Telescopes - A Guide To Selection

NEWTONIAN NOTES
Designing and Building a High Performance
Telescope from Commercial Components
by
Peter Francis
Picture 1, 7 X 50 Finder with star diagonal
Picture 2, 10" Mirror mount with 9 point flotation
shown with mirror and mounted in tube
Picture 3, Spider and diagonal holder with
diagonal as seen from the primary mirror
Picture 4, 1¼" and 2 " Focusing mounts
Newtonian Notes
INTRODUCTION
The performance of any telescope depends on using a balanced set of components matched
to the needs of the telescope user and his or her interests. Indeed, the type of telescope used,
refractor or reflector, low f-ratio or high f-ratio, Newtonian or Cassegrain, large or small aperture is
not simply a matter of which is better but rather which is better for following your interests. As the
title suggests, this booklet is limited for the most part to designing Newtonian telescopes by selecting
and using commercial components to match your needs.
Newtonian telescopes can be assembled with mismatched components and still work well
enough to show views of the sky. Many Newtonian components are available to suit a wide variety of
needs. By matching the components in your telescope more carefully you can improve the
performance with little or no increase in cost. This is the difference between average telescopes and
prize winning telescopes.
The format of this booklet is to look at each component separately and relate its selection and
use to the performance. Thus you will not find a section on photography but rather photography is
related to particular components and their use. At times the discussion gets fairly complicated. Don't
be upset if you don't understand everything on the first reading. On the other hand if you have
studied each topic carefully but are not sure you understand all the details don't feel too concerned.
Learning the beginner level of telescope design will greatly improve performance. Moving through
the more advanced level details will give only limited improvement in performance (although when
you have become advanced in telescope building those little details become more important to you).
In some cases you may want to make one or more of the components from scratch. Feel free
to try your hand at making anything you can. The basic performance requirements are given in each
section for most components. Additional material is available in publications listed at the end of each
topic along with a general brief description of the recommended material each reference covers.
Also this booklet is not intended to be used alone. A number of books are available to cover
additional material and related topics. "All About Telescopes" by Brown is recommended for all
levels but especially for beginners. If you are making your own mirror or most other telescope
components from scratch see "Making Your Own Telescope" by Thompson. A wide variety of
photography is covered in detail by "Outer Space Photography" by Paul. These books are basic
starting points for additional material especially for beginners. The last two have additional
references for more material if desired. "Amateur Telescope Making" is suggested for those above
beginners and all three volumes are recommended for advanced amateurs.
2
Newtonian Notes
TABLE OF CONTENTS
INTRODUCTION..................................................................................................................................................2
Telescopes - A Guide To Selection........................................................................................................................6
Limits of Telescope Performance.........................................................................................................................9
Special Limits of Rich Field Telescopes.............................................................................................................16
Eyepieces and Other Magnifiers........................................................................................................................18
Telescope Tubing..................................................................................................................................................21
Selecting And Using Focusing Mounts...............................................................................................................27
Checking for Focuser Bottleneck.......................................................................................................................30
Mirror Mounts......................................................................................................................................................32
Selecting Diagonal Size........................................................................................................................................34
Offsetting A Diagonal..........................................................................................................................................37
Supporting the Diagonal......................................................................................................................................40
Guide to Selecting Finders and Guide Telescopes............................................................................................46
Positioning And Mounting Ring Mounts And Similar Accessories.................................................................48
Using the Main Telescope as a Guide Telescope...............................................................................................49
Construction of a Lenseless Schmidt Camera...................................................................................................53
Importance of Balancing.....................................................................................................................................55
Telescope Design Example..................................................................................................................................56
Putting A Telescope Together and Adjusting....................................................................................................58
© by Kenneth Novak 1976,1979
3
Newtonian Notes
NEWTONIAN NOTES, Second edition
Designing and Building a High Performance Telescope From Commercial Components
by Peter Francis
Published by Kenneth Novak & Co. Ladysmith, WI 54848
© by Kenneth Novak 1976, 1979
This 6" f8 is the first telescope made by one amateur. The primary mirror was ground and
polished at the Adler Planetarium in Chicago. The pipe mount was adapted from "Making Your Own
Telescope". The tripod is made of pipe. The heavy mount ' is hard to carry but has excellent
stability. A finger tap at the focuser causes the telescope to vibrate but it damps such vibrations out
in less than two seconds. This is excellent performance for stability. The guide telescope is used for
photography.
This 10"f4 rich field telescope is made from commercial components. It is in a 40 inch long
tube and only weighs 30 Ibs. complete. It is shown on a permanent pier but can be mounted on a
commercial telescope mount made for 8" telescopes with 1½” diameter shafts. Since the mount has
folding tripod legs and is light in weight for maximum portability it weighs only 65 lbs. Thus when
used as a portable telescope the entire unit weighs only 95 lbs. and easily fits in a car or stored
away. Moreover vibration from a finger tap dampens out in less than two seconds. You don't need a
lot of weight for good performance.
4
Newtonian Notes
This 4¼" telescope has two focusers. The one
at the upper end of the tube is for the f4 Newtonian
with a 3½ degree field. Removing the mounting
screws on the Newtonian focuser allows it to be
removed, converting the telescope into an f16
Cassegrain for medium to high magnifications. The
Cass secondary is permanently mounted and
adjusted on a spider. The tube length is only 17"
long and is very light weight. The design can be
used with commercial components. It is ideal for a
compact telescope for both rich field and high
magnifications or can be used attached to a large
telescope to be used as both a finder and high
magnification guide telescope of large aperture.
5
Newtonian Notes
Telescopes - A Guide To Selection
Beginners in amateur astronomy are faced with a number of different telescopes to choose
from and too often little information on selecting the one to buy. The best telescope for any person
depends on what astronomical objects are most often viewed, the size and weight that can be
handled with tolerable ease, and the cost. These factors vary with each person. Beginners often
start knowing only how much money can be spent. Many different telescopes are available to suit
the needs of most amateurs. Matching the telescope to the observers needs is important. The
recent experience with Comet Kohoutek is a case in point. While the public media can be faulted for
leading the public to believe they could expect the comet would perform to very optimistic
predictions, (in fact a race was on to find a scientist that would give out the most optimistic prediction
then give it a lot of press coverage) the public was unprepared to choose the type telescope most
suitable to view the new comet. The beginning amateur astronomer that rushed out to get any
telescope they could get usually got the wrong type telescope either for the optimistic predictions or
the more realistic predictions for a much brighter than average comet. For experienced comet
observers using the proper equipment Comet Kohoutek was not the “flop" the public media said it
was when it did not live up to the press releases. A small 2 inch refractor used at 7 power assembled
from war surplus optics at a cost of $20 showed Comet Kohoutek head clearly with a faint tail at least
5 degrees long stretched out over the entire field. Many beginners that spent 10 or even 100 times
as much on a long focus telescope saw little more than disappointment.
Assuming the optics are good, the most important factor in seeing as much as you can through
the telescope depends on the aperture size. The aperture size represents the light gathering power
of the telescope. And since light gathering power is an area measurement it increases more than the
simple diameter. A telescope with twice the aperture diameter will have four times the light gathering
power. (For example, a 6 inch aperture has four times the light gathering power of a 3 inch
aperture.) The aperture size also represents resolution or how much detail that can be seen. Thus if
you double the aperture size the resolution is also doubled at least in theory. However the resolution
is limited by atmospheric conditions which become a more important factor in larger telescopes but
will affect the viewing in any telescope. Even if the sky is clear the atmospheric conditions can
cause stars to "twinkle" to the naked eye or through the telescope. Twinkling stars may inspire poets
and dreamers but they are bad for astronomers both amateur and professional. For any given type
telescope an increase in aperture also increases the cost and weight at a rapid rate. The telescope
becomes less portable, more difficult to store, and less convenient to use. But you can select the
telescope type that is more portable if that is important. (For example, the full length for a 6" f15
refractor is about 8 feet but the full length for a 6" fl5 Cassegrain is about 2½ feet.)
Magnification also depends on aperture but less directly. Most beginner amateurs are too
often influenced by the maximum magnification claims made for telescopes. Actually any telescope
can be made to magnify at 10, 000 power but you wouldn't see anything worthwhile. You can see
stars, moon and planets with your naked eye. The purpose of a telescope is to collect more light so
you can see more detail. This does involve some magnification but the maximum magnification
practical to use depends on how much light you collect, how much resolution you can get, and on the
"seeing" conditions of the atmosphere. You can see more with a telescope because it collects light
which is concentrated to enter your eye. Magnifying the image spreads the light over a larger area
making the image less bright. In other words, magnification works against the light gathering power
of a telescope. The greater the magnification the fainter the object becomes. In practise,
magnification is used only enough to make the object large enough to see the detail it is possible to
see. Additional magnification will make the object larger but less bright. It is often possible to
magnify the image so it is so dim it can't be seen.
Sometimes the maximum magnification is given as 100 power for each inch of aperture. A
more reasonable and practical maximum is 50 or 60 power per inch of aperture. For general
telescope use on most telescopic objects in most locations of the country the "seeing" conditions on
the average will limit magnification to 25 or 30 power per inch of aperture. On some nights when the
"seeing" is very good to excellent higher magnifications are possible. But other nights 15 power per
inch of aperture may be the limit. Higher magnification is good only if it lets you see more. Most
astronomical objects don't require high magnifications and many of the most popular must have low
magnifications. Buying a telescope based on the high magnification numbers claimed often results
in the wrong telescope for the observing desired.
6
Newtonian Notes
There are two basic types of telescopes, refractors and reflectors, with a special group that are
combinations of both. Refracting lenses work by bending light rays as they pass through the glass.
Eyeglasses and magnifying glasses are simple examples. Reflectors are mirrors such as used for
shaving. Concave mirrors can collect light and bring it to a focal plane without passing through the
glass. The glass is used only to hold the shiny reflective surface coating. The only purpose of the
refractors objective (or main) lens and the reflectors mirror primary is to collect light and bring it to a
focal plane. The eyepiece you look through magnifies the image at the focal plane. Calling a
telescope a reflector or refractor refers only to how the light is collected.
Refractor telescopes have the objective or main light collector at the upper end of the tube and
the eyepiece at the lower end. Refractor type telescopes are commonly used in small sizes. Most of
the optical surfaces are enclosed inside the tube to minimise maintenance. Refractors cost more per
inch of aperture than reflectors. And since the light passes through the glass lenses, the light that
forms the image is separated slightly by colour just as a prism does. A good refractor objective is
colour corrected to a great extent by using two lenses for the objective. The amount of colour
correction depends on the f-ratio. Refractors of f15 or higher are quite well colour corrected although
not as good as a reflector. The long focal length of f15 refractors makes the length of the telescope
quite long. And since the eyepiece is at the lower end of the tube the entire telescope is mounted
high. This is to position the telescope so the eyepiece is comfortable to view into. The tall tripod and
long tube make most good refractors more difficult to carry and store especially in larger sizes.
Lower f-ratio refractors can be made and used but the lower f-ratios are less colour corrected.
This by itself does not make lower f-ratios "bad". Since higher magnifications will also magnify the
uncorrected colour the lower f-ratio refractors are limited to lower magnifications. Lower f-ratio
refractors have much shorter tube lengths and therefore more portable. They are good, useable
astronomical telescopes for those purposes not requiring high magnifications. Common binoculars
are in fact low f-ratio refractors used at low magnifications.
The most common reflectors are the Newtonian type. A parabolic primary mirror collects the
light and brings it to a focus while a flat mirror of elliptical shape is used to divert the light beam out
of the tube to a comfortable eyepiece position on the side of the upper end of the tube. A Newtonian
telescope mount is fairly close to the ground. Because the light collected by the mirror does not pass
through the glass it does not suffer from uncorrected colour. It can and usually is made in lower fratios since it is not limited by colour problems. It does suffer from coma which limits the
performance at low f-ratios. All types of telescopes have limits. Selecting a telescope is a matter of
matching telescope limits to your viewing objectives. High f-ratio refractors and Cassegrain
telescopes are good for medium to high magnifications. Low and moderately low magnifications are
not generally available. General purpose telescopes have f-ratios from f6 to f10 are most commonly
used especially by beginner amateurs not sure of which telescopic objects will be of greatest interest.
Very low magnifications are generally out of range and high magnifications require very short focal
length which can be a problem to use comfortably especially for observers with eyeglasses. General
purpose f-ratio telescopes are recommended unless a specific interest in very low or very high
magnifications are required for observing.
Low f-ratio telescopes f5.6 or lower are generally called rich field telescopes. The name rich
field comes from the fact that lower f-ratio, telescopes have wider fields allowing larger sections of
the sky to be observed. They are designed for very low to medium magnifications but are most
important for very low powers. The low power and wide field give bright images and maximum
visible colour suited to star fields, extended objects such as galaxies, nebula, comets, etc. Since
many of these objects are often not bright and are extended or spread out over the sky they are
difficult to view through many telescopes. Magnifying an extended object spreads the light over a
larger image size reducing the brightness often to the point of disappearing. A rich field telescope is
designed to collect a lot of light and magnify it by relatively low powers. This concentrates light in a
smaller bright image. Extended objects require less magnification anyhow so the light collected is
better put in brightness rather than size. While rich field telescopes are a special purpose telescope
they are important because they are best suited to many of the most popular telescope objects. In
addition the very short tube makes them the most portable Newtonian or refractor.
Rich field telescopes are not well suited for anything more than medium power (about 25X per
inch of aperture). High magnifications of the moon, planets, and double stars are best left to higher fratio telescopes. However such high magnifications also require better than average atmospheric
conditions. Low powers can be used most often under average or less than average conditions if the
sky is clear. An f4.5 to f5 is suggested as a good rich field telescope for most amateurs. Lower fratios will give slightly lower magnifications but coma is more of a problem. Coma will limit the
7
Newtonian Notes
useable field more in lower f-ratios. Eyepieces generally don't work as well below f4.5. Above f5.6
the field is restricted by mechanical parts and the lowest powers are not available. The best f-ratio
for a rich field is a matter of personal opinion. Some amateurs do not consider anything over f5 to be
a rich field.
Cassegrain telescopes are reflector telescopes with the high f-ratios of refractors packed into a
short tube length of a rich field telescope. As in a refractor, the focusing mount is at the lower end of
the tube where a star diagonal directs the light to the eyepiece for comfortable viewing. The short
tube length is achieved by using a low f-ratio primary mirror usually f4 or f5 but instead of a flat
secondary mirror as a Newtonian telescope would have, the Cass secondary has a convex curve. In
effect the Cass secondary is a built-in mirror type Barlow which magnifies the image from the primary
by 3 to 5 power. This also multiplies the f-ratio of the primary so an f5 primary and a 4X secondary
gives an effective f-ratio of f20. Most Cassegrain telescopes are special purpose types used for
medium to high magnification while keeping the tube length very short and portable. They are well
suited to lunar and planetary objects if high magnification is desired. The high f-ratio Cass telescope
allows high magnifications with longer focal length eyepieces having longer and more comfortable
eye relief and generally better image quality.
