE ADJUSTMENTS AND ALIGNMENT
Position the telescope under a diffuse light source such as a fluorescent fixture and point the tube straight up. Remove 2 Allen head screws holding the RA bearing
cover and remove it. Insert a long focal length eyepiece into the visual back holder. Adjust the focus knob for a light cone that is just large than the RA bearing
retaining screw. See Fig. 13. If the optical tube is not aligned to the Dec axis, the light cone will be offset to the left or right. Next, LUBRICATE the thread on two
4/40 1" Allen head machine screws and thread into the ends of the optical tube shaft mounting plates. If these screws don't thread in smoothly, chase the threads
with a 4/40 tap, otherwise there is a good possibility the screws will seize and break. Next, slightly loosen the 3 optical tube shaft mounting plate bolts on the side
where adjustment is needed. For example, if the light cone is offset to the right, you will loosen and adjust the left side. See Fig. 14.
When the light cone is centered, tighten the 3 mounting plate screws. Now is a good time to install and precisely set the declination setting circle with respect to the
optical tube. With the light cone centered over the RA bearing retaining screw, apply primer, then Loctite to both sides of the setting circle spacer and threads.
Thread onto the shaft and position the setting circle for exactly 90 degrees. Reinstall the RA bearing cover.
INSTALLING AND ADJUSTING THE DEC DRIVE ASSEMBLY
With the huge reduction in friction within the Dec drive as a whole, the worm float may not need to be reduced below the factory settings as the current spring tension
may very well be adequate to maintain worm contact with the worm gear. No retrograde motion was observed and excellent worm to worm gear contact was
maintained during motor reversal. Initial backlash setting of 20 seems to be more than enough on a well balanced telescope. If the worm drive platform moves
radially with respect to the worm gear, see a discussion about setting the lash for the Dec drive elsewhere on this web site.
FINAL CHECKOUT
Balance the telescope and release the RA lock and the Dec clutch. Remember that even a slightly unbalanced optical tube will initiate rapid motion of the tube about
the Dec axis. So hold on to the tube until you get the feel of the low friction action. Start by moving the telescope by hand across the full range of declination and
compare the smoothness to that of the RA axis. Attach the Dec cable and power up the telescope. Watch the worm gear platform carefully and check that the worm
gear maintains intimate contact with the worm gear after reversal action. If all is well at this point, you have now completed your Dec drive system modification
which has dramatically reduced friction and improved performance. The orthognality of the mount has also been checked and adjusted and the optical tube has been
aligned with the RA axis. These adjustments will insure maximum pointing accuracy. As a bonus, the declination setting circle has been adjusted to the optical axis.
All this in less than 1 1/2 hours. (Michael's time not Doc G's.)
Addendum by Doc G: I am very excited by the excellent detail Michael has given for this rather complex modification of the LX200 Dec bearings. Even if one does
not do the modification, the description of the techniques for adjusting the fork mount are of considerable value. I have thought about this sort of modification ever
since my first Dec drive went out. I am most pleased to have the chance to publish Michael's work here. I imagine we will, over time, make some adjustments to the
text and images. Your suggestions and comments are welcome.
APPENDIX 16 March 1998
Return to Beginning
A major problem with removing the clutch plate screws has been experienced by several persons. The techniques described below should help significantly. Use an
allen wrench that is in good condition with a handle so that appreciable force can be applied against the screw when trying to remove it. Heat the screw directly with
hot part of the torch flame so as to soften the Loctite. It might take 20 seconds or so. If the screw is overheated, it might be wise to let it cool slightly while applying
torque to remove it. The screw will shrink slightly but the Loctite will stay soft. If the screws are damaged replace them with new ones. You might want to use torx
screws as replacements.
The bore in the fork arm should be 0.001" to 0.003" oversized. The bearing will be easily held in place with Loctite since this is a slow speed application. (I hand
honed mine to a smooth fit that went together with finger pressure - Doc G) If you under bore, you may need to cool the bearing to get it in. The fit does not have to
be that tight.
There has been some variation in shaft size found. If the shaft is 1.00" down to 0.994" it should fit fine in the bearings recommended. If the shaft is too much
undersized that is a problem that can be solved by using the next size smaller bearing which is 0.984". This complicates the job since the shafts need to be turned
down. It is better to avoid this problem by accepting a bit of extra clearance. For this application tight tolerances are not necessary since the bearings are very
heavily loaded in one direction.
It is important to have adequate lateral preload on the thrust bearings. The removal of the nylon washers takes out about 0.80" total. Thus must be replaced with
thrust washers. The forks when assembled should grip the tube very firmly. (No play at all with several pounds of pull applied.)
It has been pointed out that in some cases the washers under the bolts holding the forks might have become cupped. Inspect them and if they are damaged, replace
them with slightly thicker washers. This will allow fine adjustment of the fork positions.
With no clutch plates installed the tube will oscillate several (up to 12) cycles, showing almost no friction. With the clutch and worm gear in place the friction of the
clutch will of course reduce the freedom of this swing. As it should, since it is locked up in operation.
After modification the bearing friction has been almost completely removed. The declination drive mechanism should now be readjusted for optimal performance.
This is done by adjusting the drive platform position to tight tolerance as described in some detail on this web site. See: LX200 Mechanical Fixes/Repair of
Declination Drive.
APPENDIX 21 March 1998 Return to Beginning
This appendix includes a few more comments from Doc G's experience with rebuilding the declination drive on his 10" LX200 and a few comments received from
others who have successfully done the rebuild.
It has become clear that several persons who have done this modification have excellent skills in the machine shop. Those who try this should judge carefully which
techniques are best for them to apply. Special thanks to Jim Mettler, who worked on Don Dillinger's telescope and Bobby Middleton for their feedback. Don writes, "I
would not even consider the "Hart forkotomy and declinoplasty" were it not for the significant ongoing and refractory performance variations that I found unacceptable
-- not to mention the fact that I could talk Mr. Mettler into doing the surgery for me!" Well put (Doc G). As another bard would say "If 'tis done when 'tis done, then
better 'tis done quickly."
A good substitute bearing is the SCE1616PP INA. It is slightly smaller with sealed ends. Your scope may have "pre-damaged" clutch plate screws. If so you can slot
the screw with a Dremel cut off wheel and remove, after heating, with a slotted screwdriver. Both fork arms can just as well be removed without marking.
Reassembly and realignment is quite easy using standard machine shop tools and techniques. The bearings can be pressed into the bored mount using a mill to
insure getting them started squarely.
An accurate, sensitive level (0.005" per foot) and a piece of precision shafting can be used to do the fork arm alignment. The procedure is somewhat involved, so will
not be described in detail here. Basically the telescope RA base should be leveled. This can be done on a tripod. Then the shaft can be leveled with respect to the
RA base by raising one of the fork arms with respect to the other until the shaft is level. (I also used this technique, Doc G) One arm is placed and the other arm
shimmed up to get alignment and tightened. Remove the arm that did not need shimming, it is as low as possible toward the base and will be restored to the same
location when the OTA is put into place and the arm reinstalled.
Re-install the dec clutch, gear and motor drive. Using the level, place it across the corrector plate end of the OTA and level. Set the dec setting circle to 90 degrees
and tighten. Using a good level makes the above operations accurate and objective. The perpendicularity of the dec axis with respect to the RA axis is the important
parameter in this adjustment.
Upon first light, the results were good. After standard Meade polar alignment was completed, pointing accuracy was within 3 to 4 arc minutes over the whole sky.
Drift, without precision drift alignment, was 10 arc minutes in 2 hours. This is a fine result, thanks Don and Jim for your comments.
I (Doc G) have worked with the newly modified 10" in the basement laboratory and tested it for balancing capability and load carrying capacity. The scope is shown in
the following photograph with a considerable load of equipment. The C-5 is quite heavy and is carried in a sturdy mounting yoke. Notice that the tube is coarsely
balanced with two 2 pound Meade weights and fine balanced with the Losmandy 2D balancing bar. A 5 and 2 1/2 pound weight are on the Losmandy bar. With this
heavy but balanced instrument, only a few ounces of weight moves the scope easily and it stays where pushed in all directions. The motors run the scope without
strain. All in all a very satisfactory unit mechanically.
For information about the C-5 mount see: Guider Scope Mounts
Appendix February 2003 Return to Beginning
From: Mike Dodd <mike(AT)mdodd.com> Date: Feb 2003
I own a later model 10" LX200 that has metal bearings, so I don't need to perform the modification. However, the information about disassembly and reassembly/
alignment are helpful because I'm going through the procedure to align the forks and OTA.
It turns out that the 1" steel shaft that's mentioned for leveling the forks (also mentioned in the appendix citing Jim Mettler) is not 1" any more. Mine measures
0.984" - which is exactly 25mm. It seems that Meade went overseas for the steel bearings.
In any case, the 1" shaft is needed for the old Nylon bearings, but if the scope has factory-installed steel bearings, buy a 25mm shaft.
Also, some people have had problems removing the Dec clutch plate because of the adhesive on the end of the Dec shaft. One person described using a wheel puller
and a torch to loosen the adhesive, and another used a small claw hammer. I just put a 1/4" bolt into the clearance hole in the clutch plate (not threaded into the hole
in the shaft), then heated the center of the clutch plate with a propane torch. After it was hot, I tapped the bolt sideways with a hammer and the plate popped right
off.
Return to Beginning
Return to LX 200 Mechanical Index
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Return to Declination Rebuild Article
Declination Drive Adjustments
(without rebuild)
The following is a description of how to adjust the declination drive motor and worm mechanism. This should be done before
consideration is given to rebuilding the declination drive bearings since it might improve performance enough to avoid the much
more major rebuild process. The adjustments are in two parts. The first is a description of precision adjustment of the declination
platform without modification of the mechanism. The second is a tactic that allows for high precision adjustment with a minor
modification of the system.
All information supplied by Michael Hart, 13 May 1998.
(editing and commentary by Doc G)
BACKGROUND
In an earlier MAPUG-Astronomy post, I described adjusting the worm carriage spring tension to minimize retrograde motion by
REDUCING carriage spring tension just enough to lift the worm into contact with the worm wheel and NO MORE. The purpose of
the carriage spring is to variably remove backlash in the Dec (and RA) drive. A small amount of carriage float is necessary to
handle any eccentricity of the worm gear. The amount of carriage float is adjusted by the carriage set screw stop. If all of the
float is removed with the carriage stop set screw, it is likely the worm will bind against the worm wheel at some point. The idea is
to remove excessive carriage travel, but still allow for a bit of worm wheel run out.
I will describe a rather simple procedure I use to enable precise adjustment of the Dec drive worm which usually allows excellent
tracking of exposures at the 3000 mm focal length. On my 12" scope, I found most of the run out in the facing between the inside
Dec clutch plate and the 1" optical tube shaft. The carriage spring easily handles this, but extra tension not needed causes
retrograde motion. I was able to selectively add or remove retrograde motion by increasing or decreasing carriage spring force. If
the Dec drive is excessively noisy or retrograde motion is experienced, reduction of carriage spring force can help.
RETROGRADE MOTION -- DO I HAVE IT?
For those unfamiliar with retrograde motion, this describes a drive (usually the Dec drive) that moves a bit in the opposite
direction selected by the keypad. (before resuming in the correct direction) This is best observed at moderate to high power with
a reticule eyepiece. Autoguiders are confused by retrograde motion. In effect, a small oscillation is set up as the autoguider
program attempts to correct for retrograde anomalies. (in fact, guider software has great difficulty when this problem exists - ed)
This motion should not be confused with backlash which is do to a normal mechanical problem with the gearing system and which
can be corrected with the backlash setting provided in the LX200 software.
ADJUSTING THE WORM and WORM CARRIAGE PIVOT
Proper worm carriage pivot (hinge) adjustments allow weaker carriage springs to do a better job of reducing backlash without
adding retrograde motion. In practice, I adjust the carriage pivot set screw (which presses on a ball bearing) for a tiny bit of
resistance to the weight of the carriage assembly (worm, carriage, gears and motor) and applied a drop of oil to the ball
bearings. For the earlier post, I removed excessive spring force by compressing the spring fully several times with a pliers. I
balance the optical tube (weights not necessary, but useful). Then, I installed the Dec drive using the two machine screws loosely
tightened. I held a straight edge up from the center of the wormhole to the center of the fork arm and centered the worm and
marked the point on the Dec drive just below the worm wheel. This point was used to locate a screwdriver (small lever) to apply a
bit of pressure under the Dec drive assembly directly under the center of the worm wheel. Finally, I rocked the Dec drive
assembly against the worm wheel to find and feel the precise point the worm properly engages the worm wheel. Carriage travel is
adjusted to allow just enough travel to allow a typical range of expected declination positions.
This process is MUCH easier than writing it here- total time to adjust the Dec drive is under 3 minutes with experience.