While both rich field and Cass telescopes are generally considered special purpose telescopes
they are often used to compliment each other. Rather than have a single general purpose telescope
usually with a fairly long tube many amateurs purchase both a rich field Newtonian and a Cass
telescope tube assemblies to fit interchangeably on one telescope mount. Since both tubes are short
they are more portable and easier to use. More important though is the extended performance and
magnification range of the two telescopes beyond that available in a single general purpose
telescope. The rich field is best for very low to medium power and the Cass for medium to very high
powers with better viewer comfort. Where portability is important this also allows the use of larger
apertures. As a general rule, a telescope mount designed for an f8 telescope can be expected to
give similar performance for the next larger aperture if a short tube is used as in a rich field or Cass
telescope. Thus a 6"f8 telescope mount can be used for an 8"f5 rich field or an 8"f20 Cass telescope
for exceptional portability. Alternatively, if very high telescope mount stability is more important than
portability you can use a rich field or Cass telescope of the same aperture on a telescope mount
designed for a general purpose f8 Newtonian.
Cadioptic telescopes are usually variations of Cass telescopes that use both reflector and
refractor optics. A large refractor lens is used at the upper end of the tube to enclose most of the
optics and add image correction. While this also adds additional light loss and more cost permits a
Cass design of f8 to f10 with a very short tube and well suited to general purpose use.
8
Newtonian Notes
Limits of Telescope Performance
In selecting a telescope or selecting parts for a telescope it is well to consider the limits of
performance. Some limits are external such as weather which can not be controlled. Optics have
limits of performance from many factors. The diameter of the primary, will limit maximum resolution
and light gathering power but for many amateur telescopes the atmospheric conditions will limit
resolution. The accuracy of the optics is important but most amateurs can not test the optics they
buy and advertising claims are difficult to compare since the only way to be sure you get the
accuracy advertised is to check the optics to determine what accuracy you actually have. The
amateur is left with only the reputation of the firm he buys the optics from to be sure the optics are
the quality desired. Other limits of telescope performance are easy to control because they are
based on careful selection of the parts used to make the telescope. A little knowledge of what to look
for in a part can lead to a better choice more suited to your needs. Guides to selecting specific parts
are given in sections describing each part. What is covered here are more general limits on
telescope performance over which you have some control.
Most amateurs are aware that the parabolic mirror of a Newtonian has an optical defect known
as coma. It causes images of stars near the edges of the field to lose their round shape to become
fan tailed appearing more like a comet than a star. This distortion in most telescopes is not readily
observed unless the telescope is not fully collimated. Most general purpose telescopes have a large
field of good performance where coma is limited. Only when long focal length eyepieces of wide
field will show noticeable coma. The Coma Limits chart gives the image size for selecting eyepieces
and the corresponding real field of view.
If you know the angular diameter of the field you want the telescope to give, you can calculate
the image size required at the focus using the formula below. If you know the size of an object to be
photographed (from reference books) the prime focus image size at film can be calculated using the
same formula.

L=
A⋅F
3438
Where L = linear image size at the focal plane
A = angular diameter in minutes of an arc
F = effective focal length of the telescope
For angular size in seconds, divide by 60; for angular size in degrees, multiply by 60. Both F and L
are in the same units.
If you are designing a telescope to get a specific field of view at 100% illumination the above
formula will tell you the linear image size at the focus but the Coma Limits chart should be checked.
The formula above has no limits but telescope mirrors have practical limits. A general purpose
telescope might be designed to give a ½ degree (30 minute) field or a rich field telescope might be
designed to give a one degree (60 minute) field. Using the above formula will tell you the linear
image size to use in selecting the diagonal size, focusing mount, and eyepiece to allow the field of
view to be seen by your eye in the completed telescope. Generally the full field of view with the
longest focal length eyepieces will be used at 50 to 75% illumination at the edges. This does not
change the formula. If you select the angular size of the full field at 50% illumination the formula will
give the diameter of the image size at 50% illumination. The field of view and image size at 50%
illumination are usually much larger than at 100%.
Many amateurs take it for granted that the light reflected from the primary mirror will reach the
eyepiece. In fact, the amount of light reaching the eyepiece is designed into the telescope. If you
guess at telescope design you can easily be wrong. For a Newtonian telescope the amount of full
brightness is limited by the diagonal size and focuser size. When mechanical parts block part of the
full brightness from reaching the eyepiece the effect is called vignetting. The amount of vignetting is
expressed in a percentage of full aperture light that does pass through. For example, a 6 inch
telescope vignetted to 50% illumination means the light passing through is limited to 50% of full
aperture or 3 inches. It is not desirable to have a 6 inch telescope but use only 3 inches of the
aperture. Almost all optical instruments have some vignetting.
9
Newtonian Notes
Even expensive cameras don't try to put full 100% illumination out to the corners of the film.
Some vignetting is accepted because of problems in designing full illumination everywhere.
In a Newtonian telescope full illumination over the entire field of view requires a relatively large
diagonal and focuser particularly at very low f-ratios. And in many cases full illumination is a nearly
useless waste. The important consideration is how much illumination the eyepiece can use. That in
turn depends on the focal length and the field of view of the eyepiece. Because most telescopes use
a number of eyepieces with different linear image sizes the telescope design is a bit complicated.
Some amateurs take an average linear image size required by the eyepieces they use while others
take the linear image size of the eyepiece they use most often. Typically amateurs most often use a
32mm Erfle on a rich field telescope, 18mm Kellner or Orthoscopic, on a general purpose, and a
12mm on a high magnification Newtonian. A telescope designed for planetary use or splitting double
stars will use mostly short focal length eyepieces of less than 12mm. The largest circle of 100%
illumination the eyepiece can use is about 3/8". It Is rather simple to provide full illumination to short
focal length eyepieces.
10
Newtonian Notes
Figure 1 Linear image sizes for full field of view for various eyepieces in approximate actual size.
Grades of optical quality in linear and angular size vary with
the aperture and f-ratio. Shown here are approximate sizes for an
8" f6. See the Coma Limits Chart for other sizes.
Figure 2 Grades of optical
quality
Unvignetted 100% illumination Is designed into the telescope mainly to suit the eyepieces
used. Illumination will decrease toward the edges of the field of view. If you select 100%
illumination over the entire Grade B field of view for the 8” f6 telescope and use mostly a 12mm
Orthoscopic eyepiece you could not use all the illumination. The field of view for the eyepiece is
much smaller. It would be better to redesign with a smaller diagonal to reduce 100% illum.
On the other hand, long focal length eyepieces with wide fields of view use a large linear
image size at the focus. A 32mm Erfle eyepiece can use a linear image size of about 1 3/8" - fully
one inch larger than for a 12mm Kellner or Ortho. The amount of sky seen through the eyepiece is
measured in area. For the 32mm Erfle the area of sky is 13.5 times larger than with the 12mm.
Vignetting is more unavoidable. A good telescope design can avoid the most serious vignetting and
work out a "best compromise" of acceptable vignetting. Because the area of sky available to the
eyepiece is so huge compared to short focal length eyepieces, some vignetting is nearly unavoidable
in a rich field telescopes with long focal length eyepieces. Since some vignetting is almost always
possible in any telescope it is a matter of considering how much vignetting, in percent of full
illumination, and how much illumination you need, in linear image size.
For short focal length eyepieces to get high magnification you can use all the brightness you
can get. Since high magnification spreads the light out to form a large image, the brightness of the
image is quite low. Not providing 100% illumination for the field of high magnification eyepieces
simply throws away aperture and badly needed image brightness. For low magnifications the
available light is not spread out as much, so the brightness is much greater - often four to sixteen
times brighter. Rich field telescopes themselves give bright images at the focal plane. Long focal
length eyepieces combined with a rich field telescope will preserve maximum image brightness.
Medium magnifications using eyepieces of 12mm to 25mm focal length can use larger image
sizes of 100% illumination than used for high magnifications. Too often a general purpose telescope
designed for medium magnifications with occasional use with shorter focal length eyepieces will tend
to be designed with an oversized diagonal to get a rather large circle of 100% illumination. Most
general purpose telescopes can get along quite well with a ½" image size of 100% illumination,
especially for beginners. For rich field telescopes at low magnifications the problems of
compromising vignetting are more difficult. A 32mm eyepiece is most popular for rich field use but
designing a telescope to put 100% illumination to the edges of the linear field the eyepiece can use
would require a very large diagonal. And a large illumination size would be wasted on all shorter
focal length eyepieces with smaller fields. The brightness in a rich field drops off more gradually
toward the edges of the field than in longer focal length telescopes. In addition the image brightness
11
Newtonian Notes
can be 16 times greater than at high magnification. 75% of full illumination will cut the brightness to
12 times the image brightness. 75% illumination at low magnification is tolerable and usually
unnoticeable to many experienced amateurs. This is a particularly important compromise
considering the vignetting Problem. 2 inch focusers can help reduce vignetting and short 2 inch
focusers can help even more.
It is also important to consider the optical quality of the image. The Coma Limits Chart on
page Error: Reference source not found gives linear and angular image size by grades of optical
quality. Grade A is perfect image quality with no visible coma within resolvable limits. Grade B is
the circle of excellent image quality with little detectable coma. Generally you must have nearly
perfect weather conditions to be able to see any coma with a trained eye. Grade C is the circle of
good image quality where the coma is visible but not troublesome. Beyond the Grade C circle is the
grade D image quality, troublesome coma but can be useful in some applications. Coma gets worse
the farther from the center the object is. Stars have definite comet-like tails while diffuse nebula and
sky sweeping are less affected. High magnification requires the best image quality you can get Grade A. Since short focal length eyepieces have small linear fields it is fairly simple to match a
general purpose or long focus telescope with a large Grade A image quality to any short focal length
eyepiece you can use. For rich field telescopes at low magnification the Grade B image quality is
often better than Grade A at high magnification because high resolution is not "pushed" and because
of atmospheric conditions have a much greater effect at higher magnifications.
In using the chart for Cassegrain telescopes you must look under the f-ratio for the primary
mirror and multiply the linear and angular image size by the magnification of the secondary mirror.
For example, an 8"f20 Cass with an f5 Primary and 4X secondary is looked up in the chart under 8"f5
and all numbers are multiplied by 4. The same is true for Newtonians using a Barlow. For example,
an 8"f6 with a 2.4X Barlow is looked up in the chart under 8"f6 then all numbers are multiplied by 2.4.
It is important to understand that illumination of the field seen through the eyepiece depends
on the parts used to assemble the telescope. The diagonal determines the size of the field
illuminated to 100% possible for the given mirror size. Beyond the circle of 100% illumination, the
illumination tapers off gradually in general purpose telescopes and very gradually in rich field
telescopes. One exception is very tall focusing mounts not properly designed for Newtonian
telescopes. In some cases they do not allow full illumination to reach the eyepiece and in other
cases they can be used on f8 telescopes but not lower f-ratios without bottle necking the illumination.
Focusing mounts generally pass the 100% illumination OK but are a bottleneck to the edges of the
field. One exception is the very tall focusing mount. Focusing mounts reduce illumination very
rapidly. Since Newtonian telescopes use both a diagonal and a focusing mount the combined effect
on illumination of the image at the focal plane is to have illumination brightest at the center, gradually
becoming less bright away from the center, then rapidly dropping in illumination at the edge.
Increasing the diagonal size increases the brightness over the entire field (up to 100%
illumination at the center) by increasing the diameter of 100%, 75%, 50%, etc. illumination circles.
But you will still have the rapid drop in illumination at the same place at the edges of the field.
Increasing the focusing mount size from 1¼” to 2” will not generally increase the circle of 100%
illumination at the center but can increase the circle of 75%, 50%, etc. illumination toward the edges
of the field. A 2" focusing mount can make better use of most diagonals. However if the diagonal is
undersized for 1¼" focusers it will also be undersized for 2" focusers so the 2" focuser will add
nothing. Also in some cases of high illumination over the entire field may require both a larger
focuser and a larger diagonal.
For the most part, it is the focuser that most often limits the field of view and the illumination in
the field. Before you select a wide field of view and high illumination check to see what image size
your eyepieces can use (chart in All About Telescopes). This size depends on apparent field of view
and the focal length. It is a waste to provide a diagonal and focuser to get 75% illumination to the
edges of a 1 3/4" field image size when your eyepieces require at a maximum of only one inch field
image size. Also note eyepieces using shorter focal lengths use less of the edges of the field
because they get most or all of the light they can use from the center of the field.
12
Newtonian Notes
Figure 3 Illumination circles
Above are examples of image illumination for 8” telescopes of various f-ratios using several
diagonal sizes for both 1¼” and 2" focusing mounts. All assume the same average telescope tubing
size and the focal plane 9" from the center of the diagonal. This allows a comparison of the effects
of different diagonal sizes and focusing mount sizes. Because of the many variations in the actual
tube diameter, focusing mount height, diagonal sizes available, amount of "in" travel desired, etc.
used in telescope design not to mention all the primary mirror sizes, it is not possible to give
drawings or charts to cover all variations. Exact image sizes depends on specific design. As a
general rule, shorter focusers, smaller tube sizes, and shorter "in" travel will increase the diameter of
each size illumination circle even though the diagonal size remains unchanged. Taller focusers,
larger tube sizes, and long "in" travel will decrease the diameter of each size illumination circle even
though the diagonal size remains the same.
13
Newtonian Notes
Figure 4 Illumination circles
Above are examples of rich field telescopes in popular models. The 6"f5 shows effects of
focuser bottleneck but could well work fine with a 1.83" diagonal to give 100% illumination to a 3/8"
circle with both focusers. The 6"f4 with 2" focuser shows the effect of diagonal bottleneck. The 2"
focuser greatly increased field of view and 75% illumination. This is a good illumination pattern for
any f4 telescope - a larger diagonal unneeded. Many amateurs are tempted to use a larger diagonal
but consider the widest field of view with 32mm Erfle or 6Omm Kellner. Illumination is 80% to 85%
at the very edges! The 12½" f5 and 10"f4 show typical advantage of using a 2" focuser on rich field
telescopes even if you only have 1¼" eyepieces. The illumination patterns for 4¼" are ray traced
assuming a 5" OD tube and focal plane above the side of the tube. The 4¼"f4 shows the
considerable design problem of bringing full illumination to the eyepiece even with a large diagonal.
Actually the 1.52" diagonal used with the 4¼"f5 is a large diagonal compared to the aperture of the
telescope. A very short focuser to put the focal plane closer to the tube would be very helpful. It also
shows how large the 75% illumination circle can be with a small circle of 100% illumination.
Illumination in rich field telescopes drops off very gradually toward the edges of the field. This
makes it a practical compromise to accept a rather small circle of full illumination and still have at
least 75% illumination for very wide field eyepieces. The illumination patterns shown here were
made by ray tracing as shown in the drawings on page 53 of "All About Telescopes". However it is
usually more simple to select an image size of 100% illumination and use the formula for diagonal
size. Then check with formula (5) for focuser bottleneck especially for very tall focusers.
The Coma Limits chart gives several linear image sizes in various grades and therefore
several fields of view. It is a matter of personal choice what quality of image is used. But since the
maximum field of view of a telescope is determined by the parts used to build the telescope then
generally only one eyepiece will give the full field of view. Often this eyepiece is not the most
14
Newtonian Notes
frequently used. The low power eyepiece may be used only for sweeping the sky to find objects to
observe. Image quality is less important as long as you have a wide field of view to cover large
areas of sky with ease, images bright enough to be seen with ease, and not too much distortion so
objects can be recognised by shape with ease. Thus you may select 50% illumination over the entire
grade C field for your low power, long focal length eyepiece used for sweeping. Beyond grade C is a
region of considerably distorted images which may be observed through a telescope but since coma
distorts stars to look like comets and generally smears resolvable detail it is not even very good for
sweeping.
Shorter focal length eyepieces usually use only a smaller center portion of the field. For
serious observing the field of your most used eyepiece may then be of 75% illumination over the
entire grade B field of view. At high magnifications the field can be of 100% illumination over the
entire grade A field of view. Because high magnifications require maximum image brightness and
image quality, a well chosen set of parts can give wide field for sweeping and a bright, sharp image
for high magnifications. You may want to go through the process of selecting the most suitable parts
for three eyepieces; low power sweeping, medium power for most common observations, and high
power for detail work when weather permits. You may find you are getting many parts of large size
simply because you wanted a very large field for just sweeping. Unless you want to do a lot of
sweeping it would be better to consider a smaller field of view for sweeping as long as it did not limit
your general most commonly used observations.
It is also important to be reasonable in what you expect a telescope to do. Expecting a 10" f8
to give a 2 degree field will cause problems in finding suitable parts, especially eyepieces. Low fratio telescopes are used to match maximum wide fields with available parts. The useable field of
view is smaller in lower f-ratios but the image is smaller to get a given field size. In other words, as
the f-ratio increases the image size increases faster than the field of view of any given image quality.
Higher f-ratio mirrors give larger, better quality fields of view but the large image size makes it
difficult to get parts large enough to use the large image. Thus a 7/8 inch image size in a 10" f4.5 will
give a one degree field at 36 power with a 32.mm eyepiece. A 7/8 inch image size in a 10" f8 will
th
give a one-half degree field at 63 power with a 32mm eyepiece. The f8 will show only ¼ the sky at
th
¼ the brightness with the same image size and eyepiece. To get the same one degree field of view
in the f8 the image size must be increased to about 2 inches but the brightness of the image will be
th
unchanged at only ¼ . This leaves the image brightness much less than a rich field telescope can
give.
Rich field telescopes are designed to get brighter images and make it easier to get wide fields
of view. The Coma Limits chart may lead you to think that a long f-ratio telescope will give a better
quality image of the same field of view if you only get large enough parts. But the important part of a
rich field telescope is to make images bright so faint objects are easier to observe. Long f-ratio
telescopes are better used on small, bright objects requiring higher magnification.
One limit of telescope performance is the pupil size of your eye when observing. In the
section on diagonal size part of the formula to find magnification relates the exit pupil to the focal
length of the eyepiece and the field.
focal length of telescope
‚ magnification = focal length of eyepiece =
apparent field
dia. of mirror
=
real field
dia. of exit pupil
For practical purposes the maximum pupil size of your eye is 7mm but a telescope with very
low magnification can give an exit pupil larger than will fit into your eye. The effect on telescope
performance is to limit the maximum useable exit Pupil to 7mm which in turn limits the real field and
the apparent field. At 3.5 power per inch of aperture the exit pupil reaches 7mm. Lower
magnifications will not have wider fields view but the light is less magnified so it will be brighter. A
10"f4 telescope used with a 60mm Kellner will give 17 power but the exit pupil is 15mm. Some
books claim this is wasting aperture. True, the full field does not fit the eye, but the telescope
aperture is to collect light and very low magnifications will brighten the image.
15
Newtonian Notes
Special Limits of Rich Field Telescopes
From the above considerations of telescope performance you will note many limits such as
coma and minimum magnification affect rich field telescopes the most. A well designed rich field
telescope requires a more careful compromise in the design and selection of parts and the f-ratio
itself. While a rich field telescope is considered a special purpose telescope it performs best on
astronomical objects very popular with many amateurs; comets, galaxies, nebula, star clusters. The
compact size is often important if the telescope is to be portable. Thus for many amateurs a rich
field telescope is best both from the standpoint of objects to be observed and the standpoint of
convenience to the observer.
The very high image brightness is important to observe the colours of stars and .deep sky
objects. The human eye requires brighter light levels for colour vision. Less than the minimum for
colour vision is seen in black and white. More important is the effect of magnification to spread what
light there is over larger areas with increasing magnifications. Low magnifications keep the light
concentrated to make the object appear as bright as possible. A faint comet barely seen in an f8 can
show up brightly with a long tail in a rich field.
Once you have decided to get a rich field telescope the question becomes "Which f-Ratio?".
The question gets into the area of personal opinion on what compromises to make. In f-ratios less
than f4 the coma limits the field in linear and angular size considerably and the primary mirror is very
difficult to make optically good. Only in special cases are f-ratios less than f4 made. In most cases
the practical limit is an f4 to f4.5. The tube length required is quite short for good portability and
images are very bright at minimum magnifications. But the primary mirror is still difficult to make
optically good and while good Orthoscopic, Erfle, and Konig eyepieces will perform quite well they
are not at their best. At f5 these eyepieces will perform well and the primary is easier to make to
high surface accuracy. If you are buying optics the surface accuracy is typically better but the cost is
less. The diagonal size is smaller so the cost is less and there is less light loss due to obstruction. At
f5 the angular and linear image size is considerably larger because coma is less. Here it is important
to remember that the minimum magnification is usually as easy to get in an f5 as in an f4. But
moving from an f4 to an f5 means the tube length will be increased by one mirror diameter. In a 6
inch telescope the tube will be 6 inches longer. Also the image will be a bit less bright. In most
cases these compromises are easily acceptable, since it makes the telescope better for medium
magnifications.
At f5.6 the telescope is more of a general purpose telescope suitable for medium
magnifications but the tube length is longer and the images less bright. Since image brightness
drops off rapidly with increasing image size an f5.6 is considered the upper limit for rich field
telescopes. Most rich field telescopes are made f5 as the compromise most are willing to accept
although it is easy to find amateurs that prefer higher or lower f-ratios.
Rich field telescopes have illumination circles that change rapidly with changes in diagonal
size, focuser height, “in” travel, and tube size. For example, the 6"f5 and 6"f4 telescopes with 2.14"
diagonal shown above assumed a 7 inch OD tube and 4 inches from side of tubing to focal plane.
This is a rough average for combined focusing mount height and "in" travel. (A 3 inch high focuser
with 1 inch of "in" travel, a 3 3/4 inch high focuser with ¼ inch of "in" travel, or a 4 inch high focuser
with no "in” travel.) Suppose now that a taller focuser is desired or more "in" travel is desired to add
just one inch in height above the side of the tube.
Compare these illumination patterns with the same telescope on the previous page. The effect
of focuser height is greatest in f4 telescopes. The center of the f4 is almost 100% illumination (about
97% at the very center) then illumination drops off gradually to the 75% circle. The effect of added
focuser height increases the distance from the focal plane to the diagonal and works the same as
making the diagonal smaller.
16
Newtonian Notes
Figure 5, 6"f5 with 2.14" diagonal (1¼" & 2"
focuser). Added inch of focuser height
Figure 6, 6"f4 with 2.14" diagonal (1¼" & 2"
focuser). Added inch of focuser height.
The focusing mount size has a greater effect on the center illumination if the diagonal is
sufficiently large. This effect and the effect of added focuser height and "in" travel decrease with
higher f-ratios. It makes designing a rich field telescope require more compromises and makes parts
selection a more critical choice. Adding one inch of focuser height can mean the diagonal size must
be increased and a 2 inch focuser must be used. In the 6"f4 with added inch shown above any
increase in diagonal size would not bring 100% illumination to the center of the field but would
increase slightly the 75% illumination circle for the 1¼" focuser. If you used the 2" focuser on the
6"f4 and used a 2.60" diagonal the circle of 100% illumination would be a very respectable 5/8 inch
diameter. Adding only one inch of focuser height made both the diagonal and the focuser a serious
bottleneck. It is your choice of compromise and personal opinions differ.
One compromise is in selecting the 100% illumination size for f4 rich field telescopes.
Because of the coma limits and limits of mechanical parts the 100% illumination size is often ¼" to
3/8" although some select 0" to ½". Even with the smaller size almost all of the useable and
viewable field with most eyepieces is illuminated to at least 75%. For f5 telescopes the coma is less
limiting so the 100% Illumination size is often 3/8" to ½" although some chose 0" to 3/4". If you know
the linear image size that will pass through the telescope and can be used by the eyepiece, the
following formula will give the angular field of view.
where L = linear field of view
A = angular field of view
F = effective focal length in minutes
ƒ
A=
3438 L
F
For angular size in seconds, divide by 60; for angular size in degrees, multiply by 60. Both F and L
are in the same units.
17
Newtonian Notes
Eyepieces and Other Magnifiers
The objective lens or primary mirror of a telescope has the purpose of collecting light and
bringing it to a focus at the focal plane. The image at the focal plane can be seen by placing a
ground glass or sheet of thin white paper in the light path while moving in and out to focus. (Works
good on the moon.) At the Newtonian focus the image that appears has no magnification. The image
can be recorded on photographic film in a system called Prime Focus Photography. But in most
cases a telescope is used with some magnification. The purpose of a telescope eyepiece is to
magnify the image at the focal plane. The amount of magnification depends on the focal length of
both the eyepiece and the objective lens or primary mirror.
Most eyepieces have at least two lenses, often four lenses, and sometimes five, six, or more
lenses. Each type of lens arrangement is usually given a name to specify general performance for
each type. However for each type eyepiece the optical designer can vary the design to get the colour
correction, field, etc. required for the purpose the eyepiece is being designed. For example,
microscope eyepieces usually are made for very long f-ratios and narrow fields. Telescopes are
usually much shorter in f-ratio and wider fields are desired. Thus a microscope eyepiece can not be
expected to work well on a telescope although it might by accident of design. Another example is the
colour correction of an aerial camera lens purchased as government surplus can be for infra-red and
therefore unsuitable for amateurs to adapt for visual or regular film use. The limits to optical design
in eyepieces should be considered in selecting eyepieces for your telescope. For example, if you
want an eyepiece with a wide field a Konig, or Erfle design should be selected. These eyepiece
types have extra lenses or special designs to get good correction at the edges of a wide, flat field but
usually at additional cost.
Each type eyepiece has a limit on the lowest f-ratio of the objective or primary mirror which
can be used and give good performance. An f8 refractor or Newtonian (or longer) will work well with
a Huygens or Ramsden type eyepiece although the field of view will be somewhat narrow. Many
reflectors are f6 or less so the Huygens or Ramsden types don't work too well. But you can use
Kellner, Plossl, Symmetrical, Orthoscopic, Erfle or Konig types for good performance on f6
telescopes. For f5 telescopes the Orthoscopic, Erfle, and Konig type eyepieces are best to use.
Since their good performance begins to deteriorate rapidly below f4.5 some compromise is needed
for f4 telescopes. This is generally a fair compromise if an f4 telescope is preferred. Below f4 it is
difficult to find any eyepieces that work acceptably well.
Eye relief also varies with eyepiece type. Eye relief is the distance from the eye lens of the
eyepiece to the focus position where your eye is placed for viewing. It is usually expressed as a
percentage of the eyepiece focal length. 25% to 30%, is a short eye relief while 75% to 80% is long.
Long eye relief is generally more comfortable to use especially for people wearing glasses. Since
eye relief gets shorter with shorter focal length eyepieces you may find them less comfortable to use.
When higher magnification is desired without using shorter focal length eyepieces several
methods are available to increase the effective focal length of the primary. Magnification depends
on the effective focal length of the telescope so increasing the effective focal length of the primary
three times will also increase the magnification of all eyepieces. A Barlow lens is most commonly
used to accomplish this, usually by two or three times the focal length of the primary. The Barlow is
simply inserted between the eyepiece and the focusing mount when high magnification is desired.
Since it is simple to add the Barlow in this position when needed it is the position most commonly
used. The Barlow lens can be built into the telescope, sometimes in front of the diagonal but is then
difficult to remove to get low magnifications. A Cassegrain secondary is in effect a built in mirror
type Barlow reflecting and magnifying the light beam. The position of the secondary folds the light
beam to make a very compact telescope for medium to high magnifications. A Cassegrain telescope
is much preferred to a Newtonian with a built in Barlow because of the more compact size. Barlow
lenses and Cassegrain secondaries are important to eye relief. For example, if the eye relief of a
6mm eyepiece is too short to be comfortable you can use a 12mm eyepiece and a 2X Barlow to get
the magnification of a 6mm eyepiece but the eye relief of the 12mm eyepiece. It should also be
pointed out that wide field eyepieces are best used at low magnifications. At higher magnifications
the special designs of Konigs and Erfles is wasted. Both types of eyepieces are most valuable on
lower f-ratio telescopes. And since both have short eye relief they would be troublesome to use in
short focal lengths. Amateurs sometimes want short focal length Erfles and Konigs to get both high
magnification and wide field. Unfortunately it doesn't work out that way. Similarly, amateurs that
18
Newtonian Notes
combine an Erfle or Konig with a Barlow lens to get both wide field and high magnification will also
be lacking in results.
Light loss in the magnifying lenses should also be considered. Each air to glass surface will
add to the light loss. Most common eyepieces have four air to glass surfaces. Most Erfles and
Konigs have six. Barlow lenses have two. Combining a Barlow with an Erfle or Konig means a total
of eight air to glass surfaces. It is usually better to use an Orthoscopic eyepiece with only half the
light loss in magnifying with only four air to glass surfaces, and good eye relief. This is a better use
of the light gathering power of your telescope. Photography can be done using Barlow projection or
Barlow focus photography without eyepieces or camera lenses. This allows some magnification with
small light loss due to only using two air to glass surfaces to magnify. While your eye does not notice
so much the light loss the film will require longer exposure times. An eyepiece can be used for
projection for higher magnifications but the light loss is higher. It is usually possible to avoid using
Erfles and Konigs for projection photography since they have more light loss and cameras will
capture on film only fairly narrow fields of view with projection systems. Similarly, a Cassegrain
telescope can easily use prime focus camera adaptors with no eyepieces or camera lenses for fairly
large images at the Cass focus. The only optics are the primary and secondary mirrors which can
avoid ultraviolet absorption of lenses. Common black and white film is sensitive to near ultraviolet
th
for about 1/6 of the useable light range.
Terminology can be a problem especially in technical subjects. In this booklet "linear image
size" or "image size" refers to the entire field of view of all objects to be seen through the telescope.
"Object size" refers to the size of a particular object within the field of view. The size of the linear
and angular field size can be changed by changing mechanical parts. The object size can be
changed with magnification. For example, a telescope can be designed with parts to give a linear
image size of 5/8 inch which in angular field of view is about 1½ degrees. The moon is an object
with an angular size of about ½ degree. But if some part is added to the telescope that limits the
linear image size so the angular field of view is reduced to only 1 degree the moon still has an object
size of about ½ degree. In other words, the size of the moon does not change but amount of sky is
reduced.
The object size at the focus of a telescope is given by the formula;
„
O=
A⋅F
3438
where O = linear object size at the focal plane
A = angular diameter of the object in minutes
F = effective focal length of the telescope
For angular size in seconds, divide by 60; for angular size in degrees, multiply by 60. Both F and O
are in the same units.
If you are photographing at prime focus you can use the formula to determine from the angular
size how big the object size will come out on the film. Angular sizes of most larger sky objects are
published in astronomical reference books except planets. Since their apparent size changes
depending on how far the Earth is away from them see the current issue of Sky & Telescope
magazine. This formula gives the object size with no magnification (1X). Note that it depends on the
focal length of the telescope. If the focal length were doubled the object size would also double even
though there was no magnification. Thus the object size can be increased with a longer focal length
telescope but once you have the telescope the object size can be changed only by magnification.
Magnifiers make the object look bigger but at the same time reduce the field of view.