For detailed photographs of the deck drive Press Here
TIGHTENING THE WORM TO WORM WHEEL SETTING
It is possible to make a small modification to the declination drive platform which allows for precise adjustment of the drive for a
specific setting of the declination of the telescope. The principle is to tighten the worm to worm wheel contact for a small and
specific section of the worm wheel. When this is done one does not have to use a carriage spring at all. The concept is that
when guiding for imaging, the declination changes very little and so the worm moves over a very tiny portion of the worm
wheel.
This means the drive carriage can be adjusted optimally for the particular section of the worm wheel in use. Thus, I
fabricated a manual carriage adjusting screw that is tweaked for a very specific range of declination position. In this way, I am
able to eliminate all retrograde motion as well as minimize backlash for the long guiding times that are required for tricolor filter
imaging sequences. The technique is quite simple. I check and adjust the declination drive platform platform tension during a
short slew about the declination setting that is to be used until motor noise JUST increases. This indicates that the contact
between the worm and worm wheel is snug. With this setting correct, at high power (600-800X) no retrograde motion should be
observed and the scope should return to the same spot if bumped slightly. Manually rocking the optical tube verifies the setting,
in practice this minor tweaking is quite easy to do but requires a modified setting screw as described below..
.
FABRICATING A DEC WORM TENSION ADJUSTER SCREW (Photographs below)
In order to carry out this precise adjustment of the worm carriage, it is necessary to replace the carriage adjusting screw with a
new screw that has a large diameter head that can be easily reached from the side of the dec drive housing. (Through a slot cut
in the side of the housing.) The required screw will have a short threaded section topped with a large diameter knurled section.
It will look like a short, fat thumb screw. (I would, and probably will, make such a screw on my lathe. But Michael has a way of
his own. - ed.) The manual worm platform adjusting screw was made from am 8-32 X 5/8" hex head machine screw with the
tip sanded round. This screw was inserted through a 1/8" X 3/4" fender washer. A 5/8" external toothed lock washer was glued
to the fender washer as was the hex screw. The teeth stick out past the fender washer and allow my finger to grip it for fine
adjustments. I used two part plastic cement (Devcon brand) which is as strong or stronger than epoxy, waterproof, and sets up in
5 minutes. Enough material was used to encase the hex head and was sanded almost flush with the hex screw head as it must
rotate in a rather small space between the dec drive platform and the fork arm casting. The fabricated adjusting screw replaced
the regular adjusting screw. A knife was used to the trim away a bit of the plastic dec drive cover to allow access with a small
screwdriver (used as a push tool). I removed a bit more on mine to allow access with my thumb.
Of coarse, precise balance is maintained and the full potential of low friction dec roller bearings (if used) are best realized with the
outlined adjustments. Those looking for a reason to buy another mount should avoid the above procedures, however, those with
demanding imaging requirements, which exceed LX200 design parameters, may want to consider implementing the outlined
adjustments.
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Discussion of Declination Drive Design
The event that brought on this article is posted at the end.
(For dec run away experience press here.)
The declination drive for the LX 200 is discussed in some detail herewith. Since the RA drive is essentially identical, the
commentary here should also serve to describe its basic operation.
The drive is shown on the left from the front top view as it is delivered from Meade. This particular drive is a spare that I
purchased from Meade for $150.00. It is held in my personal stock of parts to replace a drive that might go out of service for
some reason on one of the two LX200 telescopes that I am responsible for keeping in good repair. On the right, the drive is
shown from the bottom. The mechanical operation of this unit has been discussed at length elsewhere in these pages. Our
concern in this discussion is with the electrical and electronic parts of the drive.
The tiny motor that drives the telescope through a gear train and worm is shown clearly. The motor shaft rotation is reduced by
the gear train in the housing on the left by 60 times where it is coupled to the worm. The worm in turn drives the worm wheel
(the large spur gear in the declination drive, not shown) which has 180 teeth. The worm turns only when a declination correction
is required. The RA drive is identical except that the hole seen in the worm shaft in the photograph on the left has a magnet in it
which operates a switch that establishes the starting point for PEC training. It, of course, turns continually at one (1) turn every
eight (8) minutes to move the RA position at sidereal rate.
The small circuit board has a dual Schmitt trigger circuit chip which converts the signals from two photo detectors to a bi-phase
signal that is sent to the computer board. The photocells detect light from a pair of small lights through the encoder mask and
encoder wheel encased in the gear box housing.
Close-up of the photocells and light arrangement are shown in the following two photographs. On the left are shown the two
photo detectors which are stuck into holes in the gear box housing and cemented in place. The leads on the photocells are bent
very sharply and connected to rather heavy wires, then clamped in a plastic clamp stuck to the housing. This is a possible trouble
spot. The leads coming out of the photocells are very brittle and are bent too sharply, in my opinion. They are cracked slightly at
the bend in this example. The leads going from the photocells to the circuit board are rather stiff and unfortunately the plastic
clamp is not positioned correctly nor is it closed tightly enough to hold the wires and thus relieve the stress on the photocell leads
as intended. This area should be inspected carefully if there is a declination drive runaway problem.
The photograph on the right shows the photocells, on the outside, and the lights on the inside of the housing. The encoder mask
and encoder wheel lie between the two. When the light path is interrupted, the encoder sends signals via the Schmitt triggers to
the computer where they are counted to establish differential position information for the computer. The wires to the lamps are
also quite heavy and put excessive stress on the tiny light leads in my opinion.
The circuit board contains, in addition to the integrated circuit chip, two potentiometers which are used to adjust the level at
which the trigger circuits operate. These can only be adjusted with use of an oscilloscope which is used to observe the signal while
the motor is turning the encoder wheel. A set of test terminals is provided for this adjustment.
In one case of declination drive runaway, the cause was breaking of one of the leads to one of the photocells. I tend to believe
that many declination drive runaways, that are not do to bad connections, are due to failures in the declination drive circuits in this
assembly. Unfortunately, since the declination drive motor, encoder and all the rest of the system including the computer are part
of a closed loop control system, it is difficult to find the problem without appropriate test jigs and procedures.
Return to Beginning
Dec Run away Experience
Date: June 1998
From: Doc G
Well, this should give all you mapuggers who have had runaway dec drives a chuckle. It has just happened to me. (alleged dec
expert??) This morning I went to the Madison Astronomical Society site for our annual picnic. I got there two hours early to
clean up the observatory and get the LX200 polished and running for Solar observation. When I turned on the scope. blewwy!!
running dec with no control whatever. Dec plugs check all three of them! Nothing Turn it on and turn it off a few times.
Nothing!! Talk to it. Nothing!! Talk in no uncertain terms. Nothing!! Kick the pier. Nothing!! Kick the pier harder. Nothing!!
(bent toe!)
Took off the dec drive cover and wiggled everything carefully. Nothing!! Wiggled everything harder. Ah Ha a momentary stutter.
Turn off the scope. Check every wire visually very carefully. Ahh Haa!! broken wire on one of the encoder photocells. The wire
was broken right at the cell. (in fact the lead was broken right at the seal) Very difficult to re-solder. But did re-solder using
tweezers and hot soldering gun. Turned on scope. All fixed!! This fix required lots of luck and a tiny bit of talent. :-)
The scope tracked the Sun with minor centering corrections for hours, even without changing the frequency of the drive. Great
spots with nice detail using the Thousand Oaks filter. When it works, boy it really works.
Doc G
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Return to LX200 Electrical Analysis Index
Return to LX200 Mechanical Analysis Index
Operation of the Precision Error Correction
(PEC) in the LX200 Drive
For a discussion of using a CCD imager to train the PEC see this link. PEC-CCD Training
For an example of the data obtained after PEC correction is applied see this link. PEC-WORM Data
Introduction
The purpose of the Precision Error Correction (PEC) is to correct for mechanical irregularities in the RA motion of the telescope
caused by imperfections in the drive. This is a very clever idea, much used in professional telescopes and I commend Meade for
implementing it quite elegantly in the LX200 design. A basic problem with all telescope drives, including professional drives, is that
there are mechanical irregularities in the gears and worm which drive the worm wheel that points the optical tube. These
irregularities occur with a period determined by the speed of rotation of the gears. For the LX200 design, which has a worm wheel
of 180 teeth, this period is 8 minutes long.
The basic concept is to correct the speed of the drive motor to compensate for the fact that the worm drives the worm wheel a bit
faster or slower as it rotates. This is done by going into a training mode and manually training the computer to compensate for the
mechanical defects. This is a good but not perfect solution to correcting mechanical problems in the worm to worm wheel
interface. It can not do a perfect job because correcting for one turn of the worm takes account of only one tooth of the worm
wheel. It can however remove as much as 90% of the irregularity.
Basic Operation of the RA Drive
In order to understand the details of how the computer can be used to correct mechanical irregularities in the worm, it is
necessary to understand how the RA drive works in the first place. Thus a rather detailed description of the RA drive is given first.
If you are thoroughly familiar with this material skip to the PEC operational details below. (PEC Operation)
The LX200 Right Ascension drive (RA drive) consists of a small DC motor which drives a small gear reducer train, which turns the
worm which in turn drives the worm wheel that points the telescope on the RA axis. The RA axis moves 360 degrees in a sidereal
day. Since the LX200 worm wheel has 180 teeth, the worm that drive it rotates at one turn every 8 minutes. The small gear
reduction train has a reduction ratio of 60 times so the drive motor rotates once every 8 seconds. This is a very slow speed for a
DC motor and thus it has to be controlled by a digital signal. The motor shaft has on it an encoder which has 90 openings and is
viewed through a bi-quad mask. The bi-quad encoder has two output signals (from photocells) which are in quadrature. These
signals are deciphered with appropriate logic to give two new signals. One of the signals indicates the direction of rotation and the
other tells how fast the encoder shaft (motor shaft) is rotating. When the RA axis is moving at sidereal rate, the encoder puts out
45 pulses per second.
Speed control is maintained by the computer which puts out a command pulses at a nominal rate of 45 pulses per second. The
motor shaft must move the encoder disk to match this rate. When the motor gets behind, the computer delivers more current to it
to speed it up and vice versa. In this way, the computer can control the motor shaft position very accurately. Notice that the
computer sends the command in pulses per second and the motor shaft must respond. This is the key concept which explains the
ability of the PEC to control the motion of the worm. The computer can adjust the number of pulses to speed up or slow down the
motor.
Note that this drive is NOT a stepper motor drive. It is a DC motor with a shaft encoder. The effect of the encoder is to make the
motor move from position to position in a way similar to that of the stepper. But the principle of operation is quite different.
With a pulse rate of 45 per second, each pulse corresponds to 0.333 arc seconds of motion of the optical tube assembly (OTA).
This sounds very accurate, and it is. But, is must be emphasized that the control element is on the motor shaft. There is
considerable lash in the gear reduction and worm drive mechanisms. Thus the basic accuracy is not transferred to the motion of
the OTA. The mechanical inaccuracies are the reason the PEC is needed. With the computer generating the correct 45 pulses per
second, the OTA will move with a constant sidereal rate but will wobble back and forth with respect to the stellar sphere, typically
by as much as 50 arc seconds, due to mechanical irregularities in the drive. In modern professional telescopes such irregularities
are compensated for with computer correction systems not unlike that used in the LX200 system.
For those interested in details, when the worm turns one time, the first gear in the reduction train turns 4 times, the second turns
36 times and the motor shaft turns 60 times. The gear ratios are 44 to 11, 36 to 9 and 30 to 8 respectively.
All in all, this is a very fine and very precise way to control a telescope. It can be seen that a GOTO action can be initiated by
knowing a location and moving the OTA to a new location by having the computer command the motor shaft to turn a given
number of times. The computer gets the necessary move information from internal tables and even corrects for atmospheric
refraction. The declination drive works in an almost identical way.
How the PEC Corrects Mechanical Problems
A major irregularity in the mechanical drive system is the fact that the worm is not perfect. Making a nearly perfect worm would
require very high precision and a very high cost manufacturing process. In the LX series of telescopes it is typical to find that the
wobble caused by worm irregularities is often 50 arc seconds. This is not a severe problem for viewing but is impossibly large for
imaging of any sort. There is no telescope which does not have to have some sort of guiding used to keep it on target to within a
few arc seconds or so. This includes the best professional telescopes. Fortunately, with the rather fine control provided by the LX
computer controlled drive it is possible to greatly reduce one of the worst of the mechanical problems. This is the mechanical
irregularities in the worm which repeat for every turn of the worm shaft.
This is done in the following way. The worm shaft is indexed by means of a small magnet inserted in the shaft which triggers a
pulse at what is set as the beginning of the worm cycle. The 8 minute period of the shaft is divided into 200 intervals of 2.4
seconds each. When the telescope is put in the "Learn" mode, it emits a beep at the start of the cycle and a beep every 2.4
seconds. It is the job of the operator to guide on a star as accurately as possible for the 8 minutes by pressing the East and West
keys on the keypad. This is usually done at a high magnification of 300X or so. The computer interprets the key presses and stores
this information in a file. After the training (Learn) cycle is complete, the computer will play back this correction record in
synchronism with the worm shaft and thus slow it or speed it to correct for the original irregularities.