Special wide field of view eyepieces can increase the field of view but need more linear image
size. The linear image size needs parts large enough to let the larger angular field of view to fit
through the telescope and be seen. In photography a magnifier can double the object size but the
angular field of view is cut in half and you use only half the linear image size. Planets are always
quite small in angular size. Using the above formula you will find that without magnification they are
tiny specks. So it is worthwhile to use a lot of magnification and give up some of the field of view.
Objects with large angular size need wide fields of view so you must use lower magnification to fit the
object size on the film. The object size can be no larger than the image size or you will only see part
of the object. Rich field telescopes are designed to fit a large angular field into a fairly small linear
field so with some eyepieces you can see large areas of the sky including large objects.
19
Newtonian Notes
If you are photographing using some magnification, use the above formula to find the object
size without magnification. Then multiply this size by the magnification you are going to use.
Remember that in photography the object size is limited by both the image size the telescope and the
magnifier is designed for and by the film size. For example, the moon with an angular size of 30
minutes through a telescope with a focal length of 48 inches will give an object size at prime focus
of .41 inches. Note that a 6” f8, 8" f6, and a 12" f4 all have the same focal length so give the same
object size. If the telescope is designed to give a linear image size of .75 inches you can photograph
the entire moon. However adding 5X magnification will make the object size 2.05 inches so you can
only photograph .75 inches of the moon at one time but the sizes of the craters are five times larger.
The light gathering power of a telescope is limited. Increasing magnification spreads the light
intensity gathered by the telescope over a wider area. Doubling the magnification doubles the
apparent object size but spreads the collected light over four times the area, making the brightness
th
only ¼ . All other things being equal, the exposure time is four times longer. For visual use your
eye may adjust to the lower brightness some but you can magnify an object until you can't see it any
more. Rich field telescopes are designed to be used on larger, fainter objects to put a lot of light into
a fairly small image to give objects very high brightness. If desired, additional magnification can be
used where objects are bright enough to be seen at higher magnification.
Telescope Eyepieces by Selby, Amateur Telescope Making-book three, Scientific American 1964. Extensive details and
limitations of eyepiece design even with perfect optics. Also has extensive list of additional references to follow up.
Eyepieces You Can Make in All About Telescopes by Brown, Edmund Scientific 1967. Gives linear field of telescope
image with various eyepieces. Also see What Eyepiece is Best? in the same volume. Shows various eyepiece designs, fratio limits, eye relief, effect of apparent field, etc.
20
Newtonian Notes
Telescope Tubing
All the optics in a telescope are supported in proper position by the telescope tube or similar
structural framework (which is also called a telescope tube). In most telescopes the telescope tube is
usually nothing more than a length of tubing. As simple as the telescope tube is, it is easy to neglect
the importance of the tube. It must carry the weight of the optics and maintain accurate optical
alignment. In a long telescope the primary mirror or objective lens will apply its weight a long
distance from the tube cradle causing the tube to flex. It may not seem the tube could flex enough
but the optical alignment is quite delicate. For many years the development of large telescopes was
limited by the ability of the tube to support the optics.
Figure 7, Tube currents
Figure 8, Bending strip stock into rings
In addition the tube is used to exclude heated air from passing through the optical path from
outside sources. Most of this heated air is the direct result in the temperature drop after the sun goes
down. Heat is stored in stone, concrete, water, glass, earth, metals, etc. as well as air. A flow of
warm air entering the optical path will cause optical distortions if the entering air flow is warmer or
cooler. As a rule it is best to have the air temperature inside the tube the same as the outside air.
But in the case of closed tube telescopes (refractors, Schmidt-Cassegrain, Maksutov types) the
temperature can be quite different, and cause no problems because the air is kept from mixing. It is
the mixing of air with large temperature differences that causes optical distortions. In large research
telescopes fans are used to mix the air to keep the temperature differences as low as possible. With
large open framework tubes it is not possible to physically exclude warm air from entering the optical
path so temperature of the parts is adjusted so all parts are at very nearly the same temperature to
avoid air currents of unequal temperature.
Solids and liquids usually hold much more heat than gases. Metals can change temperature
faster than stone or concrete. As the air temperature drops in the evening rather quickly, heat is
released from metals fairly rapidly while heat is released from concrete and glass rather slowly. If
you set up your telescope on a concrete patio the stored up heat will be slowly released to cause
optical distortion most of the night. The same is true if the observing path is over buildings, parking
lots, or highways. On the other hand, if the telescope is set up in an area covered by grass and trees
no large amounts of heat are released during observations making the viewing better.
The area the telescope is used in will determine the ground air current problem. But the tubing
also affects currents close to the telescope. The metal telescope mount holds a lot of heat and gives
it off as the air temperature drops. Body heat of the observer can be a problem if the heat can rise
up and enter the tube. And although you may feel cold your body gives off heat continuously
because it doesn't cool down much. Concrete or rock close to the tube will give off heat that may
enter the lower end of the tube and rise up through it. In some cases you may find it better to put a
dust cover over the lower end of the tube to avoid this. If you find you have a tube current problem it
is very easy to try out on several nights to see if there is some improvement. By the way, in some
21
Newtonian Notes
cases a dust cover over the lower end of the tube will reduce formation of dew on the optics inside
the tube. A wood covered walk area around the telescope can also help. Covering the lower end of
the tube will still allow warm air to rise up and out the upper end of the tube.
Some sources of tube currents are parts inside of the telescope. If you use uninsulated metal
tubing about half the heat given off by the metal will be inside the telescope tube. Years ago this
problem was solved by adding a thin layer of cork to the inside of the tube for insulation. Most of the
heat is then released outside of the tube. Nowadays you can use flock paper or black plastic foam
sheet which also reduces internal reflections as well as tube currents. By using non-metallic tubing
(fibreglass, cardboard, wood, plastic, etc.)you can avoid insulating the inside of the tube as usually
recommended with metal tubing but non-metals are generally less stiff or rigid so are less able to
support weight. For example, Youngs modulus of elasticity for aluminium is 10, 300, 000 lbs / sq. in.
and a stiffness of 107, 290, 000 in. Youngs modulus of elasticity for fibreglass is 500, 000 to 1, 500,
000 lbs./sq. in. (Handbook of Chemistry and Physics-Properties of Commercial Plastics)and a
stiffness of 7,690,000 to 23,070,000 in. Thus aluminium is 4.6 to 13.9 times more stiff than
fibreglass. The uncertainty in the values come about because fibreglass tubing is a mixture of glass
fibres and plastic. While most aluminium tubing is 98 percent aluminium the composition of
fibreglass tubing can vary widely between manufacturers, specific formulas, and individual batches.
Most non-metallic tubing is made with thicker walls than metal tubing to help make up for the
lower stiffness. Doubling the thickness also doubles the stiffness. And you should consider how
much stiffness is needed by the telescope. Most seamless aluminium tubing has thicker walls than
needed because it is difficult to make large diameter aluminium tubing with very thin walls. For 8
inch aperture telescopes or smaller the wall thickness of seamless aluminium tubing actually needed
is only about .050 inches. Also remember that telescopes most popular today use quite short tubes
compared to the long refractors. A shorter tube means you need less stiffness. In addition, nonmetallic tubing dampens vibrations better.
While most commercial non-metallic tubing is made of fibreglass, other materials can be used.
Spiral paper tubes are available from some telescope suppliers and from concrete contractor
suppliers. Wood makes a good telescope tube but does require some skill to make. A simple square
tube is easiest to make. Six, eight, or more sided tubes are more difficult. Round wood tubes can be
made by forming thin plywood (about 1/8" thick) in curved sections then overlapping sections in top
layers. Or you can make a 12 sided (or more) tube then turning the outside down on a lathe.
Needless to say, making a round tube is quite difficult without the tools and skill required. But a
round tube is preferred to allow the tube to rotate. Square and other multi-sided tubes can be made
to rotate by adding curved segments to the outside of the tube to form a circular profile.
Some amateurs choose to make their own fibreglass tubing. A fabrication method is described
in "Standard Handbook of Telescope Making" by Howard. Since fibreglass tubing is fairly difficult to
make most amateurs compromise and fibreglass a spiral paper tubing. Spiral wound fibreboard
tubing is available locally from concrete contractor suppliers and sometimes from concrete
contractors. They are generally known under the trade name of "Sonotube" but there are other
brands. Since they are designed to be used as concrete column forms it is important to take care in
selecting the tube. The inside or the outside is often coated with wax or sheet plastic to reduce
moisture absorption from wet concrete. For telescope use this coating is difficult to paint or to recoat with fibreglass. A layer of fibreglass will fill in the spiral joints, increase stiffness, and reduce
moisture absorption. But if only coated tubes are available the wax or plastic coating usually must be
removed before painting or fibreglassing. Most coating is wax. Removing the wax usually requires
the coating to be heated with a heat gun or set in the sun on a warm day and the wax scraped off. If
the tube is to be fibreglassed the surface can then be roughened with coarse sandpaper for better
surface grip. If too much wax remains on the surface the sandpaper will cake up with wax very
quickly. Go back to scraping the wax off before continuing to sand.
If the tube is plastic coated very often the plastic is a type that can't be painted or fibreglassed.
The plastic can be removed with coarse sandpaper. If the plastic is inside the tube it is usually too
difficult to remove. You can line the inside of the tube with adhesive backed flock paper (Edmund
No. 70, 621) which sticks quite well. Since it is rather time consuming to remove either a wax or
plastic coating it is better to start with uncoated tubing if you want to paint or fibreglass the tube. But
the uncoated tube is more difficult to get in some areas.
22
Newtonian Notes
Figure 9, Wooden tube
Figure 10, Block strip
Figure 11, Glue form
Metal tubing can be made in any diameter from one to five feet and in most common
telescope lengths from large sheets of metal rolled into shape. Many smaller tube sizes can also be
rolled depending on metal thickness and length. However since seem-less aluminium tubing is
available in sizes 10 inches and smaller most often a tube is rolled from sheet metal to get a special
wall thickness or diameter. Tube rolling requires equipment of a large sheet metal fabricating firm.
Smaller firms that do gutter and down spout work or heating and air conditioning duct work usually
don't have the rolling equipment in the right size and capacity to handle the job and often don't have
aluminium sheet metal most rolled tubes are made of. The sheet metal thickness to use for rolled
tubes is up to the telescope builder to select. As a rule the wall thickness is ½% to 1% of the tubes
diameter. The wall thickness depends on tube length, weight, and personal opinion of the builder.
Most 8 inch telescopes don’t have too much weight to handle so a well made aluminium tube is more
23
Newtonian Notes
likely to use a wall thickness of ½% of the tube diameter. On the other hand, a 20 inch telescope will
have a long tube even if it is a rich field telescope and the optics and optical support system will be
quite heavy so is more likely to use a wall thickness of 1% of the tube diameter. If you are not sure
of yourself in selecting the wall thickness to use it is worthwhile to note that most aluminium tubing
used for 8 inch to 20 inch apertures use a wall thickness of ¾% of the tube diameter or something
fairly close with generally very good results. Professional quality telescopes ($10, 000 or more) in 16
inch aperture or larger are usually made of rolled aluminium tube with internal ribs welded in place to
give better stiffness with less weight. This allows thinner wall thickness but cutting and welding rings
inside the tube is difficult and/or expensive for most amateurs.
After the tube is rolled from sheet metal a joint must be made the length of the tube. The joint
can be welded and ground smooth on the outside. If that is too expensive you can add a strip of
aluminium on the inside of the tube bolted or riveted in place. By countersinking the screws or rivets
you can cover the heads and the outside seam with auto body putty. Sanding leaves the outside with
a smooth "seamless" finish after painting. In some cases if the metal is thin enough the rolled tube
can be joined with a lock seam. It will leave a hump of folded metal usually on the outside of the
tube. Steel sheet can be used to make rolled tubing but be sure it is thick enough to support the
optics. A 24 gauge galvanised steel hot air duct may be easily available but in most cases is not
adequate without considerable reinforcing. The cost of having tubing made of rolled sheet metal
varies widely among firms that can do the work.
Tube reinforcing rings are often used on telescopes. Normally fibreglass tubing is available
with metal reinforcing rings. Unless the fibreglass is quite thick these rings should not be considered
an optional accessory merely for decoration. The metal rings are important to give adequate support
to the optical system while keeping the weight and the cost of the fibreglass tubing reasonable. With
tubing of any material assembled into a telescope you may find additional support is needed to
reduce vibrations and stabilise components. Reinforcing rings are most likely to improve
performance without major rebuilding. First it should be determined, if possible, what part of the tube
needs reinforcing.
The three most common places additional support might be needed is at the mirror mount, at
the cradle, and at the spider. A fourth common source of vibration is in the telescope mount because
of a shaky base but this does not involve the tube directly. If the tubing is made of a material that
lacks sufficient stiffness a problem may exist at the tube cradle. For good performance with most
non-metallic tubing the tube cradle should be 25 to 50% of the length of the tubing. The long cradle
reduces the flexure of the tube under normal loads or vibration. Since most amateurs have little
control over cradle length you may find it desirable to add reinforcing rings near the cradle either
inside or outside the tube. Another method of adding support is to have heavy sheet metal rolled to
fit the outside diameter of the tube for about ¼th the circumference and of suitable length. This
usually can be added between the existing cradle and the tube, with little trouble. Felt or similar
material should be used to protect the tubing.
The mirror mount end of the tube may need additional reinforcement but usually the most
common place the tube needs extra support is at the spider end of the tube. Low secondary vibration
depends on a good spider and a good support for the spider. If a sturdy spider must be used to
reinforce a weak tube then low vibration will be achieved at the cost of high diffraction in what you
observe through the telescope. Low diffraction spiders require a firm, stiff support to hold the
diagonal with minimum vibration.
If you find your telescope tube could use reinforcing rings you will probably have to make them
fit your telescope. In some cases you might be able to adapt the commercial reinforcing rings made
for fibreglass tubing to fit the inside of another size tubing by cutting the rings and squeezing them
down. The amount these rings can be reduced is limited. Cast aluminium rings can be machined to
fit the outside of larger tubes but can't be cut down for smaller sizes. Other rings can be reduced in
diameter by ¼ to ½ inch. In making your own reinforcing rings you are necessarily limited to the
material and equipment available. With a band saw you can cut rings from plywood to fit any tube
size. Metal rings can be made of flat strip steel or strip aluminium from some hardware stores. It is
fairly difficult to bend thick, wide strips but even a couple 1/16" X ½" strip rings can help a spider
considerably. If 1/8" thick aluminium is available it can be bent if it is not too wide. Bending metal
rings should be done over a form about 10% smaller in diameter than the finished ring. Plywood cut
on a band saw is good but a round post or pole will do. It is difficult to bend the ends of the strip so
don't try. Use a long strip and bend it to form a spiral. Then trim off the straight ends to fit the ring to
the inside diameter of the tube. In some locations the hardware stores don't have metal strip. You
can substitute threaded rod of about ¼ inch in diameter. If more support is needed you can use
24
Newtonian Notes
several rings next to each other. Carefully fit the rings so the ends butt against each other then glue
in place. Of coarse, the easiest thing to do is buy good telescope tubing to begin with and avoid the
need to patch it up later if you discover a problem.
Often for telescopes 16 inches or larger but sometimes in smaller sizes the telescope tube is
made in the open style skeleton type. Early this century large professional telescopes were made in
the Rectangular form skeleton tube. Construction is basically a series of large diameter rings
connected by rows of small diameter rods or tubes which run parallel to the optic axis. The rings are
also joined by diagonal bracing between the parallel rods or tubes. The design is important to
amateurs because it allows you to make any size telescope tube to suit your needs using commonly
available equipment. A power band saw is preferred to cut the rings with but you can use a saber
saw or even by hand. The rings are then drilled and bolted together using threaded rod with
electrical conduit, pipe, or tubing for spacers between rings. Sometimes no spacer tubes are used.
The space between rings is maintained by nuts on both sides of the ring. Spacing between rings is
generally one to two times the ring diameter.
While a large Rectangular skeleton tube is often easier to make than a fibreglass tube many
amateurs have had trouble getting the excellent support a well designed tube can give. Thus
skeleton tubes in general have been unjustly discredited. The problem has been in the amateurs
adaptation of the design, not the design itself. The most common error is to leave out the diagonal
bracing between rings. It is the diagonal bracing that gives the tube very good stiffness with low
weight. Without diagonal bracing the rings and the tubes or rods must be large and heavy to give
adequate support. The second most common problem is overbuilding. Sometimes amateurs get
carried away with their pet designs or out of fear not building heavy enough. Skeleton tubes have
been observed that were well machined and very craftsman in appearance but with overbuilding
apparent in design. Rings one inch thick by two inches wide in cross section has a nice massive
appearance but only a ¼ inch by one inch cross section was needed. A dozen parallel tubes were
used when only six were needed. The telescope mount was homemade but well designed and
machined. The problem was it was not adequate to support such an overweight tube so a great deal
of-effort produced a shaky telescope that was nice to look at in daylight but close to embarrassing to
show off when observing at night. The third problem is open tubes allow warm air currents from the
ground, telescope mount, observer, etc. to pass freely through the optical path. Stray light also can
pass into the telescope. Both the unwanted air currents and light can be excluded by adding a thin
metal heating duct or cardboard tube set inside the skeleton framework.
In more recent years large professional telescopes have adopted an improved open telescope
tube called the Serrurier Truss. It gives exceptional stiffness with low weight using less parts. The
design recognises the great importance in the diagonal members and completely eliminates all the
parallel rods or tubes while reducing the number of rings to only four. The rings are connected by
eight diagonal members, usually thin steel or aluminium tubing. Two rings are fairly close together at
the cradle or whatever is used to connect the tube to the telescope mount. Sometimes these two
rings are replaced by a single box-like structure. The Serrurier Truss has been used by a number of
amateurs for large telescopes with better results than the older Rectangular form. The Serrurier
design forces you to use diagonal supports while in the Rectangular form the diagonal bracing was
easily and often omitted.
Both the Serrurier Truss and Rectangular form of open tube can be built by amateurs but the
Serrurier Truss is more stiff and takes less parts which results in less weight and better performance.
Both require cutting rings from sheet material. Usually aluminium is used but wood and steel can
also be used. Cutting the outside of a ring is easy on a power band saw but cutting the inside
diameter can be a problem. You can drill a pilot or starting hole and use a saber saw. Or if you want
to use a band saw you can drill a pilot hole, cut the band saw blade, insert it into the hole then reweld
the blade back together. Commercial band saws have built-in blade welders to handle such
problems. For most amateurs it is not convenient to cut and reweld band saw blades. You can split
the ring by cutting through it to cut the inside diameter then welding the split after cutting (glue and
screw in the case of wood).
In the Rectangular form the rings are connected with small tubes or rods. This form of open
tube works best when rods are placed inside the small tubes. The rods are tightened to apply a
strong compressive force on the tubes. In effect the tube and rod becomes a prestressed structural
unit for greater resistance to bending. In a Serrurier Truss only thin tubes are needed. Usually a
plug is made for the ends of each tube and the plugs are then bolted to the ring.
25
Newtonian Notes
Depending on where you live, you may be able to get plastic tubing made for other purposes
and adapt it to telescope use. Plastic tubing can be divided into two groups - reinforced and not
reinforced. Non-reinforced plastic tubing such as polyvinyl chloride and polyethylene is used for
home plumbing, sewer pipe, ducts for corrosive gases, etc. It is available in many sizes with about
3/16 inch thick walls. The problem with non-reinforced plastic tubing is that it is often not stable
enough for good telescope performance. It can sag under its own weight and sag worse under the
load of mirrors and accessories. This is not a bad property for sewer pipe but is bad for optical
alignment in a telescope. In some cases it can not stand prolonged exposure to sunlight without
cracking and ultimately falling apart.
Reinforced plastic tubing is usually glass fibre reinforced polyester or epoxy resin. In most
cases additives are mixed with the plastic to prevent deterioration by exposure to sunlight. The glass
fibre can be applied by cutting in short lengths and spraying it on with the resin, fibreglass cloth fabric
applied between layers of resin, or a continuous glass filament wound on a form and coated with
resin. Filament wound tubing is quite expensive but has the greatest strength. The glass fibre
reinforcing increases strength over plain plastic tubes but most important it increases stiffness to
reduce sagging. Reinforced plastic tubing is available in 10, 12, 14, 16 inch inside diameter sizes
(among many other sizes including larger sizes) with a wall thickness of 3/16 inch to ¼ inch or more.
A number of firms specialise in making industrial fibreglass reinforced tubing and other firms handle
it. Joseph Ryerson & Son, Inc. with offices in several dozen major cities handles both reinforced
and non-reinforced plastic tubing. The cost can be very high for some types of plastic tubes.
Thin wall aluminium tubing (3/32" thick or less) is made for irrigation systems. If irrigation
suppliers are within driving distance you can get 5, 7, and 10 inch OD tubing in lengths to 20 feet
long at fairly low cost. Getting the tube shipped adds to the difficulty and cost. If you want to look for
tubing locally start first in the Yellow Pages. If you live in a smaller city or town contact the telephone
office for getting a telephone book of a nearby large city.
Structural Considerations for Telescope Makers, Sky & Telescope Magazine, June 1976. Serrurier and Rectangular tube
designs.
Amateur Telescope Making - book two, Scientific American 1965. Many tubing discussions.
Making Your Own Telescope by Thompson, Sky Publishing Corp.1973. Insulating metal tubes.
Standard Handbook for Telescope Making by Howard. Making a fibreglass tube.
26
Newtonian Notes
Selecting And Using Focusing Mounts
A focusing mount is the device on the telescope that moves the eyepiece or accessory to the
proper focus. A simple focuser is a sliding tube which holds the eyepiece. Most often a focusing
mount uses knobs to turn a set of gears called a rack and pinion which slides the focusing tube up
and down. Two types of rack and pinion focusers are used. A regular straight cut and a helical or
angle cut. If properly made a straight cut rack and pinion will work well in most applications. Helical
rack and pinion focusing is used on fine quality, professional instruments in astronomy but also on
microscopes. If the gear set is wide enough, several gear teeth engage and disengage continuously
and smoothly giving very good fine control with easy and rapid long travel required when switching
eyepieces or adding a Barlow lens. The Crayford focusing mount is smooth operating like a helical
rack and pinion but has no gears. The focusing knobs turn a friction drive which moves the sliding
focusing tube. While not yet commercially available for telescopes, some amateurs find the design
easy to make. Spiral focusers are sometimes called helical focusers but are then easily confused
with helical rack and pinion focusers. Spiral designs work with spiral or helical threads between the
focusing tube and the base simular to a large nut and bolt. The focusing tube does not slide but as it
is rotated it spirals up and down to focus. Most often they are used on optical instruments with fixed
eyepieces (such as binoculars) not requiring long focusing travel. Adapted for interchangeable
eyepieces on telescopes a device is usually added to allow rapid long travel without turning the focus
adjustment many times. Most telescopes use rack and pinion focusers.
The size and travel of a focusing mount is determined mainly by the eyepieces and
accessories used at the focus. The travel of a focusing mount is the distance the focusing tube can
be moved by the focus adjustment. The "in" travel of a focusing mount is the distance from the focal
plane toward the diagonal the focusing can be adjusted. The "out" travel is the distance from the
focal plane away from the diagonal the focusing can be adjusted. Together they use up the total
travel a focusing mount is designed for. Where you position the focusing mount determines how
much of the total travel is divided into "in" travel and "out" travel. Thus you must decide how much
"in" travel and "out" travel needed to select a focusing mount and to figure where to mount it on the
tube.
Barlow lenses usually require ½ inch of "in" travel and sometimes one inch or perhaps slightly
more. A 35E9 SLR (single lens reflex) camera used for prime focus photography uses-about 1¾ inch
of "in" travel and slightly more with a prime focus adaptor (depending on design). Eyepieces and
eyepiece projection photography require "out" travel. The amount of "out" travel depends on the
focal length of the eyepiece and the position of the focal plane set into the eyepiece barrel. The
position of the focal plane in an eyepiece is chosen by the eyepiece maker. In good sets of
eyepieces the focal plane is set at the same distance from the top of the focusing mount, no matter
what the focal length of any eyepiece is. The eyepieces can then be interchanged without changing
the focus. Very little "out" travel is used on the focusing mount. Such eyepiece sets are called
PARFOCAL. To take full advantage of them the amateur must be satisfied using eyepieces only
from one manufacturers set. Most amateurs want to be able to use other eyepieces not in the set.
Outside of parfocal sets there are no standards so the focusing mount must adjusted to suit the
eyepiece variations. The specific "out" travel required depends on the particular combination of
eyepieces. Generally an "out" travel of one inch is used, sometimes as much as two inches, but
usually 1 ½ inches of "out" travel is enough. Care must be taken to be sure enough "in" travel is
available. There is no easy way of adding extra "in" travel. Prime focus photography is easiest to
begin with and gives best results for some objects. But if the telescope has been assembled without
sufficient "in" travel it can be a problem repositioning telescope parts to give more "in" travel. To add
"out" travel an eyepiece extension can be added between the eyepiece and the focusing mount for
eyepieces that require a long "out" travel. The filter adaptor available from Vernonscope, Candor,
NY 13743 can be used as a focusing mount extension 1½" long.
Very long focusing travel can solve some problems but make others worse. Very tall focusing
mounts with long travel can increase the size of the diagonal required especially on rich field
telescopes. For f4 telescopes the minor axis size of the diagonal is increased ¼" for each inch of
added focusing mount height. This adds to the cost and increases diffraction to a limited extent. A
regular focusing mount has a travel of about two inches so an added inch of travel and focusing
mount height does little to effect the telescope cost or performance. But since an inch of added
focusing mount height also shortens the tube length required by an inch, some amateurs add three or
even six inches to the focusing mount height to shorten the tube by an equal amount. This often
27
Newtonian Notes
results in problems of excessive diagonal size, restriction of the field of view, and large off-axis
weight especially with a camera. It in best to keep the focusing mount tall enough to do the job and
nothing more. If extra "out" travel is needed for certain eyepieces a focusing mount extension can be
added.
In the United States there are two standard eyepiece sizes -1¼ and 2 inches. Foreign
standard sizes include .965 inches (Japan), .917 inches (microscope standard), and 27mm (Europe).
Most 2 inch focusing mounts come with 1¼ inch adaptors. The question often asked is why use a 2
inch focusing mount when only a few 2 inch diameter eyepieces are available. Or why are some
eyepieces mounted in 2 inch diameter barrels when most focusing mounts are 1¼ inch. The
question of eyepiece sizes goes back to the early days of amateur astronomy when refractors were
most commonly used. To provide better colour correction refractors of long focal length are
necessary. This in turn limited the field of view. When the technology of parabolising and coating
mirrors reached amateur telescope makers and particularly after Pyrex became available, amateurs
began to make larger apertures with lower f-ratios around f8. The field of view became wider if the
eyepiece or focusing mount were large enough in diameter not to constrict the field. For the
eyepieces and reflectors available at the time the 1¼ inch diameter standard became more widely
accepted for U.S. telescopes. In more recent years the availability of eyepieces and telescopes has
increased. Eyepieces with longer focal lengths and wider apparent fields often require focusing
mounts larger than 1¼ inch to give restricted fields. Lower f-ratio mirrors and complete telescopes,
particularly rich field telescopes are now popular because of the wide, bright fields of view. To get
wide fields of view the 2 inch focusing mount has been adopted as standard. However most
refractors used in the U.S. still use the .965 inch standard.
The question now is - when to use a 2 inch focusing mount? The answer depends on the focal
length and apparent field of the eyepiece. To simplify things you can use the chart in "All About
Telescopes" in section 8 on the topic "Eyepieces You Can Make", on page 173. 'The chart gives the
linear field (or diameter required) at the focal plane of a telescope image with eyepieces of many
focal lengths and apparent fields of view. Apparent field of view is related to the design of eyepieces
and is usually specified in eyepiece ads. However your eye will see the full apparent field of view
only if the focusing mount is large enough and the eyepiece barrel is large enough to let the light
reach the linear field at the focal plane. As a rule of thumb, Erfle and Konig types of eyepieces with
focal lengths longer than 25mm and Orthoscopic and Kellner eyepieces with focal lengths longer
than 32mm have their fields limited by 1¼ inch focusing mounts. In many cases it does not matter if
the eyepieces are mounted in 1¼ or 2 inch barrels because it is the focusing mount that limits the
field. But for 45mm or longer focal length, eyepieces should be in 2 inch barrels to avoid field
restriction.
The chart in "All About Telescopes" also indicates when 1¼ inch focusing mounts restrict the
field. However such rules of thumb and charts generally assume a fairly short focusing mount,
ignore effects of various f-ratios, and assume 100% illumination over the entire linear field of the
eyepiece. For practical reasons the image size of 100% illumination is often a maximum of ¾ or one
inch and is often selected to be ½ inch even though the eyepiece can use as much as 2 inches. The
bottleneck of the focusing mount and diagonal size will reduce the brightness at the edges of the field
before it will block the field of view completely. Ideally the focusing mount and diagonal should give
100% illumination over the entire linear field of all eyepieces. This is not practical because many
telescopes would require an extra large focusing mount and a huge diagonal. Only long focal length
eyepieces can use a linear field larger than ¾ inch. If long focal length eyepieces are only
occasionally used it is a waste to provide a telescope with a large linear field. On the other hand, if
long focal lengths are often used to get wide, bright fields it is important to get parts for the telescope
to let the light through to illuminate a large linear field. It is best to select a reasonable image size of
100% illumination. Use the image size in the formulas for the focusing mount and diagonal size. For
rich field telescopes the selection of best image size becomes a more difficult set of compromises.
See the special section on rich field telescopes for more information. Information on selecting image
size is covered in the section on selecting diagonal size.
It is usually sufficient to provide 75% illumination or even less on long focal length eyepieces
over the full linear field the eyepiece can use. Some compromise is needed for shorter focal length
telescopes. For example, a 10"f4 rich field designed for an image size of 100% illumination of ½
inch diameter needs about a 3.10 inch diagonal. Using the formula for minimum focusing mount
inside diameter gives 1.67 inch. A 1¼ inch focusing mount would be far too small so a 2 inch
focusing mount is recommended. A larger diagonal can be used to increase the 100% illumination a
little but the limit set by the focusing mount is not much larger. it is important to select both the
28
Newtonian Notes
focusing mount and diagonal size considering the restrictions in both. A diagonal selected for a 3/4
inch image size of 100% illumination will do little good if the focusing mount limits the image size to
3/8 inch.
29
Newtonian Notes
Checking for Focuser Bottleneck
For rich field telescopes and any telescope using a very tall focuser it is a good precaution to
check the focuser to be sure the full illumination can reach the eyepiece. To see what the largest
image size of 100% illumination will be allowed through a particular focuser you might use on your
telescope just plug in the dimensions into the formula. The formula assumes the diagonal will be big
enough. The image size you use to find diagonal sizes assumes it will pass through the focuser.