There is provision to run the training several times and to average the results to improve the correction. One "Learn" and one
"Update" session is generally enough to reduce the irregularities to about 10% of what they were. The final accuracy is typically 3
to 10 arc seconds and this may be considered a very fine result. While not perfect, this level of error makes it possible to guide
with much reduced demands on either manual guiding or with an automatic guiding system.
More Detail About How the PEC Works
In order to see what the mechanical problem with the worm is, it is necessary to consider what the mechanical errors are in terms
of their distribution around the 360 degrees of the worm. The worm reaches the same point every 360 degrees while the worm
wheel moves one tooth. It would be nice if the wobble of the worm surface was rather uniformly distributed over this 360 degrees.
Unfortunately this is not the case. Some parts of the worm surface are very smooth and regular and some parts have larger
bumps and wiggles. This is somewhat like a ramp with steeper and less steep inclines and perhaps a speed bump or two. Data
recently published on the web and my own measurements on two worms have shown that there is often smooth motion followed
by sudden shifts in the OTA and then a sudden motion back. See: PEC-WORM for a short description and data on actual original
OTA errors and corrected operation.
It is thus necessary to have a system with frequent enough corrections to compensate not only the long slow variations but to also
correct for the bumps. Rates of motion have been observed as large as 0.5 arc second per degree of rotation of the worm shaft.
This is a very fast motion when observing through a high power eyepiece. It is difficult to correct this movement when guiding
manually. This means that hundreds of corrections need to be made in a single turn of the worm shaft and they need to be made
very quickly. The LX200 system allows for 200 corrections for each turn of the worm. This is done by detecting the start of the
cycle, through the magnetic shaft switch and keeping track of the 200 sections over the 8 minute cycle in the computer.
The operator then manually guides the telescope as accurately as possible and the computer keeps track of the corrections for
each interval. There are 108 pulses delivered to the drive motor in each interval. This is done as follows:
The 8 minute worm cycle is divided into 200 bins within the computer. Since there are 45 pulses per second to drive at sidereal
rate, there are 108 pulses in each bin or 21,600 pulses per worm cycle. The values stored in each bin are nominally set to 108. On
"Erase," the values are initialized to 108. When "Learn" is selected the values are also initialized to 108 and are then modified by
the training process which is done in the "Learn" mode. When "Update" is selected the values in the bins are not reset at the start
of training. During either learn or update training, any previously stored PEC corrections are not applied to the drive. Thus the RA
axis is driven at a fixed rate of 108 pulses per 2.4 second period at a constant rate during both learn and update.
During each 2.4 second period the currently active bin has the number of user added or removed pulses combined with the
nominal number of 108. In the learn mode, this new value is stored in the appropriate bin. In the update mode, this value is
averaged with the value already in the bin. After the training, the system plays back the number of pulses in the bins which then
adjusts the pulse rate so that in the 2.4 seconds that the bin is active, the indicated number of pulses are fed to the motor drive
circuit evenly over this period. This has the effect of averaging any corrections over the 2.4 second period. This operation has the
effect of speeding up and slowing down the drive uniformly over each period as required to correct for mechanical errors in the
worm.
Rapid changes in drive motion are not tracked over the 2.4 second periods so that abrupt "bumps" or "valleys" on the worm gear
or worm wheel which occur more quickly than 2.4 seconds may result in an abrupt image shift. More frequent pushes of the E/W
buttons might increase guiding accuracy since this tactic avoids overlap between bins and prevents over correction that might
occur from one segment to the other. The averaging of the new training with the stored training (previous value) has the effect of
geometrically weighing the older trainings So, in the case of the learn mode the training value is stored but in the case of the
update the training value, it is averaged with the older value. That is, the older trainings become less significant and the most
recent training becomes one-half of the stored correction. This implies that the most recent training is the most heavily weighted
and thus the most important. A poor update can spoil an otherwise well trained worm. Note that since one can spoil good training
with a final bad training session, the averaging algorithm used is not the best. For this averaging scheme to work well, training
requires that you make more or less random errors in training and that you get better at training if you do a learn and several
updates afterward. In my opinion it would be better to playback the PEC during training and then allow for corrections, which
would be small, to be weighted into the values that are stored, thus perfecting the training in a systematic way as you do more
and more training.
Off center mounting of the worm wheel can be a problem if you train on one part of the worm wheel and then operate on another
part. This may explain why you can have a well trained PEC and then on another night find the PEC less effective. It must be
remembered that one cycle of the worm trains the entire worm but on only one tooth of the 180 tooth worm wheel. For this
reason, a permanently mounted telescope should have the drive clutches locked so that the trained part of the worm wheel is used
at all times. (only one-half of the worm wheel is ever used in this case.)
After training, the total pulse value that appears on the hand controller should nominally be 21,600. Differences from this value
are due to poor training or a crystal that is slightly off frequency. This effect may be due to a highly asymmetrical error in the
worm itself. The deviation from 21,600 represents a very slow RA drift which is not a problem while guiding manually or with a
CCD imager. Note that when the PEC is turned off, the training vanishes.
If one can reduce the worst of the irregularities by a factor of ten, which is certainly possible, the worst rates of deviation will
usually be only 0.05 arc seconds per degree of rotation of the worm shaft. Thus, with very good PEC correction in place, the
residual errors should be correctable with careful manual guiding and certainly easily corrected with automatic guiding with a CCD
imager. Without good PEC training, manual correction is exceedingly difficult and automated guiding often impossible. The
exception to this is in the accidental case that a particular drive has almost no errors. Such cases have been reported but they are
likely very unusual. Even the finest Byer's research quality gears have only a 10 arc second rating. By the time one gets to the one
arc second guiding condition, other effects such as telescope mechanical instability, wind induced motion and atmospheric wobble
will take over in blurring a star image.
Other Issues
There are are a variety of issues that relate to PEC training. One of the interesting ones is using the CCD imager guiding signals to
do the training of the PEC. There have been a number of reported successes using this technique. See: (PEC-CCD)
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Photos of the Disassembled LX200 RA Base
Here are some photos of the disassembled RA base of the LX200. The photos should be self explanatory. Notice that the two
parts have to be driven apart by tapping them so that the bearings let go of one piece. The tapered center piece is held to the
bottom part of the base with a second mounting plate turned in from the bottom. It is not certain that the post will be exactly
perpendicular to the base. It looks like the bearings should be concentric since the upper piece is made from one part and turned
as one part. The RA gear is very snugly mounted on the upper moving part and should be well centered.
The RA base is quite well built, but it is not, in my opinion, large enough to hold a 12" scope. It is probably designed for an 8"
scope and is probably satisfactory for that size and weight.
I hope these figures help LX owners better understand the design of the LX 200 base.
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LX200 Classic Main Computer Board -- R. A. Greiner
(revised April 1998) (interim report)
This is a personal interpretation of the design of a computer control system with comments that are based on my own experience.
I have taken the liberty of mentioning, at several points, my own example of how a revised design might address perceived
problems.
This is not an attempt to decipher the details of the design of the main computer board in the LX200. It is written to help owners
understand better just what is in the computer system and how it interacts with the keypad, the motor drives and other external
items. The main board (MB) is very complex. It has a powerful computer chip, memory, logic chips to interface the computer to
the outside digital world, including the keypad and encoders and analog outputs to drive the motors on the two axes of the
telescope. I do this because it is sometimes more comfortable to own and work with a piece of equipment that has been described
in some detail. All in all, the computer works very well. Some of the interface electronics has been reported to give problems
from time to time. I will offer an analysis of the operation of the MB with particular attention to the interface electronics.
Schemetic drawings are available by clicking here.
Go to: Main Board Components: Basic Computer Operation: Clock and Timer Circuits: Problems Reported: Motor
Drive Circuits: Summary
Main Board Components
The computer can calculate co-ordinates for both Polar and Alt/Azm mounting, correct for atmospheric distortion, find thousands
of objects, respond to external commands, store PEC data and do much more. The microcomputer, at the core of the system, is a
68301 Chip. This is a Toshiba chip which is a clone of the well known Motorola 68000 series. (but not one in the Motorola line)
There are two Sharp 64K memory chips and a large AT&T gate array. The latter is a collection of logic gates custom programmed
for the particular application. There are a few minor chips for input and output of serial signals, two large EPROMs which carry
the main programs and data, Ver 3.34 in this case.
There is also a crystal oscillator and two Dallas DS1202 date chips. These output Day, Month and Year information. The system is
probably booted from an Atmel EEPROM, 24CO4A but this is a detail that while not too important at the moment, is discussed in
more detail below. There are also several operational amplifiers and four transistors which are associated with the analog outputs
to the motors. One very interesting chip is a motor driver chip, UDN2993B, made by Allegro. More about this chip below. There
are two small power operational amplifiers, L2724, which drive the motors. Many of these devices and their functions will be
discussed below. The board is a very high quality, multi layer, glass board. It is quite complex and appears to be very well laid
out. I noticed only one added (soldered in) wire in the analog circuitry. A concern about the design is that it is dated and several
of the devices are now hard to get or discontinued. The greatest complexity in any digital design is in the firmware that boots and
runs the system. This is unfortunately not accessible from a casual inspection of the board.
Basic Operation of the Computer Board
The basic purpose of the computer system is to take various commands from the keypad and deliver signals to the motors that
move the telescope optical tube to the desired position. Additionally, the computer translates time and in some cases guiding
signals into signals which correct the motion of the tube. All this is done with a setup that requires only a few inputs to the
computer to tell it the starting position of the telescope optical tube. Position calculations are done on a differential basis. That is,
the next position is dependent upon the last position and the distance, length and direction, to the new position. The more
accurate the last position and the smaller the move, the more accurate the final position will be. The computer has tables of
positions of 65K stars and can also calculate the positions of the important movable objects like the planets, sun and moon.
Additionally the computer takes care of housekeeping duties; like the display of the coordinates, local time and GMT and a few
more minor but important calculations such as the worm correction data (PEC), the declination lash setting and remembering
other settings within the mode group of commands such as location.
There are actually hundreds of things for the computer to do to make the system friendly to the user. These, I feel, have been
carried out with great success. This GOTO telescope is a real joy to use in most scenarios. On the other hand some problems
have been reported and the system itself is not well described or explained in the instruction manuals. Most of the chips used are
standard, reliable and well understood. As complex as the system is, it does not compare in complexity to a PC and operating
system. In my opinion, the computer system, its interface with the keypad and with the several accessories such as the RS232
port and the CCD port work very well.
work well.
It has a very good array of commands for external computer communications which also
The Clock and Timer Crystals and Chips
There are two Dallas Semiconductor DS 1202 clock/calendar chips on the main board. They are controlled by a 32.768 KHz
crystal which controls the accuracy of the timers. One of the chips is setup to deliver local time information and the other set up
to give sidereal time for and is reset when star alignments are made The tow clocks are not connected in any way. The crystal
oscillator circuit provides clock information for the clocks. Since the accuracy of all timing is determined by a crystal, it should be
both accurate and stable. If there is a problem with the timing, it will have a serious affect on the RA drive rate as well as other
timed functions. The crystal frequency can be adjusted with an external capacitor, but accurate time/frequency measuring
equipment is required to make these adjustments. A more tedious means is to simply observe the time generated by the
telescope and see if it is running fast or slow and then tweaking the crystal with a small parallel capacitor. (if it needs to be
slowed).
The most practical action that the user can take is to simply synchronize the clock with and accurate clock, like WWV, at the start
of a session. The timer will not drift much over a period of a few hours.
Possible Problems with the Electronics
Of particular interest to the user will not be those parts and systems that operate well, but those that cause problems. The
problems that have been reported have been primarily with the electronic and mechanical interface to the telescope structure
itself. That is, the motor drivers, the motors and the encoder system. First a few words about the basic decisions required to
move a telescope in a suitably precision way. The telescope is moved by an electromechanical servo system. The theory and
design of such systems is well understood both theoretically and practically. In the case of most modern telescopes motion is
accomplished by stepper motors or by DC servo motors. A basic design decision that has to be made is which sort of drive system
to use. Stepper motors have the advantage that they are easily incorporated in computer driven systems. They have several
disadvantages as well. It is hard to get very large ratios of the slowest to the fastest drive speed. In the case of a telescope, one
would like as much as 1000 to 1 ratio. (which is what the LX200 does) Perhaps it would be wise to settle for several hundred to
one to simplify the design. But even several hundred to one is tricky to get with stepper motors. Another disadvantage of stepper
motors is simply the fact that they step. This basically means that they constantly shake the mechanical structure. They have to
be made to step fast enough under the slowest drive conditions so as to not shake the telescope. This means about 3 to 5 steps
per arc second of optical tube motion. The LX200 does 3 steps per arc second. (It is not a stepper motor but a sort of quasi
stepper system., see discussion following.)