You must compare the illumination for the diagonal with the illumination size that will fit through the
focuser. Only the smaller of the two will reach your eyepiece. But you will know which is a
bottleneck so you can decide if it is desirable to change something and test its effect. Perhaps a 2"
focuser is needed for 100% illumination. Or perhaps you are using too large a diagonal for the
focuser you want to use.

I=
R⋅F - H⋅M
F - H
Where I = images size of 100% illumination
F = focal length of mirror
M = primary mirror diameter
R = restriction diameter
H = height of focal plane above side of tube (or focuser height plus “in” travel)
For image size of 75% illumination use above formula but divide mirror diameter in half = M.
The widest field used by eyepieces is in long focal length eyepieces. Thus the focuser tube is racked
"out" and is usually not the bottleneck. With the focuser tube racked "out" the bottleneck is usually
the bore diameter of the focuser base casting. R = 1 3/8" for most short focusers for l¼" eyepieces.
In better quality focusers for 1¼" eyepieces the base casting is bored to 1½" to reduce vignetting so
R = 1½". In focusers for 2" eyepieces R = 2 3/16" approximately. Use one of these R values unless
you can measure a focuser directly. But to make a good buying choice this calculation should be
made before you spend your money. If you are not sure what size a focuser is you must assume the
smaller size to be on the safe side. After solving formula (5) you should have the Image size I come
out to a positive number. If it comes out a negative number (with a minus sign) it means that the
focuser will not let full illumination through to reach the eyepiece. This problem is more likely to
occur in low f-ratio telescopes using l¼" focusers. However some firms persist in putting focusers on
the market without proper design or ray-tracing or formula checking to be sure the full brightness will
reach the eyepiece. One model checked allowed only about a 1/10 inch image size of 100%
illumination at f15 and nothing at any lower f-ratios The focuser is being sold at a premium price
which often is taken by amateurs to mean that it is of high quality. Actually it is junk at f-ratios less
than f15. Let the buyer beware. Use the formula (5) or ray-trace before you spend your money.
Figure 12, Focuser
30
Newtonian Notes
Figure 13, Vignetting
Vignetting of Image Forming Rays by focusers : Amount of vignetting depends on f ratio,
focuser size, height of focal plane above focuser base, and focuser design. Typical good designs are
shown. Since focuser vignetting is mainly at the base end it is common to use 1¼" eyepieces in a 2"
focuser to avoid vignetting at the eyepiece end, particularly with low f-ratio telescopes and eyepieces
with wide fields of view (large linear images)
31
Newtonian Notes
Mirror Mounts
The mirror mount is used for mounting the primary mirror in the tube of the telescope. For
Newtonian telescopes there are a number of variations in design but they all work about the same.
Most commercial mirror mounts are made of aluminium for light weight, corrosion resistance, and
rapid adjustment to changing air temperature. Other materials can be used to make a good mirror
mount. Steel, cast iron, and brass work well but the weight is close to three times as much as
aluminium.
A primary mirror mount is made of two basic sections - the mirror cell and the tube adaptor
frame. The mirror cell supports the mirror from the back, the edges, and the face. A good mirror cell
is adjustable to suit the actual mirror diameter and thickness which can vary considerably from
nominal sizes. The tube adaptor frame is fitted and bolted to the inside of the telescope tube. The
mirror cell and tube adaptor frame are connected with collimation screws that allow the position of
the primary mirror cell to be adjusted. Don't be confused with big phrases like "primary mirror
collimation adjustments". It only means you can adjust the mirror to point straight up the telescope
tube. The mirror cell, tube adaptor frame, and collimation adjustment together are usually called a
mirror mount although the terminology can vary.
In a mirror cell the mirror is supported on the back of the mirror. The points of support can be
nothing more than wherever the mirror sits in the cell but more often the points of support are definite
points or raised areas on which the mirror rests. There are several advantages to definite raised
points. A mirror back on a flat surface will naturally come to rest at three points. But these points are
not definite and can cause the mirror to shift position to another three points when the telescope is
pointed to mother area of the sky. Making three definite points built into the mirror cell design can
eliminate rocking. This is simply done by spacing the three points an equal distance apart (120
degrees) near the outer diameter of the mirror.
Also to be considered is the fact that the mirror will sag under its own weight. It is true the
amount of sag is very small but the surface accuracy of the mirror is very high. Thus the amount of
sag can cause optical distortion. There are several ways of reducing mirror sag. One is to make the
th
mirror blanks thick. The generally accepted rule is to have the thickness at least 1/8 the diameter.
th
Most amateur sized blanks are now well standardised to a 1/6 ratio of thickness to diameter. This
allows enough for grinding and extra to be on the safe side. But the extra thickness adds weight and
cost so adding thickness has practical limits.
If you support a mirror at the center, the maximum sag will be at the edges. If you support a
mirror at the edges of the outer diameter, the maximum sag will be in the center. To reduce mirror
sag to a minimum using three points of support on the back, the points are set on a circle about 70%
of the diameter of the mirror. This puts half of the weight of the mirror inside the point circle and the
other half outside the point circle. In larger amateur mirrors the problem of mirror sag is more
serious because the three points becomes farther apart with increasing size and weight. To correct
this, large amateur telescopes and most professional telescopes use a multiple point back support.
In most amateur telescope sizes this is accomplished by using a 9 point flotation system where each
of the three points is subdivided into three points mounted on a floating pad that is free to adjust itself
automatically to the back of the mirror. This distributes the mirrors weight equally over nine points.
With nine points for support each point is much closer to the next to reduce sag. it is a matter of
personal opinion when is use a 9 point flotation system instead of a regular 3 point support. The
issue will not be argued here. If you feel the need for better support in a medium to large mirror
mount you can build or buy one of the 9 point systems available.
A mirror cell supports the mirror at the edges to keep the mirror from shifting sideways. Since
mirror cells are made of materials that expand and contract much more than the mirror does when
the temperature changes, it is important to make some provision in the edge supports to take up the
difference in expansion. Too firm of an edge support will cause strain in the mirror and thus produce
optical distortion. Too soft of an edge support will avoid strain but will allow the mirror to shift
sideways thus losing collimation.
Mirror clips rest against the face of the mirror. In normal telescope use the telescope is
pointed at or above the horizon. With properly designed edge supports the mirror rests in a stable
position in the mirror cell with no pressure on the mirror clips. The only time the mirror clips actually
32
Newtonian Notes
hold the mirror is when the telescope tube is pointed below the horizon - normally only during
transportation or storage. In other words, the mirror mount should hold the mirror properly in normal
use without the need for mirror clips. Mirror clips are usually made of metal but are faced with cork
or soft plastic at the points it rests against the mirror face. This soft material allows the mirror cell
and mirror to expand and contract to normal changes in temperature without strain to the mirror. It is
important because the easiest way to put strain in a mirror is with mirror clips not properly designed
or set too tight. Experienced telescope makers take care to get a good mirror then often set the
mirror clips with a sheet of paper between the mirror face and the mirror clip. After the mirror clips
are set the paper is removed leaving a slight gap.
th
th
A good mirror with a 1/8 wave or 1/16 wave optical surface will not perform at its best if the
mirror clips strain the mirror out of shape by ½ or even one wave. To check for mirror strain you can
use the Ronchi Grating test (such as in Coulter Optical Co. Performance test kit)or adapt the
telescope for the Foucault test. Mirror strain will show up as astigmatism so some reference should
be used to show what astigmatism looks like in the test you give. The Ronchi test is recommended
because it is simple to set up and judge. Mirror strain is more likely to be seen at lower
temperatures. You may begin an evening of observing with a test for mirror strain and find none.
But several hours later the temperature has dropped 5 or 10 degrees and find the mirror being
strained. The same is true for unusually cool observing nights. It only takes a minute or two to check
for strains once you have the test materials and know what to look for. It should also be noted that
mirror cells made of wood may change with the humidity.
If you do find mirror strain in the primary mirror don't get too upset. The strain usually does no
permanent damage and the strain can be eliminated. But it does require removing the mirror mount.
First check to be sure you have correctly adjusted the mirror cell to the mirror. If it seems the
adjustment is correct then add a layer of cork to the mirror clip and carefully readjust.
Mirror mount collimation is usually through three collimation screws. But if you are mounting
the mirror mount in the tube yourself it is possible to make collimation easier by positioning the mirror
mount with reference to the focusing mount. Either of the two positions will work the same. The idea
is to adjust the “left" and "right" screws first to line up the mirror on one axis. Then either the “top" or
"bottom" screw is turned to adjust the other axis until cantered. The "top" and "bottom" position is
important because it can adjust the mirror to move along one axis only. The "right" and "left" screws
are adjusted first because they move the mirror on both axes at the same time. Without the "top" or
"bottom" position all three screws would move collimation in both axes at the same time by differing
amounts. Thus it is more difficult to figure out which direction the mirror will move when one screw is
adjusted.
Making Your Own Telescope by Thompson, Sky Publishing Corp.1973. Describes making your own primary mirror mount.
Amateur Telescope Making-book one, Scientific American 1966. Describes the need for 9 point flotation systems and
shows some early designs.
33
Newtonian Notes
Selecting Diagonal Size
In making a Newtonian telescope a diagonal is selected often by "rule of thumb" or "suggested
sizes". The actual reasons for using one size rather than another are usually unknown to the
amateur. Most amateurs know the diagonal size depends on the focal length of the primary mirror
but in fact also depends on a number of other parts including tube size, focusing mount height, and
the image size. Years ago the focusing mount height was for the most part close to 3 inches tall
because little else was available. Eyepieces were available in much more limited focal lengths and
apparent fields. In more recent years the size of focusing mounts available ranges to over 12 inches
tall. Eyepieces are now available with longer focal lengths and wider fields that need larger image
sizes. All this makes the "rule of thumb" less accurate and in some cases leads to poor telescope
design. The newer telescope parts have expanded the world of amateur astronomy. But poorly
chosen parts often put a bottleneck in the optical path to limit the full performance of the finished
telescope. Too often the diagonal is undersized or in some cases oversized.
The following are the reasons and methods of selecting the diagonal size. If these details are
a bit too complicated for you it is still safe to stick to suggested sizes for many telescopes. But if you
are using a focusing mount more than 3½ inches high or more or you are using long focal length or
wide field eyepieces, or you want to use prime focus photography, then it is best to select the
diagonal size more carefully.
The first problem in selecting the diagonal size is most amateurs think of the Newtonian focus
as a point. In fact, the image is not a focal point but rather a focal plane of real size. The eyepiece
is a magnifier to increase the size of objects that make up the image you see. If a diagonal is
selected by formula or by layout drawing assuming a focal point with no image size the diagonal will
be undersized and the eyepiece will not get full illumination. In other words, you will waste some of
the light gathering power of the telescope and reduce the brightness of the image.
The correct image size to use depends on the eyepiece you use. The linear field of a
telescope image increases with both longer focal lengths and wider apparent fields. Please note that
linear field, apparent field, and the actual real field of view are all different kinds of field
measurements. The linear image size is the actual diameter of the image that a particular eyepiece
design can use when measured at the focal plane of the telescope. The linear field or image size
assumes no mechanical parts are limiting the field. For example, a 6Ornm Kellner or Orthoscopic
eyepiece has a linear field of about 2 inches in diameter but if it is mounted in a 1¼ inch barrel or
uses a 1¼ inch adaptor then the three field measurements are all much smaller because the linear
field can be no more than 1¼ inches. The apparent field is determined by the optical type or design
of the eyepiece, Erfle, Kellner, Huygens, etc. It is what the angular field size appears to your eye
thus the name apparent field. The real field of view you actually see through the telescope is the
angular size of the sky the telescope can cover or bring into view in one position. For most
telescopes the real field of view is one degree (two moon diameters)with a long focal length eyepiece
and perhaps one-fourth of a degree at high power. The apparent field is important because the
larger it becomes the larger the real field of view becomes. For example, a 20mm Ramsden
eyepiece with an apparent 35 degree field may perform well in a telescope. But a 20mm Erfle
eyepiece with an apparent field of 70 degrees can give double the real field of view and allow you to
observe four times the area of sky as you could see with the Ramsden. The focal length is the same
so the magnification is exactly the same but the wider apparent field lets you see much more. The
problem with the Erfle or other wide field eyepiece is added cost for the special wide field design and
the linear image is doubled when you double the apparent field.
Ramsden and Huygens may work well in a telescope but Kellner, Orthoscopic, Erfle, and
Konig eyepieces are preferred by most amateurs because the area of sky is two to four times larger.
However this may double the required image size measured in linear diameter at the focal plane.
Unless the diagonal size, focusing mount size, and other parts are large enough to allow the full
image size to form, you will not benefit from the larger apparent fields of the more expensive
eyepieces. A good set of eyepieces represents a large investment that usually stays with you even
though you may some day get a larger or smaller telescope or a second telescope. Using undersized
telescope parts that bottleneck the field will only limit the performance of both the telescope and the
eyepieces. The correct image size the telescope should be designed for depends on the apparent
field and focal length of the eyepieces you use. The chart in "All About Telescopes" by Brown in the
section "Eyepieces You Can Make" gives the linear field image size for many eyepieces. Since most
34
Newtonian Notes
telescopes are used with several eyepieces you can select the image size of your most frequently
used eyepiece or an average of several eyepieces. There are a number of ways to find
magnification so the following formula can be used to relate a number of telescope elements to each
other.
†
magnification =
focal length of telescope
focal length of eyepiece
=
apparent field
real field
=
dia. of mirror
dia. of exit pupil
Another problem is in most cases it is not practical to fill the entire field of all eyepieces with
100% illumination. Most telescopes have a small field in the center of 100% illumination with the
brightness tapering off toward the edges to 50 or 75% illumination. This drop in illumination is not
generally noticeable to your eye. Shorter focal length eyepieces only use the smaller center field so
the lower illumination at the edges of the field affects longer focal length eyepieces. It is not really
worthwhile to provide full illumination for one long focal length eyepiece when most of your other
eyepiece can't use as large a field. This is especially true if the long focal length eyepiece is used
only occasionally.
Often it is better to lake an average of the focal lengths of your eyepieces. If your average
eyepiece is a 16mm Kellner or Orthoscopic the field required is about ½ inch. If your average
eyepiece is a 25mm Kellner or Orthoscopic the field required is about 7/8 inch. In most cases filling
the entire field of long focal length eyepieces with 100% illumination would require a huge diagonal.
The light loss would be high along with the cost. Thus a diagonal is usually selected to put 50 to 75%
illumination at the edges of the larger fields of the eyepieces used. It is a matter of personal
judgement how large to make the image size of 100% illumination. For beginners an image size of
½ inch is suggested when you plan on using eyepieces mostly of less than 22mm focal length (stars,
moon, planets and other general use); an image size of ¾ inch when you plan on using eyepieces of
24 to 60mm focal length (comets, deep sky objects, etc.);and an image size of one inch for prime
focus photography with 35mm camera. The beginners formula below has been simplified and
assumes ½ inch of "in" travel in the focusing mount for a 2X barlow.
First solve D = T + 2H + 1, then use in
where N = minor axis size of diagonal
M = primary mirror diameter
T = tubing diameter
I = image diameter of 100% illumination
H = height of the focusing mount
F = focal length of primary mirror
‡
N =
M ⋅D + 2⋅I⋅F - I⋅ D
2⋅ F
If you are making your first telescope and not really sure what eyepieces will be most used or
which astronomical objects you will prefer viewing, then it is best to select an image size of about ½
inch diameter of 100% illumination for general purpose use. Once the diagonal size is calculated it is
not necessary to get the exact size diagonal that came out of the formula. In most cases, simply
selecting the nearest available minor axis size from any desired firm offering a full range of sizes is
sufficient.
If your telescope will use a 2.4X or more power Barlow, prime focus photography, or for any
other reason use more "in" travel than ½ inch, then it is best to select diagonal size using a more
general formula below. The formula is basically the same as the one above but has a bit more
35
Newtonian Notes
complication because the "in" travel must be selected and put into the formula. Also remember that
longer "in" travel usually requires a focusing mount with a longer travel or extension tubes.
First solve D = ½T + H + b, then use in

N =
M⋅D + I⋅F - I⋅D
F
where N = minor axis size of diagonal
M = primary mirror diameter
T = tubing diameter
I = image diameter of 100% illumination
b = “in” travel of focusing
H = height of focusing mount
F = focal length of primary mirror
To be a bit more accurate you can add .050 inch for the anti-chip bevel.
No matter which formula you use, remember there is nothing wrong with using a 3/8” or 5/8"
diameter image size instead of ½". The suggested image sizes given here are only suggestions for
guidance and not fixed rules. You may choose to use ¾" image size for prime focus photography but
none of the edges of the film will be fully illuminated. If the object being photographed fills only half
the film a ¾" image size is large enough. But for objects that are large enough to nearly fill the film
should have full illumination to get a uniform exposure. For visual use if you select a small image
size of 100% illumination you may have difficulty locating faint objects. Looking for a comet, galaxy,
nebula, etc. of low brightness often requires sweeping the sky in the area the object is expected to
be. A long focal length eyepiece will give a wide field with bright images so it is a good choice for
sweeping. But if the eyepiece gives a one degree field the small image size of 100% illumination
th
may give full brightness to only a ¼ degree circle in the center or only 1/16 the full field of view.
The edges of the field may then be at only 25% illumination. The faint object must sweep near the
center of the field to be seen with maximum ease. Your eye may see the large field giving the
impression of covering a large field but faint objects will be easy to miss when they pass through the
field nearer the edges.
36
Newtonian Notes
Offsetting A Diagonal
An elliptical diagonal set at 45 degrees will reflect a circular beam of light by 90 degrees. In a
telescope the light beam passing from primary mirror to the focus is not a straight cylinder but rather
a tapered cone. In effect the half of the diagonal closest to the primary is in a larger beam of light
than the half of the diagonal closest to the spider. From geometry it is known that a plane through
both a cylinder and a cone are elliptical in shape. The difference is the center of the ellipse in a cone
is offset from the centre line of the cone. Technically, a diagonal in a telescope should be offset. If it
is not, the illumination at the edges of the field of the eyepiece is not equal. It is not generally
practical to fill the entire field with the maximum 100% illumination but the edges of the field are
commonly about 50% illumination. Not offsetting will decrease illumination on one side and increase
illumination on the other. For common f8 Newtonians the effect is very slight. You generally don't
notice the drop in illumination from 100% in the center to about 50% at the edge. A few percent
difference in illumination at the edges of the field from the left edge to the right edge is even less
noticeable. At lower f-ratios the difference becomes increasingly greater. For this reason some rich
field telescopes are assembled with the diagonal offset to equalise illumination. For longer f-ratios
personal opinion and the desire for technical accuracy will lead some amateurs to offset the diagonal.
In practise most amateurs do not offset on any f-ratio telescope for a number of reasons: (A) The
technical reasons for offsetting are not understood; (B) The methods of offsetting are too difficult or
not understood; (C) The effects of not offsetting are not readily noticeable so they are ignored simply
out of personal choice.
‰
S =
D⋅N
4F
where F = focal length of primary mirror
N = minor axis size of diagonal
D = diameter of primary mirror
R = distance from center of rube to the focal plane
S = Offset from tubing centre line
R = ½ the tube diameter plus height of the focusing mount when fully racked "in" plus the "in" travel
of the focusing mount.
S = Offset from tubing centre line - this is the distance the diagonal is shifted away from the focuser
side of the tubing and the distance the mirror cell is shifted toward the focuser side of the tubing or
the sum of both if both are shifted.
The two diagrams of illumination shows the effect of centring and offsetting the diagonal.
Illumination circles are shifted slightly to the right and are slightly out of round for the centred
diagonal. The effect has been exaggerated to make it possible to see the effect in the drawing.
There is no effect on collimation.
Once you have the amount of offset calculated the problem is to adjust in some way the
telescope components to provide the offset needed. Spiders and secondary holders are usually
purchased and constructed for a centred diagonal. It is possible to adjust the spider mounting screws
to offset the center of the spider. The amount of adjustment possible depends on the specific
combination of spider and inside diameter of the tube and also the type of mounting hardware on the
ends of the spider vanes. Almost all spiders for tubing 7 inches inside diameter or less usually come
with male mounting studs on the vanes. For larger tubing sizes the mounting hardware can be either
male or female except where the tubing clearance is very limited in which case male hardware must
be used. If the male mounting hardware is used then adjustment nuts are on the outside of the tube.
The offset adjustment is generally very limited. If female mounting hardware are used on the vane
tips then machine screws inserted from outside the tube hold the spider and provide the adjustment.
If necessary, two shorter screws can be used on one side and two longer screws on the other. If this
does not give sufficient offset then two of the spider mounting holes can be enlarged to allow the
female hardware to pass through the tube wall. Larger washers usually must be added to cover the
37
Newtonian Notes
enlarged hole. Female hardware can allow more offset than male hardware but often the amount is
too limited.
Figure 14, Diagonal offsetting
Another method of offsetting is to offset the mirror mount leaving the spider centred. This
method shifts the optic axis away from the center of the tube. Again the amount of offset depends on
the combination of components. A mirror mount-made for 6 7/8 inch tubing set in tubing with an
inside diameter of 7 1/8 inch with shims at only one of the three mounting screws will give an offset
of 3/16 inch. More offset is available if two of the mounting legs can be cut down another 1/8 inch or
so. For a mirror mount made for 9 3/16 inch tube but used in 9 ½ inch inside diameter tube the
offset is about ¼ inch plus any additional by trimming the mounting legs. For a mirror mount made
for 17½ inch tube but used in 18 inch inside diameter tube the offset is 3/8 inch. The maximum
amount of offset available by trimming the casting off at the mounting bolts depends on the mirror
cell design. The outer ring frame style cell can be distinguished by the large ring frame that goes
around the outside of the mirror. This style mirror cell can be offset by a limited amount depending
on the diameter of the ring around the mirror. The open back frame style cell has no ring around the
outside edge of the mirror. The only casting beyond the outside diameter of the mirror is for the edge
supports for the mirror. The frame is entirely in back of the mirror with a smaller ring at the
38
Newtonian Notes
collimation adjustment bolt circle. The open back frame style cell can be offset more than other
styles if the offset is in the direction of the open space between two edge supports. As a rule of
thumb, the approximate amount of mirror mount offset can be figured by subtracting the smallest
tube the mirror mount will fit into from the actual inside diameter of the tube. For the outer ring frame
style mirror cell-divide in half. For the open back frame style the offset is ¾ths. In both styles more
offset is possible if the casting is trimmed. But the amount it is possible to trim will depend on the
specific mirror mount and tubing used. It is difficult to figure before buying the mirror mount.
If the mirror mount is offset and the diagonal left centred in the tube, then collimation is best
done with a mask over the end of the tube. A simple sheet of cardboard is sufficient with a hole the
same size as the mirror or a little larger offset by the same amount and in the same direction as the
mirror mount is offset. A pin hole collimation device will show the primary and secondary mirrors and
the tube all centred when collimated. Collimation will then automatically place the diagonal at the
correct 45 degree angle. If the mirror mount offset is not enough you can choose to ignore the
difference or offset the spider in addition to the mirror mount. In either case a mask over the end of
the tube should be used. The hole in the mask is then offset the same amount as the mirror mount
only. If the spider offset and mirror mount offset are the exact amount total required the pin hole
collimation device will show concentric circles and the diagonal will then be set at 45 degrees.
Without full offsetting it is difficult to judge what the collimation device should show. The offset
image and the angle of the diagonal are difficult to measure. This leaves only the method of
checking collimation with the focusing mount fully racked in and fully racked out.
39
Newtonian Notes
Supporting the Diagonal
A diagonal in a Newtonian telescope must be supported at or near the center of tube. At the
same time it should not block too much light from reaching the primary mirror yet be sturdy enough
to avoid vibration in the image. There are several methods of supporting the diagonal in this
position. The simplest and least expensive is the single vane diagonal holder. It is a metal rod
attached to the base of the focusing mount base at one end and has a metal plate to which the
diagonal is cemented at the other end. This type is useful, only for smaller diagonals and smaller
tube diameters. As the weight of the diagonal and the length of the rod increases, so does the
vibration of the image. A large, heavy diagonal on a long rod can vibrate so much as to make the
telescope unusable. Usually the single vane diagonal holders are made so they can hold only
smaller diagonals.
For most telescopes a more substantial support is needed. Usually some form of spider is
used to support a diagonal holder with suitable collimation adjustments. The choice of which spider
design to use depends on a number of factors. Two of the most important, diffraction and stability,
often work against each other. The diffraction pattern that is most visible in telescope use is caused
by straight supports or vanes which produce "spikes" extending from bright astronomical objects.
Other parts of the telescope also produce a considerable amount of diffraction but spider vanes give
a pattern of diffraction concentrated into a small area of the spikes. Eliminating vanes will not
eliminate diffraction. For each straight diagonal support extending from the diagonal to the tube wall
there are two diffraction spikes 180 degrees apart. A single vane diagonal holder thus makes two
spikes on the image of bright objects. The length of the spikes depends on the brightness of the
object and the thickness or diameter of the vane. Since a single vane rod must be fairly large in
diameter to support the diagonal the spikes on objects are fairly long and bright.
If a three vane spider is used to support a diagonal holder the task is given to three arms
rather than just one. This also allows considerable tension to be applied to the vanes which can add
to the stability of the image. Thick, narrow vanes can give good stability but considerable diffraction.
Thin, narrow vanes give much less diffraction but poor stability. Thin, wide vanes give good stability
and low diffraction but are more expensive. For a three vane spider with each vane 120 degrees
apart the diffraction spikes on bright objects are six in number and 60 degrees apart.
A four vane spider has one more vane which means more diffraction but with the vanes 90
degrees apart the diffraction pattern of opposite vanes will overlap each other so there are only four
spikes 90 degrees apart. Each spike is a little brighter and longer but there are less spikes. Since
four vanes support the diagonal holder the vanes can be made thinner than on a three vane spider.
The additional vane will add some diffraction none the less. It is a matter of personal opinion
whether you prefer six spikes a little less faint of four spikes a little brighter. Overlapping diffraction
as on a four vane spider does not double the spike length or brightness. The difference in diffraction
spikes is not great between three and four vanes spiders because the nature of diffraction spikes is to
be concentrated to a limited area near the image of a bright object. Moreover there are relatively few
objects in the sky bright enough to produce noticeable spikes. Most objects bright enough to produce
noticeable spikes with a four vane spider will also produce spikes with a three vane spider. So using
a three vane spider will not eliminate spikes.
The question of diffraction must also be tempered with stability. Four vane spiders have the
added leg of support and each leg is 30 degrees closer to the next. Because non-metallic tubing is
less rigid than metal tubing it can support less tension which gives spiders their stability. Because
non-metallic tubing such a fibreglass are more popular to use and the difference in diffraction is not
great, the three vane spider is now rarely used even though they cost less to make. Most large
professional telescopes use four vane spiders to support the secondary holders.
All this is not to say that three vane spiders are "no good" because of too much vibration in the
image. The problem is in the more careful design and application required for three vane spiders.
Designing spiders to give good stability, moderate diffraction, and minimum obstruction has been
somewhat difficult. Two of the most common problems found in telescopes are unstable telescope
mounts and unstable spiders. Many telescopes have either or both of these problems. The result is
excessive vibration of the image. Simply adjusting the eyepiece focus can cause the image to
vibrate for five or even fifteen seconds after the telescope is touched. A telescope made of well
designed components, including portable telescope mounts, will show vibration for a second or two.
The added problems of three vane spiders not correctly designed and used only makes vibration
40
Newtonian Notes
worse. Three vane spiders fell into almost rare use in favour of four vane spiders. Larger telescopes
with heavier secondaries probably need the extra support of four vanes. But the common sizes of
amateur telescopes can use a three vane spider with good performance if it is properly designed and
used.
For three vane spiders a good design with low image vibration should include very wide, thin
vanes to keep diffraction from each vane low. And since non-metallic tubing is commonly used it is
advisable to add metal rings inside the tube, outside the tube, or both. Fibreglass tubing is sold
usually with metal rings for the outside of the tube as an optional accessory. But for either three or
four vane spider performance these rings should be used.
This brings us to the area of making any spider give good performance. Most spiders are not
rigid by themselves but depend on spider vane tension applied by the tubing to make the spider rigid.
Making a spider rigid without the tension from the tube requires the vanes to be about 1/8 inch thick
but this in turn makes the diffraction spikes more prominent in both width and length. For thin vane
spiders of low diffraction the vanes must be in some tension applied by the tube. Making the vanes
wide can reduce the tension required to some extent. For very thin metal tubes and most nonmetallic tubing metal support rings should be used. The end rings for fibreglass tubing should be
within three inches of the spider mounting screws. For larger spacing and other kinds of non-metallic
tubing you can make your own rings out of steel or aluminium strip, 1/8 inch thick or so, available at
some hardware stores. The strip is clamped to a wood form at one end then bent around to form a
ring. Bend more than one full circle because the ends of the ring are flat, and because the wood
form is usually made an inch or so smaller undersized to allow the ring to spring open a bit closer to
the tubing inside diameter. Wide rings are difficult to bend this way by hand, but you can make two
narrower rings of metal of one-half to one inch wide. Position one ring on both sides of the spider
mounting screws. This avoids the problem of getting a spider to fit the outside diameter of the tube
yet have clearance inside the tube to add the extra thickness of the rings. The two narrow rings can
be added to an existing telescope after an image vibration problem is discovered without changing
the spider.
Another method of improving spider performance is to use wide rim washers against the tube
at the spider mounting screws. They distribute the force on the tube wall over a larger area. Wide
rim washers 1¼ inches in diameter are available for common 3/16" and ¼" screw sizes at some
hardware stores or auto supply stores but are often called "fender washers" or "plaster washers". In
some cases both wide rim washers and metal rings should be used. But both can be added after the
telescope is assembled and a problem is discovered, usually without too much trouble.
If you would like the less bright diffraction spikes a three vane spider gives you probably will
have difficulty finding a commercial spider or have difficulty making your own. However you can
consider the use of a four vane spider. A four vane spider will have the diffraction spikes overlapped
only if the vanes are spaced equally apart around the circumference of the tube or 90 degrees apart.
But instead of spacing the vanes 90-90-90-90 degrees apart you can space the spider vanes of a four
vane spider 75-105-75-105 degrees apart. A commercial four vane spider can be used simply by not
spacing the spider mounting holes equally around the circumference of the tube. This is suggested
for thin vane spiders only since it requires bending the vanes at the center body. It is the holes in the
tube that determine the angular spacing of the vanes, not the angle the vanes are set into the center
body.