The Motor Drive Circuits
A DC motor drive has some advantages. It has very high torque for its size, is inexpensive, has maximum torque at stall, has
smooth motion and can be operated over a very large speed range. The disadvantage is that there is no easy way to tell how fast
it is rotating and how far it has gone. It requires an external encoder to provide this information to the computer driving it. One
way is to provide a tachometer feedback. This basically analog system has been used but has some of the usual disadvantages of
analog systems such as accuracy and drift. There is a very cleaver way to get the advantages of the DC motor and those of the
stepper motor. The system is well known and well understood. The LX200 uses it. The DC motor shaft is provided with an
encoder that generates a large number of pulses as well as direction information as it turns. Then the direction and distance the
motor has moved the telescope are kept track of by the computer. The LX200 has such a bi-phase encoder built into both the
declination and the RA drives. It becomes a question of how this system, which is basically excellent, is actually brought off in
practice. Because of the speeds required, there will be a gear reduction box between the motor shafts and the worms that drive
the main gears. Since the gear box in the LX200 design is not within the computer/encoder feedback loop, it must be of very high
quality. Unfortunately it is not of high quality and the motor is too small for the application. (in my opinion) But remember, the
entire motor drive for the LX200 sells for $150.00. A very strong gear/motor drive system, such as I am building for my new
mount will cost $200 for the motor and encoder and $150 for the gear deduction unit. Both drives require the main gear and
worm mechanism in addition. It is hard to make a breakdown for the LX200 drive but the motor itself is very modest and thus
might cost only a dollar or two. These considerations are not so much a criticism of one design over the other as they are of the
economics of making and selling lots of something over a no holds barred design and making of just one model. I believe the
LX200 design would fare better with a motor that was several times stronger and with a higher quality gear train.
When one part of the design is changed, particularly made stronger and larger. There are consequences for other parts of the
system. In this design it is the power operational amplifiers that drive the motors that would be under stress. These amplifiers,
L2724 made by SGS Thomson, are dual operational amplifiers made to run on a single sided power supply. There is one chip for
each axis. The L2724 is in a 9 pin SIP package with a heat sink tab. It is rated at 28 volt max power supply and an output
current of 1 amp max. The power dissipation is rated at 10 watts. (50C case temp.) The chip is designed to drive small DC
motors. The circuit shows an internal snubber diode. (positive side only) With DC motors, I prefer to see external snubbers
used. The chip shows thermal protection on the chip. On the face of it, this looks like an adequate chip to drive the LX200
motor. (But it is not over designed by any means I would say)
Some failures have been reported. Only an extensive evaluation of the circuit under the most extreme conditions expected would
tell the full story. My feeling is that the chip was designed for use with VCR and CD capstan and take up motors. These do not
operate under the stressful conditions experienced by the telescope drives, especially stalled or near stalled operation. I have
looked over the detailed specifications on the motor driver chip to determine the suitability of using this chip in combination with
the power operational amplifiers that are used. While the design is a bit unusual, probably do to the limited selection of chips
available at the time of the design, there is no reason to believe that the design is flawed. Never-the-less, I am skeptical about
the design and would not do it the same way. The electrical signal at the motor in the LX200 is incredibly noisy and has switching
artifacts of large size on it. It looks like some additional filtering should have been used. One of the most mysterious parts of the
MB design involves the conversion of the digital information to motor driver commands. I have to assume for the moment that
the computer calls for a set number of pulses for the axes to move to get to a new location. The motors report back to the
computer how many tics the encoder has made and when the correct number have been made the computer stops the motor.
The mechanical feel of the motors is that they are controlled to one tic accuracy. (This does not of course move the telescope 0.3
arc seconds because of the lash in the gearing but it has the potential to do so.) The motors are in any case very tightly
controlled.
Exactly how the computer sends the logical command to the motor is still not clear to me. The chip that is the center of this
operation is an Allegro USD2993. This is a dual H-bridge motor driver chip. It is, by the way, discontinued though Allegro reports
that they have a substitute for it. (whew!) The data sheet for this chip indicates that it is logic controlled and puts out some sort
of PWM signal. The signal is amplified by the power operational amplifiers and sent to the motor. The circuit is complex enough
that I have not been able to decipher it completely, The signals in various places are pulse like and sort of PWM looking. The
final output however looks voltage limited. This element of the design will have to have more analysis done on it.
Summary
Summarizing, it looks like the design of the output stages driving the motors is adequate under normal drive conditions but may
be marginal under extreme conditions such as stall or near stall operation. The larger, stronger motor drives that I have designed
for my own new mount will be driven by a significantly higher power DC amplifier using discrete components with substantial heat
sinking. This more powerful amplifier design will not be costly, perhaps only several tens of dollars. It is important to remember
that a more powerful design for the electronics, motor and gear train will yield a design that can deliver large forces to the worm
and main gears in the drive. Thus, mechanical slip clutches need to be installed to prevent damage to the remainder of the
mechanical system under conditions of jamming. You don•t want to loose a camera, diagonal or finger, after all. (particularly not
the observers)
Now we can see that the basic operation of the system that the computer takes into account all of the command inputs, looks in
the tables, determines the current position and sends a signal that moves the telescope to the new position. This signal is the
number of degrees in Dec and RA or in Alt and Azm that the telescope must move. The motor responds by rotating and sending
back to the computer pulses from the encoder. When the right number of pulses has been received by the computer, it stops the
motor. For the LX200 the number of pulses is about 3 per arc second of motion. The computer has simply rotated the motor
shaft the correct amount. It does not directly measure where the tube is pointing because there is no encoder on either shaft.
When a CCD guider is added, the situation is quite different. In this case, the CCD guider looks at a star and tells the computer
how it needs to move the tube to re-center the star on the guider chip. In this case, the tube is in the feedback loop so that
control can be made as accurate as the CCD chip can manage.
One of the problems in the system is that of lash in the mechanical parts of the system. Mechanical lash is a dead zone in the
feedback loop. Control about the dead zone can be unstable. This causes jumps and jiggles in the operation. A tight system, one
with very little mechanical lash, will behave more smoothly. The idea of my new design is to tighten the system mechanically as
much as possible. That includes reducing the mechanical lash to a very small amount. Because the friction and other forces
increase when this is done, stronger motors to drive the system and higher powered amplifiers are required. The result will be a
smoother running mechanical structure that will move firmly and precisely. (I hope)
Overall summary: I believe the computer and basic design concepts of the LX200 are very good. The shortcomings of the LX200
are, I think, entirely in the size and strength of the drives and their gearing. This is manifest by the apparent result that the larger
LX200s, which used the same drives as the smaller ones, have more problems. A coupling of the computer system of the LX 200
to a drive and mount of greater mechanical integrity would be close to ideal, I believe. I would hope that the 16Ó LX200 with its
larger bearings, gears and drives would behave very well. I hope and trust, that my new design will do well. : - )
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LX200 Electrical Index
Keycodes
Hotplugging
LX200 Classic Keypad Operation
This is an analysis of the LX200 Classic keypad including a description of its various components and how it communicates with
the LX200 main computer. Some commentary on the use of long cables is included.
The pad has 19 buttons. There are 20 connections between it and the converter chip (no surprise there) It looks like all of the
connections go to a 40 pin chip. The chip has an oscillator (crystal) to control the frequency (period) of the digital circuits in the
keypad. There are also a few 74 series logic chips. One of them, an SN74LS14N, (Hex Inverting Schmitt Trigger), has one output
and one input connected to the cable. The other two wires in the cable connect ground and positive voltage. The voltage on the
line is full battery voltage. This is regulated down to 5 volts with a 7805 type three terminal regulator. Additionally there are a few
chips which drive the display.
The 40 pin chip might be a commercial keyboard chip which generates 8 bit words. It is hard to tell about it, because it has no
manufacturer identification. The parallel words from the chip are probably converted to serial with the 74HCT166 and sent to the
computer in the base. It looks like the return serial signal is converted to parallel and sent to the display. The keyboard is a sort
of dumb controller with the main computing done in the computer in the base ot the telescope. There is a tiny computer-like chip
in the keypad that drives the display.
I have set up the keypad with a precision oscilloscope and measured the signals transmitted to and from the keypad to the
computer. One of the lines sends signals to the computer and one receives them from the computer. These signals are of course
in serial form. The code is a simple high/low signal with a time period of 100 microseconds. (10 kHz pulses) I did not try to
translate very many of the signals, this could be done, but it appears that a single 8 bit word is sent each time a key is pressed.
For example West sends 100010101, East sends 101011101, North sends 110001101 and South sends 100110101. This is
consistent no matter what speed mode is set. The speed key and every other key sends a different code to the computer in the
base. When the keypad is in a display mode such as time or coordinates, the computer sends a string of code continuously to the
keypad. Thus it appears that the computer essentially controls the display. The keypad is relatively dumb, doing no more than
sending the key strokes and running the details of setting the display. The pulses are not especially short, 100 micro seconds, and
are crystal controlled in the keypad via the keypad to parallel word converter chip. The wave form of the pulses is very good with
a cable of 25 feet. I do not see why a longer cable could not be used except for the voltage drop in the cable. i.e. The poor
ground it causes.
Note that the commands to the computer are made up entirely of the 19 codes generated by the keypad. The great variety of
actions are accessed by sequences of key presses. Each key has functions which depend upon previous key presses and the
particular mode that the computer is in at the moment. Thus just a few keys can control many functions. This is not very efficient
since some times simple and often used functions have to be accessed through 3 or 4 key presses. There are in addition some
key sequences that are not documented in the Meade instructions. These are described briefly at the end of this discussion.
The keypad draws 74 ma at 5 volts. Thus it consumes 0.37 watts. The rest of the heat in the keypad comes from the three
terminal regulator. In the case of a 12 volt supply this is 0.52 watts and for an 18 volt supply it is 0.96 watts. The voltage drop in
the cable is as expected from the resistance of the cable (standard flat 4 wire telephone cable, #26 gauge) which is 4 ohms per
100 feet. With a current of 74 ma 100 feet gives a drop of 0.3 volts. As it turns out several ground points on the telescope are as
much as 0.2 volts different. The designers have not been very careful about grounding in the electronics to the case.
The ground drop may be of concern since it subtracts from the response voltage window of the Schmitt circuit in the SN74LS14N.
I think the concern about ground voltage drops is real. On the other hand, I do not see a problem with extending the cable with a
wire that provides a good ground. I think the SN74LS14N can drive 100 feet or more of cable with a capacitance of 15 pf. per foot
all right. It has a fanout of 10. The specifications for the chip indicate that it should be able to drive several thousand picofarads.
I am a bit concerned about the heat dissipation in the 7805 three terminal regulator. With an 18 volt supply the dissipation is
about 1 watt. The chip gets quite hot to the touch since it is not provided with any heat sink at all. This chip is the major source
of heat in the keypad. All the warmth is right at the location of the regulator. I am devising a small heat sink for this device since
I insist on operating at the 18 volt supply recommended. I think the entire design of the LX200 electronics was not looked at
from a synergistic viewpoint. The several separate parts do not quite fit together in some cases. (my personal opinion) (If I had a
circuit with grounds not at the same voltage, I would find out why, damn fast.)
In summary, I see no reason to get too concerned about the keypad. It works well. The only problem which might arise with
wear is the functioning of the buttons. They do seem to have a reasonably good snap action however. I have not had trouble
with them or with any functions of the keypad. I have noticed sluggish display response when the temperature got to 0 degrees
F. (not more sluggish that the observer however)
I have heard rumors of reports of keypad failure do to a long cord. I would expect failure to show up as incorrect codes being
passed rather than massive failure of the keypad. It is possible that too much voltage drop in the ground line will cause code
errors. But I would not expect damage to the keypad. Is it possible that damage with a long cable, as rumored, was do to
improper design such as reversing the power supply? This would definitely destroy the voltage regulator and possibly the circuitry
in the keypad. There is no protection whatever within the keypad for an error of this sort. There have been a number of reports
of using long cables without trouble. I recommend caution. Coiled cords of 15 to 25 feet, normally used for telephone handsets,
should be satisfactory. If a cable of greater length is made, it must be wired correctly. A cable with a good ground should be
utilized or a separate ground from the keypad to the control panel with low resistance should be used. The shield and drain wire
in a long cable should be adequate.
Undocumented key presses are accessed from the blank mode. When in blank mode, pressing enter results in "Set Brightness"
which refers to the brightness of the keypad itself. When 7 and 2 are presses in order a menu is available as follows: 1 Learn, 2.
Update, 3. Erase, 4. User 21600, 5. Motor Test 0, 6. Demo. The purpose of these functions seems to be some sort of testing of
the telescope. This much is known. The motor test runs both motors a bit, then runs the altitude motor, bypassing the electronic
stops. With this you can slew the scope into the base on purpose. Pressing the N, S, E or W keys cancels the motor test function
and stops the motors. The Demo function is for an auto tour. The time between slews can be set and when it times out the
scope slews to the next predefined object in the general area and so forth. It is not advised to try to use these functions.
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LX200 Classic Keypad Codes
I have looked at the code sent from the keypad to the computer for all of the buttons on the keypad. I have set the keypad into
various modes and set it at various selections within the menus. Each key sends the same code no matter the setting of the
keypad. That is, the starting point. The computer interprets the key presses depending on what sequence it sees. And the
computer interprets the meaning of the code sent depending on previous key presses. (or sequences of presses)
The code has nine bits. It always starts with a high (1) and ends with a high (1) when the key is pressed and a low (0) when the
key is released. Every press sends a code and every release sends the same code but for the final bit. Here are the codes. Only
the code sent on the presses is given, for release code change the final 1 to 0.
North
110001101
South
100110101
East
101011101
West
100010101
Enter
101001111
Mode
101011001
GOTO
100111001
Next
111011101
Prev
110001011
Guide (0)
100010011
Center (1)
101110011
Map (2)
110110011
CNGC (3)
100110011
Find (4)
111010011
Focus (5)
101010011
Star (6)
110010011
Slew (7)
111110011
Ret (8)
111100011
M (9)
101100011
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LX200 Keypad -- Plugging it Hot (turned on)
I suggest great caution in doing this. I would in fact go so far as to say "DO NOT PLUG OR UNPLUG THE KEYPAD WHILE THE
SYSTEM IS TURNED ON!!!"
Here are the reasons. Because of the design of the power supply in the keypad and the fact that two digital signals are also
transmitted through the connecting cable, there is a distinct probability that hot plugging might destroy the receive chips in both
the keypad and the computer main board.
The problem is that the full power supply voltage of 12 to 18 volts is connected through the cable. The plug is not the correct type
for hot plugging. A hot plug must have a ground connector (pin) which makes connection before any other connections are made.
The telephone type plug is not of this type.
What can happen is that the power line might be connected momentarily before the ground. This momentarily applies, through the
electronics, the full supply voltage to the input and output terminals of the send/receive chips in both the keypad and the
computer board. These chips are designed to operate and do operate at a 5 volt maximum level. In this instant, the chips could
burn out. There is also the possibility of static electricity discharges taking place when the cable is plugged in at either end.
I believe that the system was designed to be turned on and off only with all connections in place. Meade gives this warning. This
may or may not be a good design decision, but it is a fact. (I think it not good)
Also, if for some reason the keypad cable looses its ground, the same result may be effected. My personal feeling is that the
modular connector is not the best choice for this application.
As an aside, I might note that many pieces of equipment have this same problem. When hot connection is desired, an appropriate
plug with a good first on, last off ground must be used. All cables carrying power voltages should have this type of plug. In 40
years of consultation on electronics problems I would estimate that half the problems relate directly to power supplies and
improper grounding.
Summary: Do not hot plug your LX200 equipment.
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Miscellaneous Information About the LX200 Classic Ports
A number of ports on the LX200 control panel are described below. In some cases these are not clearly described in the
instruction manual.
There is a port labeled AUX. This port was originally designed to be used with encoders which were available for the LX200
many years ago when it was introduced. When the encoders did not sell, they were eliminated and the port was relabled AUX. A
very modest power is available from this port. It only provides +5 volts at some unknown, but modest current. I believe that a
modest logic circuit could be operated from it if it required only tens of milliamps. There is no other known use. The signals which
might be available at this port are undocumented. They seem to be logic level input and output signals such as one would expect
from standard encoders.
There is a port labeled RS232. This is a standard 232 port but it contains connections for two 232 ports. One is connected
with the standard 232 cable which in turn connects to a computer. To use both of the ports, a special cable must be made up.
The connections for this cable are as follows. The pins are numbered from the left to the right looking at the front of the LX200
control panel. (Note this is NOT the strange numbering shown in the Meade operation manual.)
pin 1
pin 2
used to
pin 3
pin 4
pin 5
pin 6
+12 volt power
Ground Note that this ground is not the same as the return power socket ground, there is a 0.1 ohm resistor which is
measure power supply currect between the two and thus some 0.05 volts potential.
Misc. Transmit
LX200 Transmit
LX200 Receive
Misc. Receive
The normal LX200 transmit and receive lines go to the computer receive and transmit lines. The second set of lines on pins 3 and
6 then can go to another serial port on the computer. They go to the computers receive and transmit pins respectively. DB type
connectors which connect to most computers are either DB9 or DB25 types which fit the serial connectors on the computer.
Modular plugs and the necessary crimper as well as wire are available from electronics supply houses such as Radio Shack or
Newark Electronics.
There is a port labeled Focuser. This port provides power to the Meade focuser with two speeds controlled from the keypad.
It might be noted that the low speed works ok if the telescope is powered from 18 v but may not work for 12 v. Some other, nonMeade, focusers will also work from this output.
There is a port labeled Reticule This port provides both variable intensity and blinking modes for the Meade 9 mm illuminated
eyepiece. (or similar eyepieces) Note that the ground terminal on this connector is not at the same potential as the return ground
on the power supply socket. There is a 0.1 ohm resistor between the two which is used to measure the power supply current.
There is a port labeled Keypad. This port of course goes to the Keypad. The keypad should not be plugged in or unplugged
when the telescope power on. Permanent damage can be caused to either or both the keypad and computer boards. Excessively
long cables, greater than 50 feet, should not be used unless a heavier ground wire is supplied. The four wire cable carries ground,
power to the keypad and the two signal lines. The signals to and from the keypad are serial digital signals. These are discussed in
detail elsewhere on this web site.
There is a port labeled Power On earlier telescopes this port was labeled 12 volts DC. It is now labeled 18v DC. The scope
seems to work with 12 volts but the higher voltage is recommended by Meade so as to get full power out of the drive motors.
Note that the return line from this socket is not at the internal circuit ground. There is a 0.1 ohm resistor between the socket
return connection and the internal ground which is used to measure the power supply current.
There is a port labeled CCD which is to be connected only to a CCD imager to control the telescope in the guiding mode. The
cable for this connection is special. One of the wires in the cable must be cut or a connector pin removed and the cable itself is
reverse order from a normal 6 wire cable. Take great care to use the correct cable.
It is not only possible but convenient to guide the LX200 through the CCD port. The PEC training of the worm
irregularities can also be done through this port.
In order to do this a keypad must be made which has four momentary switches. The CCD port takes a standard 6 pin modular
plug. This is connected to a standard 6 wire telephone type cable and four momentary switches are connected between the four
control lines and ground. The control lines are normally high (+5volts). The N, S, E, W motions are activated by shorting one of
the four control wires to ground through one of the momentary switches. The pins in the socket are numbered from the left to the
right looking at the front of the LX200 control panel. The keyway is to the bottom and the pins in the socket at the top. These
pins correspond to the following connections. Note this is NOT the strange numbering used in the Meade instruction manual.
pin 1 Do Not Use (do not connect external wire)
pin 2 This is the Ground pin Note this pin is not at the same potential as the return groun for the power supply for reasons
described above.
pin 3 This is the WEST motion pin
pin 4 This is the SOUTH motion pin
pin 5 This is the NORTH motion pin
pin 6 This is the EAST motion pin
A wire of almost any desired length can be used. Mount the four momentary switches on a small control box and connect the
wires to the switches in the order suggested above. Connect the other side of each of the switches to ground. The control box
can be a small plastic box like those found at Radio Shack. Switches can also be found there. The 6 wire cable can be made up
by attaching a modular plug to a length of cable if you have the crimper tool. Otherwise, a 25 foot extension cord can be used
with one end cut off and stripped back appropriately. The advantage of using the CCD port is that the keypad is very simple to
construct.
There is a port labeled Dec Motor which takes the cable that goes to the declination connector on the fork. Note that there is
a second connector on the inside of the fork which should be checked from time to time to avoid intermittent circuits. The wires in
this cable carry the power to the motor, the power to the demodulator circuit board on the declination drive and the encoder
signals from the encoder wheel to the computer.
Return to Beginning
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Return to LX200 Ports
Technical Note on Plugs and Cables for the LX200
General comments are first and specific cable connection schemes at the end of this note.
Special note about grounds in the LX200. There is a 0.1 ohm resistor between the return line on the power connector and the
internal circuit ground. The 232 ground pin is connected to the internal ground. Thus it is not at the same potential as the return
line (negative) on the power plug. This can cause a problem if the LX200 and a computer are connected to the same battery. The
common power ground connection between the two will form a ground loop with the 232 ground connection. The potential
difference is probably only about 0.05 volts but its existence should be recognized.
Many of the control signals on the LX200 are carried by telephone type connectors known as modular plugs/sockets. These are
usually 4, 6 or 8 pin plugs/sockets. These signal cables can be extended to at least 100 feet and often much longer. I have tested
cable lengths up to 150 feet for the following cables. All 232 Serial cables including the computer to LX200 cable, the serial cables
to the various imagers. The keypad cable requires special attention given below.
Some of these cables are available in complete form from Radio Shack or Newark Electronic supply houses. Cables over 25 feet in
length will have to be custom made since none are generally available. A standard handset coiled cord will work for the keypad.
This is a four wire cable with 4 pin Modular plugs at each end. The cable is connected as a normal, straight through cable.
So called modular jack extension wires have a 6 pin modular connector at each end but are generally only a 4 wire cable. These
will work for any of the purely serial communications ports but will not carry ground and power signals since these signals are on
pins 1 and 6 which are not connected or do not exist in these four wire cables.
Custom cables can be made to any length by purchasing bulk 4 or 6 wire flat modular telephone cable and attaching plugs to each
end. The cable and plugs are available from Newark electronic supply or possibly other suppliers that stock telephone supplies. In
order to attach the connectors, a special crimping tool is needed. There is a Telco tool that will crimp both 4 and 6 pin connectors
which should be considered. It is plastic and thus for light duty use. But it is only $18. With the proper crimping tool the
connectors are very easy to apply. Without, it is all but impossible.
It is possible to put the connectors onto the wire in two orientations. A right way and a wrong way. You must get this correct.
When the cable is a standard cable, the color coded wires will go from left to right at one connector and in the opposite direction
on the other when the connectors are looked at side by side pointing in the same direction. Viewed another way, the connectors
and wires go straight through with no twists when laid out straight.
NOTE: the cable that goes from the imager to the CCD socket on the LX200 IS NOT a standard cable. DO NOT substitute for the
coiled cable supplied. A special cable of greater length can be made but do not attempt this unless you are sure of what you are
doing. If a new cable is made, the connections must be EXACTLY like the coiled cord cable supplied for the purpose. This requires
a cable with a twist and one pin removed at each end.
The modular plugs/sockets are made to carry low currents and like all plugs should be worked from time to time to keep down
oxidation on the contacts. Regular use, plugging and unplugging, should not cause problems and will generally be good for the
connectors. Signal circuits are often called "dry" circuits because they carry little current. Even slightly oxidized contacts can cause
problems in "dry" circuits. Thus, if these plugs are seldom used they should be worked from time to time to keep them clean.
Standard contact or switch cleaner can be used but excess should be wiped away.
Recently there have been complaints about declination drive intermittent electrical problems. Many do not realize that there is a
second plug on the inside of the fork where the declination cable plugs in. This plug should also be worked from time to time as
well and cleaned. The declination cable carries motor power, ground and DC supply power as well as the two signals from the
encoder of the motor shaft. Interruption of any of these signals will cause a dead or runaway declination drive depending upon the
type of fault.
The keypad cable requires special consideration. Several persons have reported replacing the cable with up to 150 feet of standard
flat telephone cable with good success. Others have reported failure and burnout of the keypad electronics with cables of such
great length. Meade advises to not extend the cable great lengths. I have been successful with a cable of 100 feet length.
However, there is some concern that the ground wire connecting the keypad to the main control panel will have too much drop in
it due to the current drawn by the keypad. I have measured the voltage drop carefully and found that there might be a problem
with cables much longer than 50 feet. Thus, if you need to make a keypad cable longer than 50 feet, I recommend you use an
additional ground wire, which might be the shield and drain wire in the case of a shielded cable. The small gauge wire in typical
telephone cable might not provide an adequate ground connection.
I hope this primer on cables will help solve some cable/connector problems for LX200 owners.
Specific cable connection schemes for the 232 cable
There are a number of confusing diagrams and very confusing designations for cable connections given in the LX200 operation
manuals. Specifically, on page 29 of the manual (page 28 in the older manuals) there is a diagram of the connector socket for the
CCD connector on the LX200 panel. This connector is shown with the key way to the bottom and the connection wires to the top.
This is in agreement with its orientation on the LX200 control panel. However, notice that the connector wires (pins) are numbered
in a unique manner. They are numbered from the left to the right as 1, 4, 2, 5, 3 and 6. This is contrary to what would make
common good sense. i.e. 1, 2, 3, 4, 5 and 6. The table is correct for the numbering used by Meade on their socket diagram.
On page 85 or 86 of the manual they have another drawing of what presumes to be the socket on the LX200. This diagram is
inconsistent with the diagram on the earlier page. The upper drawing shows the key way at the top and the pin numbers at the
bottom. This diagram is wrong! The lower drawing shows the pins at the top, which is correct if the key way (not shown) were on
the bottom (as it is mounted on the LX 200 control panel). In the lower drawing, the pins are numbered 1, 4, 2, 5, 3 and 6 which
is consistent with the earlier numbering, even though peculiar) The table given is correct for the pins as they are numbered in the
lower drawing. Note that the Figure is called Fig. 61 in the text. It is actually Fig. 29.
To summarize the correct 232 connections: Looking at the socket with the key way to the bottom and the pins (contact wires) at
the top, the second wire from the left is the ground wire and goes to pin 5 on a DB-9 connector and to pin 7 on a DB-25
connector. The fourth pin from the left is the data transmit pin from the LX200 and the data receive pin to the PC. It is connected
to pin 2 on the DB-9 connector and to pin 3 on the DB-25 connector. The fifth pin from the left is the data receive pin on the
LX200 and the data transmit pin from the PC. It is connected to pin 3 on the DB-9 connector and to pin 2 on the DB-25 connector.
No other pins should be or need to be connected. It is especially important not to connect the first pin from the left to anything
since this is the positive hot supply bus of the LX200.
Return to Beginning
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LX200 Classic Command Set
Intended for professional programmers, the LX200 Command Set is used to write custom software for remote operation of the
telescope with a personal computer. Each command is listed in a section appropriate to its type. Each entry in the command list
includes the command name and any parameters which must be appended, any values which might be returned and a description
of the command. The parameters and the return data are shown in a manner that indicates their format. These formats are
listed below along with examples of how the data might actually appear, the allowable range of values and a short description.
This command set is presented here as it appears in the Instruction Manual dated 1996
Ver 0196-2
a. Command Set Formats
HH:MM.T Example: 05:47.4
Range: 00:00.0 to 23:59.9
Hours, minutes, and tenths of minutes.
sDD*MM Example: +45*59
Range: -90*00 to +90*00
Signed degrees and minutes (the (*) represents ASCII 223 which appears on the hand controller
as a degree symbol).
DDD*MM Example: 254*09
Range: 000*00 to 359*59
Unsigned degrees and minutes.
HH:MM:SS Example: 13:15:36
Range: 00:00:00 to 23:59:59
Hours, minutes, and seconds.
MM/DD/YY Example: 02/06/92
Range: 01/01/00 to 12/31/99 (see description)
Month, day, and year. The two digit year indicates the following: 92 through 99 = 1992 through 1999
and 00 through 91 = 2000 through 2091
sHH Example: -5 Range: -24 to +24
Signed hour offset.
NNNN Example: 3456
Range: 0000 to 9999
Four digit object number.
sMM.M Example: -02.4
Range: -05.5 to 20.0
Signed magnitude value.
NNN Example: 134
Range: 000 to 200
Three digit object size (minutes).
DD* Example: 56*
Range: 00* to 90*
Higher' parameter (degrees).
TT.T Example: 59.2
Range: 56.4 to 60.1
Tracking 'frequency'. Info
(missing command set format line in manual)
Example: CNGC1 976 SU DNEB MAG 3.9 SZ 66.0'
Range: n/a
Object information.
Ok
Example: 1
Range: 0 or 1
Status value returned after setting values. If the value is legal 1 is returned, otherwise 0 is returned.
b. General Telescope Information
Command ACK (ASCII 6)
Returns A, L, P or G
Gets alignment status, A for alt-azm, L for land, P for polar, G for German mount polar.
Command :GR#
Returns +HH:MM.T#
Gets the current Right Ascension.
A Mapug-Astronomy post indicated that for the 3.34L ROMS the format is +HH:MM:SS# for RA
And the format for Declination is sDD*MM'SS#. Thus adding seconds and arc seconds.
Command :GD#
Returns sDD*MM#
Gets the current declination.
Command :GA#
Returns sDD*MM#
Gets the current altitude.
Command :GZ#
Returns DDD*MM#
Gets the current azimuth.
Command :GS#
Returns HH:MM:SS#
Gets the current sidereal time.
Command :SS HH:MM:SS#
Returns Ok
Gets the sidereal time.
Command :GL# :Ga#
Returns HH:MM:SS#
Gets the local time either in 24 hour (GL) or 12 hour (Ga) format.
Command :SL HH:MM:SS#
Returns Ok
Sets the local time. NOTE: The parameter should always be in 24 hour format.
Command :GC#
Returns MM/DD/YY#
Gets the calendar date.
Command :SC MM/DD/YY#
Returns Ok (see description)
Sets the calendar date.
NOTE: After the Ok, if the date is valid, two strings will be sent. The first will contain the message
"Updating planetary data," the second (sent after the planetary calculations) will contain only blanks.
Both strings will be terminated by the (*) symbol.
Command :Gt#
Returns sDD*MM#
Gets the latitude of the currently selected site.
Command :St sDD*MM#
Returns Ok
Sets the latitude of the currently selected site.
Command
:Gg#
Returns DDD*MM#
Gets the longitude of the currently selected site.
Command :Sg DDD*MM#
Returns Ok
Sets the longitude of the currently selected site.
Command :GG#
Returns sHH#
Gets the offset from Greenwich Mean Time.
Command :SG sHH#
Returns Ok
Sets the offset from Greenwich Mean Time.
Command :W1# :W2# :W3# :W4#
Returns Nothing
Sets the current site number.
c. Telescope Motion
Command :Mn# :Ms# :Me# :Mw#
Returns Nothing
Starts motion in the specified direction at the current rate.
Command :MS#
Returns 0, 1, or 2 (see description)
Slews telescope to current object coordinates. 0 is returned if the telescope can complete the slew,
1 is returned if the object is below the horizon, and 2 is returned if the object is below the 'higher'
limit. If 1 or 2 is returned, a string containing an appropriate message is also returned.
Command :Qn# :Qs# :Qe# :Qw#
Returns Nothing
Stops motion in the specified direction. Also stops the telescope if a slew to object is in progress.
Command :Q#
Returns Nothing
Stops a slew to an object.
Command :RG# :RC# :RM# :RS#
Returns Nothing
Sets the motion rate to guide (RG), center (RC), find (RM), or slew (RS).
d. Library/Objects
Command :Gr#
Returns HH:MM.T#
Gets object Right Assension
Command :Sr HH:MM.T#
Returns Ok
Sets object Right Ascension.
A Mapug-Astronomy post indicated that for the 3.34L ROMS the format is +HH:MM:SS# for RA
And the format for Declination is sDD*MM'SS#. Thus adding seconds and arc seconds.
Command :Gd#
Returns sDD*MM#
Gets object Declination.
Command :Sd sDD*MM#
Returns Ok
Sets object Declination.
Command :CM#
Returns (see description)
Sync. Matches current telescope coordinates to the object coordinates and sends a string indicating which
object's coordinates were used.
Command :Gy#
Returns GPDCO#
Gets the 'type' string for the FIND operation. A capital letter means that the corresponding type is selected
while a lower case letter indicates it is not.
Command :Sy GPDCO#
Returns Ok
Gets the ¥type' string for the FIND operation.
Command :Gq#
Returns SU#, EX#, VG#, GD#, FR#, PR# or VP#
Gets the current minimum quality for the FIND operation
Command :Sq#
Returns Nothing
Steps to the next minimum quality for the FIND operation.
Command :Gh#
Returns DD*#
Gets the current 'higher' limit.
Command :Sh DD#
Returns Ok
Sets the current 'higher' limit.
Command :Gb# :Gf#
Returns sMM.M#
Gets the brighter (Gb) or fainter (Gf) magnitude limit for the FIND operation.
Command :Sb sMM.M# :Sf sMM.M#
Returns Ok
Sets the brighter (Gb) or fainter (Gf) magnitude limit for the FIND operation.
Command :Gl# :Gs#
Returns NNN*
Gets the larger (GI) or smaller (Gs) size limit for the FIND operation.
Command :SI NNN# :Ss NNN#
Returns Ok
Sets the larger (GI) or smaller (Gs) size limit for the FIND operation.
Command :GF#
Returns NNN#
Gets the field radius of the FIELD operation.
Command :SF NNN#
Returns Ok
Sets the field radius of the FIELD operation.
Command :LF#
Returns Nothing
Starts a FIND operation.
Command :LN#
Returns Nothing
Finds the next object in a FIND sequence.
Command :LB#
Returns Nothing
Finds the previous object in a FIND sequence.
Command :Lf#
Performs a FIELD operation returning a string containing the number of objects in the field and the
object that is closest to the center of the field.
Command :LC NNNN# :LM NNNN# :LS NNNN#
Returns Nothing
Sets the object to the CNGC (LC), Messier (LM), or Star (LS) specified by the number. Planets are
¥stars' 901-909.
Command :Ll#
Returns Object Information
Gets the current object information.
e. Miscellaneous
Command :B+# :B-# :B0# :B1# :B2# :B3#
Returns Nothing
Increases (B+) or decreases (B-) reticle brightness, or sets to one of the flashing modes (B0, B1, B2 or B3).
Command :F+# :F-# :FQ# :FF# :FS#
Returns Nothing
Starts focus out (F+), starts focus in (F-), stops focus change (FQ), sets focus fast (FF) or focus slow (FS)
Command :GM# :GN# :GO# :GP#
Returns XYZ#
Gets site name (XYZ). M through P correspond to 1 through 4.
Command :SM XYZ#
Returns Ok
Sets site name.
:SN XYZ#
:SO XYZ#
:SP XYZ#
Command :GT#
Returns TT.T#
Gets the current track 'frequency'.
Command :ST TT.T#
Returns Ok
Sets the current track 'frequency'.
Command :TM# :TQ# :T+# :T-#
Returns Nothing
Switch to manual (TM) or quartz (TM). Increment (T+) or decrement (T-) manual frequency by one tenth.
Command :Gc#
Returns (12) or (24)
Get 12/24 hour status of clock.
Command :H#
Returns Nothing
Toggle 12/24 hour mode.
Command :P#
Returns "HIGH PRECISION" when ON "LOW PRECISION" when OFF
Toggles the High Precision Mode ON or OFF.
Command :U#
Returns Nothing
Toggles the long format ON or OFF.
When the long format is active, whenever a request to send or receive position data, the following format is used:
HH:MM:SS
Example: 05:47:45
Range: 00:00.0 to 23:59:59
Hours, minutes, and seconds.
sDD*MM:SS
Example: +45*59:45
Range: -90*00 to +90*00
Signed degrees, minutes, and seconds (the'*' represents ASCII 223 which appears on
the handbox as a degree symbol).
DDD*MM:SS
Example: 254*09:45
Range: 000*00 to 359*59:59
Unsigned degrees, minutes, and seconds.
Command :Lo N#
Returns Ok
Sets the NGC object library. 0 is the NGC library, 1 is the IC library and 2 is the UGC library. This operation
is successful only if the user has a version of the software that includes the desired library.
Command :Ls N#
Returns Ok
Sets the STAR object library type. 0 is the STAR library; 1 is the SAO library, and 2 is the GCVS library. This
operation is successful only if the user has a version of the software that includes the desired library.
f. Keypad Hand Controller Specific
Command :D#
Gets the distance 'bars' string.
Command :$Q (1-5)#
Returns Nothing
Toggles Smart Drive status.
Command :?#
Command :?+#
Command :?-#
Returns Page of Help Information
Starts (??) or moves through (?+ or ?-) Help.
Command
Command
Command
Returns
Used
: G0#
: G1#
: G2#
Alignment Menu Entry
to implement alignment menu
LX200 Demo Program
The RS-232 interface communicates with your computer at 9600 Baud Rate, Parity = None, 8 Data Bits, 1 Stop Bit.
For those who are familiar with programming, the LX200 Command Set is written in ASKII character format and
can be used to write your own programs. The LX200 Demo Program is written in Quick Basic and is intended to
demonstrate how commands can be sent to the telescope and information received from the telescope. It is not a
"polished" program and does not incorporate all of the RS-232 features available. The program is set-up to operate
on serial port 2 (COM2). To operate on serial port 1 (COM1) line 4 should be changad from "COM2 to "COM1.
The program is as follows: Please note that Meade Instruments does not support these programs, or programs that
you may write in any way. For questions relating to after-market software programs, refer back to those manufacturers.
LX200 Test Program
Once you have the RS-232 cable constructed you will want to test the cable. Below is a simple program called "LX200
TEST" that is written in GW Basic programming language and will work with IBM compatible computers. LX200 TEST
is an effective program to fully check the RS-232 line communications from your personal computer to the LX200,
allowing you to concentrate on de-bugging your RS-232 cable.
To enter the following program, first load BASIC or GWBASIC (which ever your computer system uses), then type in the
following program. When complete, be sure to save the program as "LX200TST.BAS."
10
20
30
50
60
70
CLS
DEFINT A-X
OPEN "COM1:9600,N,8,1,CDO,CSO,DSO,RS," FOR RANDOM AS #1
key1$ = INKEY$: IF key1$ = * * THEN GOTO 50
REM Key 1 loop
IF key1$ = CHR$(119) THEN GOSUB 200: REM "w" key
80
90
100
105
110
120
200
210
220
230
240
250
260
270
280
290
300
400
410
420
430
440
450
460
IF key1$ = CHR$(101) THEN GOSUB 200: REM "e" key
IF key1$ = CHR$(110) THEN GOSUB 200: REM "n" key
IF key1$ = CHR$(115) THEN GOSUB 200: REM "s" key
IF key1$ = "x" THEN END: REM To exit test.
GOTO 50 REM return to Key 1 loop
END
REM.Key 1 direction printouts to COM 1
REM west
IF key1$ = "w" THEN a$ = "#:Mw#" : PRINT #1, a$: REM GO west
REM east
IF key1$ = "e" THEN a$ = "#:Me#" : PRINT #1, a$: REM GO east
REM north
IF key1$ = "n" THEN a$ = "#:Mn#" : PRINT #1, a$: REM GO north
REM south:
IF key1$ = "s" THEN a$ = "#:Ms#" : PRINT #1, a$: REM GO south
key1$ = INKEY$:
IF key1$ = CHR$(32) THEN GOTO 400 ELSE GOTO 200
REM This stops motion (by hitting SPACE bar).
B$ = "#:Qe#" : PRINT #1, B$
B$ = "#:Qw#" : PRINT #1, B$
B$ = "#:Qn#" : PRINT #1, B$
B$ = "#:Qs#" : PRINT #1, B$
RETURN
END
To use the above program, connect the completed cable to your PC serial port and to the LX200 RS-232 Port. Load BASIC (or
GWBASIC), if not already loaded, and run "LX200TST.BAS - " Nothing will appear on the computer screen. Press any one of the N,
S, E, or W (lower case) keys on your PC keyboard, this will move the LX200 North, South, East, or West respectively. Press the
space bar on the PC keyboard to stop. Press X to exit the program. If the LX200 does not respond to the N, S, E, or W keys, be
sure the CAPSLOCK is OFF. If it still does not work, check the PC serial port pinouts for your computer to be sure they are wired
correctly to the LX200 6 line connector. With a successful check-out of the PC link with the LX200 using "TEST", you are now
ready to write your own software program using the LX200 Command Set or to use the sample program called "DEMO" that is
written in Quick Basic software language.
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Circuit Diagrams for the LX200 Classic --part 1
Here are the circuits of the LX200 telescopes. These are not official documents but are deciphered from the circuit boards
themselves. This is an incredably difficult process. It was done with great care, but there may be errors. These are presented to
satisfy interested parties who may want to know more about the types of circuitry involved in their imastruments. It is not known
if all vintages of the circuitry are the same.
The main computer board digital electronics is a .gif file of size 192K press here to see it.
To see a picture of the motherboard press here to see it.
The the main computer board analog drivers is a .gif
file of size 116K press here to see it.
The circuit board associated with the motor drivers is a .gif of size 68K press here to see it.
Circuit Diagrams for the LX200 Classic --part 2
From: "Tim" <tim_at_telescopeservice.com>
> If you want to see the circuits for the LX200 (Classic) go to
> <http://www.telescopeservice.com> and go to the Technical page.
> I have completely reversed engineered
> the main board, front panel, DEC and RA motors, and the hand
> controller and have posted the schematics . These schematics are in PDF
> format and are freely available to download. The driver chip in question
> is an Allegro MicroSystems UDN2993 which is no longer in production, so I
> would doubt that Meade used it on the GPS version. The chip is rated to
> 30V @ 500 MA, but is NOT curent limited, though it does have protective
> diodes for inductive kick.
Actually, the UDN2993, is rated at 600mA 30V. 500mA, is the typical 'application' maximum. The chips do have limited current
protection (depends on the version - Allegro claim no protection, but the 'second source' version from Sprague lists limited
overcurrent protection - 1 second max...), and provision for full limiting, but Meade elected not to use this ability (they short the
current senses to GND...). They however limit the maximum supply current available, via Q1, but though this should work, the
behaviour is dependant on the hfe of this transistor, rather than on a proper sensor.
For anyone who does damage this driver, the UDN2998, is pin compatible, higher current rating, and still available.
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Return to Bearing Replacement Article
Replacing the Declination Bearings
on the 12" LX200
I decided it was time to replace the bearings in the 12" LX200 which I had a year earlier donated to the Madison Astronomical
Society. This telescope has been living in a permanent building at the MAS dark site for two winters and a summer. In Wisconsin
this is an appreciable time subject to the complete temperature and and humidity ranges extant.
The telescope has been working perfectly in every way with the exception that the declination drive failed. This failure is described
elsewhere in this web site. After inspecting the drive I found that the gear on the end of the worm shaft had worn its key way
and was able to move several degrees on the shaft. I replaced the broken drive with a new drive which I purchased from Meade.
The telescope then came back to its normal operating condition and pointing accuracy, which was actually quite good. I felt that
the original source of the failure was that the declination bearings were causing rough motion of the declination mechanism. The
Hart bearing replacement suggestion came along at this time. I got experience in doing the replacement by replacing the bearings
in my own 10" LX200. This was convenient to do since I had the telescope in my basement laboratory for other tests at the time.
Doing the second replacement was a very easy job. I dismantled the 12" telescope at the observatory in about 40 minutes. The
most difficult part of the operation was again removing the declination clutch screws. They were both very tight and predamaged. They came out but with a medium tough struggle. I took the fork arms home, mounted them in my mill and bored out
the bearing holes in about 30 minutes. The new needle bearings were inserted and held with medium loktite.
The fork arms were returned to the observatory and the entire telescope put back together. This took about 45 minutes. The
results were even more pleasing to me than those for the 10" telescope. I feel that the amount of improvement for the 12" was
much greater than for the 10". This is, I think, because the basic mechanism, mechanics, of the 12" is much more marginal than
for the smaller 10" telescope. The weight of the 12" especially with the accessories that this 12" carries is great enough to stress
the drives provided.
The 12" carries the Meade dew hood which is quite heavy and placed well to the front of the declination axis of course. There is
also a Celestron C-5 in Losmandy rings and with a Losmandy rail which adds quite a bit of weight, fortunately very near the
declination axis. On the back is a JMI focuser, a 2" diagonal and 2" eyepiece and on occasion an ST-7 CCD imager. In order to
balance the accessories, especially the hood, it is necessary to use 7 pounds of Meade weights as close as possible to the back
plate and a Losmandy 2D rail and weight set consisting of 7 1/2 pounds of weights a maximum extension from the optical tube.
(The Losmandy 2D rail was extended to fit the 12" telescope optical tube.)
Before the bearing replacement, it was almost impossible to tell when the tube was balanced because of the excessive friction in
the declination bearings. It is now very easy to balance the tube using the two position method because it is very clear when
balance has been reached. In addition to much better balancing, the Slew setting is always set to about 4 degrees per second
instead of the maximum slew rate. I have always felt that the maximum slew rate is too high for the capabilities of the very small
drive motor and gears used on these telescopes. There is really no reason that the telescope needs to slew as fast as it will at
maximum setting. Unfortunately, the computer does not remember the slower setting when turned off.
I am very pleased with the bearing replacement and now feel the telescope is much less fragile and will serve more hands without
damage. I will report on the success of this modification in the coming months. In over a year now, there have been many
reports of successful modifications of LX telescopes using this information. A few concerns have related to some specific telescope
where slightly different use of thrust washers and final adjustemnts were required.
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In March of 2000, an important post appeared on MAPUG-Astronomy which describes some experiences with
carrying out the dec bearing modification. This addendum is an edited version of two posts on this topic from Neal
Barry. Neal has modified three LX scopes and can be considered one of our growing clan of experts.
(The following is a paraphrase of his comments by Doc G)
1. I had a difficult time getting the under-side plate of the declination clutch off. I was able to get the two screws out
(surprisingly, without heating to soften the Loktite) I then used a small wheel puller that pushed off of a 1/4-20 bolt threaded into
the clutch knob hole to pull on the clutch plate. I tightened it up real well - nothing happened. I waited a while and tightened it
a far as I felt was safe - still nothing. Hmmm... For the heck of it, I got out the ol' propane torch and heated the center of the
plate for about 45 seconds. Still nothing! As I walked away, I heard a terrific bang - it broke free! Two of the three LX telescopes
I've modified have had some sort of really nifty adhesive that bonded the clutch plate to the declination shaft in addition to the
two screws. (Ed. Apparently some significant pulling has to be done on some of the telescope clutch plates to get them off. On
the telescopes I modified they came off easily.)
2. I used an SCE2012 (McMaster p/n 5905K29) needle bearing with the addition of a PI162012 (McMaster p/n 7929K56)
hardened steel inner ring on both axes. I found with the first telescope that I modified that using a needle bearing alone would
ultimately put indentations in the original LX200 case aluminum shafts when carrying a heavy, but reasonable, load. As the
indentations 'grew' it became increasingly difficult to achieve accurate pointing. (Ed: This is a very important bit of new
information. It might be wise when doing the modification to go to the steel sleeved bearing parts.)
3. Through experience I found that I needed to bore out the LX200 arms significantly undersize in order to eliminate the lateral
slop. In the case of the bearing combination above, the SCE2012 is approximately 1.505" and is designed for a press fit into a
1.50" hole. I had to actually bore the arms to 1.496-7" to achieve a compression fit with approximately 0.0005" lateral play
between the inner ring and the needle bearing. (I used Loctite to fix the ring to the declination shaft..) This might seem like nitpicking, but the difference in pointing accuracy was quite pronounced. With any slop, the entire OTA would 'flop' over as the
telescope rotated in RA.
4. When you reattach the arms, you need to provide sufficient preload to the declination axis. (between the arms) Normally one
arm is already locked in place (see Doc G's main article) and then you accurately tighten up the other arm to a predetermined
position which should 'pinch' the OTA between the two arms. The shoulders on the two plastic bushings that were replaced by the
bearings contribute approximately 2 X 0.042" (0.084") to the declination axis preload. I used a single additional 0.031" thrust
washer (McMasters p/n 5909K49 on the arm that carried the declination drive gear and two of the same thrust washer on the
opposite side. This results in slightly more preload than comes from the factory, but I have found that a little more does not hurt
and too little can contribute to pointing errors.
(Ed. My experience has been similar. The amount of preload necessary becomes apparent when the arms are re-assembled and
tightened. Tighter is better that allowing any slop.)
With everthing fitted tightly, it is wise to align the arms accurately. The use of a 1" precision ground steel bar is helpful. Detailed
instructions for re-assembling the telescoe arms is given in the main article and should be followed carefully.
Overall, I would recommend this modification to any LX200-12" owner who has the necessary mechanical savvy to carry it out and
needs the improved pointing accuracy and/or has a heavy load of accessories on the OTA. In all three of the reworked LX200
telescopes the pointing accuracy improved to better than 2 arc minutes across the sky (with mirror flop eliminated) On my own
scope, on which I've spent a little extra time, I've achieved better than 25 arc second pointing accuracy with the help of T-point.
This is quite good for a mass-produced telescope.
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This is a brief discussion of how to
bore the declination bearing holes to
receive the needle bearings.
Both fork arms can be removed from the telescope and the bearing holes bored open to receive the new needle bearings. The
original holes are nominally 1.1 inches in diameter. Each of these holes has a nylon insert which should be removed and
discarded. Clean the residual grease from the bearing hole. The fork arm can be mounted as shown in the photo on the flat bed
of a small milling machine. The existing hole should be carefully centered under the milling spindle. A boring tool should be used
which is set to a diameter of 0.003 inches under the outside diameter of the needle bearing to be used. The bearings I used were
1.330 OD. I used a fly cutter set to 1.3 inches. The setup with the cut started can be seen in the photograph.
The Aluminum casting used for the fork arms is soft and very easy to cut. There are however lots of voids. These do not affect
the outcome. After the arm is bored, I used a bearing scraper (hone is ok) and opened the holes to fit the bearing snugly but to a
finger push fit. This took just a few minutes of scraping.
The photograph below shows the bearing partly inserted into the fork arm opening. After it is confirmed that the bearing is a
smooth fit, it can then be coated with medium loktite (blue) and pressed into place with the inside surface flush to the fork arm.
Once this is done, the bearing must be firmly packed with grease and the telescope is ready for reassembly.
The bearing replacement operation turns out to be a very easy to accomplish. I have done both a 10" and a 12" LX200 with
absolutely no problems.
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Using a CCD Imager to Train the
Precision Error Correction in the LX200
Training of the LX200 PEC with autoguiders seems to fall into to two categories, that done with the ST-4 and that with the 216XT.
The report here is from Philip Perkins on the use of the ST-4 (note: mixture of English and US spelling intentional)
The reports on the use of Pictors for this purpose are very scarce at this time (January 1999) See short comments below for
the 216XT data.
The very best information I have seen on the issue of Guiding Techniques for Astrophotography (film) have been compiled by
Philip Perkins and can be found at his fine web site <http://www.astrocruise.com> Mr. Perkins is, by the way, one of the
finest film photographers ever to use an LX200, in my opinion. I have freely extracted from some of his MAPUG-Astronomy posts
on this topic. (slight editing imposed, Ed.)
Reports from Philip Perkins, who writes about his experience with the ST-4.
For what it's worth.. I've been training the PEC on my LX200 for about 4 years now. I have found that on my LX200 accurate PEC
training is absolutely essential to achieve accurate guiding over the 3 hour exposures that I sometimes make. If I do not train my
PEC then sometimes the errors have sufficient magnitude to drive the star right out of the ST-4 guiding box. I achieved the most
accurate training last June. It is so good that I have hung on to it ever since. The big surprise is that this training was done with
the ST-4. This directly contradicts a post I made a year ago in which I said that I had not had any success in training the PEC
using my ST-4.
I think that three factors made the difference: - The seeing was exceptionally good - I chose an integration time of .3 seconds on
the ST-4 - I selected Faint Mode on the ST-4 Following the training, I did one update.
I believe that all three factors were important, but Faint Mode especially so. Faint Mode has a special algorithm that smoothes the
star image, as well as boosting its brightness. This has the tremendously useful effect of canceling out rapid star movement due to
scintillation. When we train the PEC we want to record only star movements associated with worm error, and we must be careful
to avoid programming in movements due to scintillation.
Faint Mode meant that only genuine drift corrections got programmed in, and the .3 second integration meant that errors were
corrected very swiftly. When using Faint Mode, star brightness readings should be in the range of about 8 - 18. The combination
of .3 seconds and Faint Mode means that some time may be required to select a star of suitable brightness. But it could be worth
it. I continue to be very happy with the training I achieved 6 months ago. In retrospect, the effort seems very worthwhile.
During my four years of experience in PEC training I had tried using an autoguider many times, using many different settings, with
both my original 201XT and my current ST-4. None of these attempts worked very well, and until last June I also believed that
manual training was the only viable method. But there was one very significant change that I think was primarily responsible for
the breakthrough I made last June. As stated earlier, I used Faint Mode. As well as boosting star brightness, this mode also
smoothes the star image which almost completely cancels rapid star movement due to scintillation. The effect is dramatic, and it
overcomes about the only major limitation in using an autoguider for PEC training.
I agree that in theory it should be possible to achieve excellent PEC training every time using the manual method. The problem
with the manual method is that it needs to be done by humans, and humans are all different. There are some very real and
practical limitations to the manual method from my own experience. I suspect that many LX200 owners will nod in agreement at
these points. Despite best attempts, it is very difficult to maintain split second concentration over the whole 8 minute period, twice
over. How many times does one's attention wander, or is one distracted by something? When one's concentration is restored the
guide star is often found well outside the limits.
How many times does one over-or-under shoot when making corrections? Despite years of experience in discriminating between
seeing induced motion and worm induced motion, it remains difficult to do this accurately. All too often one is left madly stabbing
the keypad while trying to bring an over-corrected star back to the centreline. An autoguider completely overcomes these foibles.
Corrections are very precise, occur very rapidly at the moment of the error, and are completely consistent over the whole 8
minute period. However there are two problems with using an autoguider, which may be overcome as follows:
Normally an autoguider is completely unable to discriminate between seeing induced motion and worm induced motion, whereas
the human eye can, to a limited degree. This is a major problem, but is very effectively overcome by using the Faint Mode of the
ST-4. As mentioned above, the effect is dramatic. Note that this solution is unavailable with other imagers. Note that binning is
not the same as Faint Mode.
The results with the ST-4 seem to indicate that it is more than rapid enough for accurate PEC training. Yet higher frequency can be
achieved by using 0.1 second exposure. Moreover, each correction is very precise compared to what an average human can
achieve. There are others out there with ST-4 experience. In fact the vast majority of the most advanced astrophotographers use
the ST-4, and many of them use sub-second integration times with their AP refractors.
My best training (using the ST-4) is somewhere around 6 -8 arc seconds peak to peak. The variance is due to recording slightly
different results at different times. It's difficult to measure accurately because of star scintillation. Under ideal seeing and expert
manual training error residuals in the 4 to 8 arc seconds area have been reported. This is exceptionally good with the known
limitations of the Meade PEC system which averages over 2.4 second intervals..
The reason why the autoguider method is so interesting is exactly because of repeatability. It takes out the human factor. I
suspect it may do so for several other LX200 users. It sounds as though you are able to achieve superb training absolutely
repeatably, but I wonder how many other people can? Not only do I wait until seeing is optimum but I program the PEC during the
time just before dawn when seeing is steadiest. Opportunities for optimum PEC training are pretty rare which is why I need to
hang on to an optimum training for as long as possible.
This topic has had a good airing now and I feel comfortable that the pros and cons on both sides (manual training and CCD guider
training Ed.) have come to light. But at the risk of wearying the patience of those not interested in PEC training I would like to
make a couple of observations.
I can't claim to have achieved anything much better than 10 arc seconds peak to peak with manual training. With the average
scintillation that we get here I find it near impossible to record this much more precisely than 1-2 arc seconds either side. It's
possible that with the training on July 8, 97 I achieved 8 arc seconds, but I also recorded 10 arc seconds on a couple of occasions.
I use a Meade 4000 series 2x barlow in conjunction with a 12.5 mm illuminated reticule Plossl. This gives about 400x, which is
about the maximum for any meaningful work under most conditions.
Discussion of Faint Mode in the ST-4
Faint Mode is potentially very important for any LX200 owner who has to work under less than ideal seeing conditions. For me,
and I suspect many others, that means most of the time. Faint Mode works by replacing each pixel value in memory with the sum
of the 3 x 3 box centered on that pixel. Hence it greatly boosts the brightness value of each pixel and it also performs an
averaging / smoothing function across the entire star image. This means three things:
- the overall star brightness is greatly increased
- if a star is in rapid motion owing to scintillation, the star's average position in that range of motion will be accurately uncovered
by the smoothing function.
- sub pixel accuracy can be reduced where a pinpoint star image is available in completely stable conditions.
It is true that the resulting star image will be expanded, but if you look at the mathematics of the function you will see that in
practice it has very little effect on the centroid calculation. As an ST-4 user you will be aware that it has a particularly
sophisticated and effective centroid calculation algorithm that is capable of detecting the centroid to within 0.2 of a pixel. However
this is only made possible by the pinpoint star images delivered by precision short focus instruments in stable conditions, where
typically the star image is mostly contained within 1 pixel. This will never be true for those of us using long focus SCTs, where the
star image will always cover several pixels, even under very stable seeing conditions. The real answer is that for SCT users, there
is no perceptible loss in guiding resolution through use of Faint Mode because the star image always covers several pixels to start
with, and the accuracy of the centroid calculation is hardly affected under these conditions.
Well, that's the theory side of it, but in practice the rationale for Faint Mode gets much better. In practice, most of us have to
contend with less than perfect seeing, most of the time. This has two main effects on the star image.
- The star's position remains fairly stable, but it is bloated and distorted by the seeing. The distortion often causes false brightness
spikes in the star's image that are offset from the centre. A short integration can mean that the star is captured in such a distorted
shape, causing the autoguider to make a false correction. The smoothing function restores the round shape of the star and
restores the brightness weighting towards the centre of the star.
- The star image is fairly crisp, but it moves ('wobbles') rapidly about its true position. This can mean that the star image is
smeared across several pixels with a brightness peak usually offset from the true centre, causing the autoguider to make a false
correction. The smoothing function averages out the brightness variations, allowing the centroid calculation to fix the centroid on
the average position.
Of course, there can be combinations of the above, where the star image both bloats / distorts and wobbles at the same time. In
such conditions the benefit of Faint Mode is even more dramatic.
The result of all of this is that in moderate or poor seeing conditions using a long focus SCT, the use of Faint Mode
_significantly_improves_ guiding accuracy. This probably sounds like a lot of theory. Not a bit of it - in the early part of 1997 I
exposed a large number of frames comparing Faint Mode with Normal Mode. The results were so compelling that from March 1997
onwards I completely standardised on Faint Mode. Every single image on my web site has been exposed with Faint Mode enabled.
I'll pick an interesting example: <http://www.astrocruise.com/m51.htm> because I have been asked a couple of times if it
was taken using an AO system. I suppose the answer is that SBIG's AO system doesn't work too well with film ;-) - no, it was
simply taken with a 'virgin' LX200 using an ST-4 with Faint Mode enabled.
I expect that most LX200 users with autoguiders have seen this problem: the guiding errors are large and frequent large
corrections are made. The LX200 is constantly over-or-under correcting, and the result is trailed stars. What's happening is that
the autoguider is 'chasing the seeing' - exactly what it should not do. Enable Faint Mode on the ST-4 and the result is dramatic guiding errors are cut in half and the whole system stabilises - the system is guiding on true star drift, as it should do.
Comments from Doc G
The above related experiences are similar to others I have seen over the past year or so both as to experience for typical self
guiding function and in use for training the PEC. I thought it worthwhile to post these comments so that a complete picture of the
dedicated use of the ST-4 can be had. It is clear that a CCD guider can be used for PEC training, but that everything has to be just
right. Mr. Perkins' experience with the ST-4 is very encouraging.
I have found training for the full 8 minutes to be tough. Eight minutes can seem like a very long time when staring into the scope,
bumping it and waiting for it to settle down, and punching the buttons like mad at the same time. In fact, at magnifications of
300X I found it takes upward of 5 seconds for the LX200 to settle down to the point where meaningful corrections could be made.
This leaves a gap of 2 or 3 sections without correction. I did manage to get a ten times reduction in the worm errors with one
learn and one update and was at the 6 to 8 arc second level. This is about is as good as possible I think. But, I would certainly like
to train my LX200s with a CCD imager. It is clear that the ST-4 does send information often enough to effect a good training.
Since the PEC averages over 2.4 second sections of time, it might only be necessary for the CCD imager to send corrections at
that rate. One must apply sampling concepts, such as the Nyquist criterion, with caution when the effective bandwidth of one
element in the control chain is so limited.
Comments On the Use of the 216XT for PEC Training
I was not very successful using the 216XT to train my LX200s, but that was about a year ago, before I did the bearing
modifications. I used the 216XT with an exposure setting of 1/4 second and choose a star bright enough to get a brightness
reading of 10 to 20 on the 216XT readout. This seemed to give corrections every second or so. I found that this was satisfactory
for guiding after the PEC was manually trained but was not useful for actually training the PEC.
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Data on Precision Error Correction for Two LX200 Telescopes
Here is a bit of data on worm drive errors found in two LX200 telescopes. The top plot shows the error curves for my own 12"
LX200 measured in August 1997 with the PEC turned OFF and also with the PEC turned ON after a very good training session. The
middle plot shows the same telescope but with a very poor training session. The lower plot shows the error curve for my 10"
LX200 with the PEC turned OFF and with the PEC turned ON after a very good training session. (The 10" was measured sometime
in Summer of 1998)
The erratic curve of the RA worm irregularities with the PEC turned OFF are shown for 200 bins (8 minutes). The deviations are
about 50 arc seconds peak to peak for the 12" and about 25 arc seconds peak to peak for the 10". With careful PEC training, as
shown for both the 12" and the 10", these deviations can be reduced greatly. The improvement factor of 5 to 10 as shown above
is obtainable with this careful training. This sort of improvement has been verified by many reports from Mapug-Astronomy
members.
It is clear that the irregularities are not smooth and nicely sinusoidal, but are very irregular. This is due to the fact that the worm
surface is ragged on a microscopic level. Remember that an arc second of pointing error is caused by an irregularity of only a
wavelength of light or so. We can safely assume that all worms show similar errors as has been reported by many on MapugAstronomy members over the years.
It should be perfectly clear from this data and the many reports from experienced astrophotographers just how important PEC
training is. We are most fortunate to have this feature on our LX200 telescopes. After careful training, it becomes much easier to
either manually guide or to guide with a CCD guider.
Data of this sort is very easy to compile. One simply watches through a calibrated eyepiece, for example the Meade 9 mm or
Celestron 12.5 mm, and calls out the error to a person who writes down the value and calls back the time for the next reading.
This can easily be done at 2.4 second intervals or any convenient interval for the 8 minutes required to get one cycle of the worm.
The exact interval is not important just so the values are taken at consistent intervals. If one has a cassette recorder this can be
done by one person calling out the errors and then writing them down on playback.
I suggest you take a sample of this very important information which can be used to judge the quality of the RA drive on your
telescope. With the telescope well, polar aligned and the PEC turned off, take about 4 cycles of data like that shown above. Plot
the four runs on top of one another. This gives you an idea of how consistent the errors are four four teeth on the worm wheel. If
the curves are very similar you can have some confidence that a training on one tooth or two will be good for more positions of
the worm wheel. The more cycles you examine, the more you know about the consistency of your worm wheel.
The same data can be taken after the PEC is trained to determine if the PEC training accomplished is good enough or should be
refined.
The derivative of these curves gives you critical information about the frequency of the corrections required to stay within a given
error criterion. With the 216XT which I use for guiding, I pick a star that is bright enough so that it reads 15 to 20 on the
brightness readout with an exposure of about 1/4 second, These settings give corrections every second or so. This is often enough
to give good guiding with a well trained PEC.
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