By using a four vane spider with the vanes set at 75-85-115-85 degrees, each vane makes as
much diffraction as a vane on a three vane spider but there are eight spikes instead of six. Like the
three vane spider, the diffraction is not overlapped so it is less bright in the spikes. In other words,
you have the diffraction and stability of a four vane spider but the diffraction is spread out without
overlapping as in a three vane spider. The diffraction spikes on bright stars will be 75-20-65-20-7520-65-20 degrees apart. There is a tendency for diffraction spikes to overlap when the spikes are
separated by a small angle. The spikes are broadened close to the image of a bright object. This is
the problem when a four vane spider is intended to be mounted with the vanes at 90 degrees but
sloppily layout of the tube puts the vanes at irregular angles producing eight spikes not quite
overlapped but rather set in close pairs of spikes. You will observe four spikes because what you see
as one spike is the brighter portion where both overlap and the added width of the diffraction close to
the image of the bright object. For this reason unequal vane spacing should be quite unequal, not
just a few degrees. You may choose to space the spider vanes 70-85-120-85 degrees apart so the
diffraction spikes are 70-25-60-25-70-25-60-25 degrees apart. Or you may choose to space the
spider vanes 60-90-120-90 degrees apart so the diffraction spikes are 60-30-60-30-60-30-60-30
41
Newtonian Notes
apart. Some stability is lost as the angle between vanes is increased beyond 90 degrees so 120
degrees is suggested as a maximum with 60 degrees a minimum.
Most amateurs prefer to set four vane spiders at 90-90-90-90 degree angles. But if you want
to experiment with more spikes that are less bright the unequal spaced four vane spider method is
one approach using a commercial spider that avoids some of the stability problems of three vane
spiders. For beginning amateurs the 90-90-90-90 degree pattern is suggested. You may not like
spreading diffraction into more than four diffraction spikes.
Figure 15, Spider configurations
These five drawings show spider designs that reduce diffraction spike brightness. Overall diffraction
and stability is compared to four vane spiders with equal spacing used on most telescopes. Three
vane spiders (not shown) have lower diffraction, more diffraction spikes less bright, reduced stability.
It is also possible to use a curved secondary support to eliminate the diffraction spikes
altogether. This does not eliminate all diffraction, only the one type of diffraction that causes on
bright objects. In fact, some curved secondary supports cause much more diffraction than a three or
four vane spider. A curved secondary support has the advantage of spreading diffraction over the
entire field more so that the diffraction is not seen as spikes. Since curved secondary supports can
have little if any tension they must be made of fairly thick metal. They are also more dependant on
the tubing being quite rigid. Designing a curved secondary support is somewhat difficult and often
improperly done resulting in poor performance. The result has been the same as with three vane
spiders - poor performance leading to unpopularity. The idea is basically good although the stability
probably is difficult to make equal to a good four vane spider.
Several types of curved secondary supports have been used. One is the two full circle type
with the secondary holder mounted where the circles meet at the center of the tube. Because of the
curves, the length of material in the optical path is much longer than a four vane spider the diffraction
is about 50% greater. The curved support must be made of thicker metal to compensate for the little
tension and the long, curved arms which are easy to flex and vibrate. The tube must be rigid metal
42
Newtonian Notes
or a metal ring added to the inside of the tube for reasonably good results. This type curved support
is difficult to make work properly without high image vibration.
Another type curved secondary support is the semicircle type. The support arms are curved to
eliminate diffraction spikes but are also short in total length for reduced diffraction. Like the two full
circle type the semicircle type is attached to the tube at only two places and requires a solid mounting
to metal tube or to a ring inside the tube, at least for reasonably good performance. Curved
secondary supports generally work better on smaller tube sizes but these sizes are common to
amateurs. With increasing tube size the distance from tube wall to secondary becomes long making
flexure of the support arms more easy and more likely with larger, heavier diagonals.
The question of how wide and how thick to make the support arms is difficult. It is a personal
judgement how much image vibration is acceptable to an individual telescope user. If you would like
to experiment with curved secondary supports a suggested rule of thumb at least for a starting point
is to use metal 1/16 to 1/8 inch thick and one-half to one times the minor axis size in width. In other
words, a 2 inch diagonal should have curved supports perhaps 2 inches wide. This may seem a bit
extreme but consider a four vane spider with good performance has vanes close to one-half the
minor axis size in width. The semicircle support arms are roughly equal to only two vanes.
Therefore the curved arms should be roughly double in width to compensate for the fewer arms in the
case of the semicircle type and to compensate for the very long curved arms subject to easy flexing
in the case of the two full circle type. The added thickness of the arms is to compensate for not
being able to place the arms in tension as a three or four vane spider usually is. In experimenting
with curved secondary support designs you may find a design that works well on 6 inch telescopes
but is poor in a 10 inch telescope. Other designs have been used for curved secondary support such
as the S-shape, etc. but the two shown in the drawings usually work the best.
To greatly reduce or eliminate diffraction spikes on any straight vane secondary holder see
Dealing with Spider Diffraction by Couder, Amateur Telescope Making-book two, Scientific American
1965. The method described will increase overall diffraction only moderately and will increase light
obstruction. It has particular advantage in that it can he used with any straight vane spider and can
be made removable.
One very important factor in the performance of telescopes could be called the secondary
overhang. The diagonal mirror is not placed on the centre line of the spider vanes. Instead, for
mechanical reasons, the diagonal is mounted off to one side toward the primary mirror. Since the
weight of the diagonal and the diagonal holder is not on the spider centre line the weight adds a
twisting force on the spider. To better understand the problem, try lifting a gallon of paint keeping
your lower arm horizontal and your elbow against your chest. In this position the weight is more
difficult to hold than with your arm at your side almost vertical. But now try to hold the paint at arms
length horizontal with the gallon of paint far from your body. The weight has not increased but the
distance from your body has increased and the effect of leverage has reduced your ability to support
the weight. In physics and engineering this effect is known as lever arm. In amateur telescope
design the effect is often ignored.
43
Newtonian Notes
Scale drawings showing two commercial spiders and
diagonal holders. The center of diagonal weight is at
the center of the diagonal near the optic axis. The
diagonal holder with the 50% greater overhang distance
is longer so weighs more, adding to the weightoverhang problem. Narrower spider vanes give less
support to the heavier loading. Thus the unit with lower
overhang can be twice as stabile in minimising
vibration. A few commercial and some amateur
designs have even greater overhang, all with the same
diagonal size. Some well designed diagonal holderspider units are incorrectly mounted with a large
overhang to compensate for errors in layout or design
problems not fully corrected in Newtonian-Cassegrain
conversable telescopes.
Secondary overhang is present in single vane, three vane, four vane, and all curved vanes
spiders and secondary holders. In addition, the more a secondary holder weighs the greater the
twisting force will be on the spider. Bulky and heavy secondary holders add to the problem. You
don't have much choice in diagonal weight. But by using good design of the secondary holder the
weight can be kept low and made close to the spider. Many spider designs don't work well because
the diagonal is too heavy and too far from the spider. This is especially true among amateur designs.
The diagonal then sags out of position and is easy to vibrate. Several decades ago it was the
practise of some to try to balance the overhang with a diagonal counterweight. More often than not
this overloaded the spider with increased weight and increased vibration. As heavy metal tubes were
44
Newtonian Notes
being replaced with lighter non-metallic tubing such as fibreglass the diagonal counterweights were
used less. The lower stiffness further reduced spider load limits and so the counterweights rarely did
any good. Books written before 1950 or 1955 still sometimes show diagonal counterweights but are
no longer recommended.
Large diagonals have the problem of increased weight and since the size is larger it must be
mounted farther from the spider. Thus rich field and large aperture telescopes require more careful
design of the secondary holder and the spider to carry the additional weight and overhang.
Diagonals larger than 3.10 inch in minor axis size should use heavy duty spiders carefully designed
to carry the heavy overhung load. If you are buying a spider and diagonal holder look for one with
low overhang and sturdy spider vanes, especially in larger sizes. If you are making your own spider
and diagonal holder be careful to keep the diagonal overhang and the weight as low as possible. For
any given thickness, brass weighs three times as much as aluminium so aluminium is preferred.
Keep thickness of any material down as much as possible. There are mechanical problems keeping
the thickness down and keeping the overhang low which is why many designs are not very good.
Design the spider to resist the twisting effects yet support the weight. if you make a curved spider
the diagonal overhang must be kept very small to keep vibration down to tolerable limits without
having to make the curved vanes very thick. With some care in selecting or making a spider and
diagonal holder you can avoid one of the most common causes of poor telescope performance. If
your telescope shows considerable vibration the spider and diagonal holder is one of the first places
to look for the cause and can indicate replacement is needed.
Making Your Own Telescope by Thompson, Sky Publishing Corp. 1973. Describes making a four vane spider and
diagonal holder.
Standard Handbook for Telescope Making by Howard, Thomas Y. Crowell Co. 1959. Describes making a curved spider
(type D), and diagonal holder.
A Note on Curved Spiders, Sky & Telescope magazine, October 1969. Describes advantages semicircle curved spider fur
lower diffraction (type C).
An Eight Legged Spider, Sky & Telescope magazine, October 1974. Describes making an eight vane spider to improve
stability and increased diffraction effects.
Star Gazing with Telescope and Camera by Keene, Amphoto 1967. Anti-diffraction screen to be added to the telescope for
use on planets.
45
Newtonian Notes
Guide to Selecting Finders and Guide Telescopes
The main telescope is used for celestial observing but most main telescopes have a fairly
narrow field of view averaging one degree with a fairly long focal length eyepiece used on a common
Newtonian telescope or be as little as one-half degree for many refractors and Cassegrain
telescopes. This makes it a problem to find the object to observe it through the main telescope. The
Big Dipper is spread over 20 degrees of the sky while our moon is about one-half degree. If you find
an object with your naked eye it is difficult to point the main telescope to the center of what you can
see with your naked eye. The view through the main telescope will show stars clearly but not enough
of them will be seen to form a pattern you know with your naked eye. Without seeing a star pattern
you know, it is difficult to know in which direction the main telescope must be turned to find the object
you want to see. To overcome the problem a finder scope is used for locating the object easily. The
finder scope is a small telescope attached to the main telescope and adjusted so they both point in
the same direction (an adjustment called finder collimation). To work well a finder should have a
much wider field of view of 4 to 6degrees. The area of sky through a finder with a 4 degree field is
16 times the area of sky through a main telescope with a one degree field.
Many celestial objects can not be seen with the naked eye so to find them you use star charts
such as published monthly in the astronomy magazines. They show the positions of interesting
celestial objects not visible to the naked eye in relation to bright stars and the horizon that are easy to
see. The bright stars serve as guideposts to all the objects in the sky. For more information on using
star charts see How to Find Sky Objects in "All About Telescopes".
It is important to be aware of these considerations in selecting a finder scope. The field of
view should be wide enough to see the bright stars of the monthly star chart without too much hunting
around. As a general rule a 4 to 6 degree field is about the best range although a wider field is often
desirable and preferred. The diameter of the finder scope lens is also important. A good general
rule is that the finder diameter should be about ¼th the diameter of the main telescope. This would
be about 35mm diameter for a 6 inch telescope and about 5Omm for an 8 inch telescope. The light
th
gathering power will then be about 1/16 that of the main telescope. Finders too small in diameter
often have problems showing fainter objects that are desired to be viewed in the main telescope.
The magnification should be at least 2½ power per inch of finder aperture and as a general
rule be about 5 power per inch of aperture. Since magnification spreads the light over a larger area
making the image less bright, it is best to limit magnification especially if the finder is on the small
side for your telescope. Increasing magnification also reduces the field of view which takes away the
purpose of the finder. A star diagonal is often important if the finder is mounted 45 degrees or more
from the focusing mount to allow rotating the eyepiece to a comfortable position. All finders used on
Newtonian telescopes are easier to use with a star diagonal to position the eyepiece so you don't
have to turn your head toward the sky when switching from finder to main telescope.
A guide telescope is usually used for photography and its best known use is to guide the main
telescope during long exposures. The camera is attached to the main telescope while it is guided
and corrected as required by the view through the guide telescope crosshairs. The aperture of the
guide telescope to be used depends on the magnification to be used and the brightness of the guide
star. Since guide telescopes are often used at 10 to 50 power per inch of aperture enough light
gathering power is needed for easy viewing. But also keep in mind most popular and easiest to
photograph objects in the sky are the brighter ones. Usually the guide telescope is collimated to the
main telescope. If the object being photographed is quite faint or is so extended in angular size as to
make it a poor guide object to guide the main telescope with, then a nearby bright star can be
selected. The guide telescope is then offset so the crosshairs follow the guide star while the main
telescope follows the object being photographed.
While most guide telescopes are used with a crosshair eyepiece, several added features are
available to make using the guide telescope easier. An illuminated crosshair eyepiece will make the
crosshairs more visible especially against the dark sky. Unilluminated crosshairs against a truly dark
sky often will be faint or invisible until a line of the crosshair happens to cross a bright object. For
guiding it is easy for the star to slip from the crosshair. Then it is difficult to put the star back on the
crosshair unless you can see them to know which direction to turn the telescope. Crosshairs are
usually illuminated by battery through a brightness control. The control allows adjustment of
brightness to suit the observing conditions and the guide object.
46
Newtonian Notes
Double crosshair eyepieces are similar to regular crosshair eyepieces except there are two
sets of closely parallel lines with each set at a 90 degree angle to the other. This forms four
crosshair intersections each to be used the same an a regular crosshair. Or the double crosshair
forms a central square which can be centred on the guide star. Some astro-photographers prefer a
double crosshair or a crosshair with a central circle to guide the telescope. The guide star can be set
slightly out of focus so the image expands to almost fill the central square or circle. Slight errors in
tracking are then easily observed as the star image touches the reticle pattern. In addition, if one
crosshair is set parallel to the declination axis motion of the telescope and the other crosshair set
parallel to the polar axis motion, then the star image will also indicate what correction is needed in
the telescope tracking. Setting the crosshair pattern is just a matter of rotating the eyepiece. For
double crosshairs you can start with the guide star at the edge of the field within the parallel lines.
With the telescope drive off, the star should drift across the field staying between the two lines.
Alternatively, the telescope can be rotated by hand on the polar axis while the declination axis is
locked. Rotate the eyepiece until the star passes between the parallel lines from one edge of the
field to the other. The star image should be expanded with magnification using Barlow lenses rather
than expanding the star image too much by setting the image out of focus.
A guide telescope is useful for photography even though long exposures are not used. With a
camera set up to the main telescope the sky can be seen through the viewfinder. However the field
of view is often very narrow. One exception is prime focus photography which uses no
magnification. Since most objects are photographed with some magnification and sometimes high
magnification (planets for example) the field of view is narrow. It is easy to loose the object from the
narrow camera view. A finder works well in locating objects if the main telescope has a low
magnification wide field eyepiece in place. A camera viewing system often has a field of view
smaller than an eyepiece. It can be difficult to relocate the object using a regular finder. But so is
removing the camera, inserting the eyepiece to locate the object, then replace the camera. A guide
telescope can be used at low power as a high magnification finder for the main telescope with a
camera. At about 20 power the guide telescope has a field of view wide enough to find objects for
the camera. But this is too narrow for use as a regular finder.
47
Newtonian Notes
Positioning And Mounting Ring Mounts And Similar Accessories
In positioning finders and guide telescopes it is important to consider the position relative to
the focusing mount of the main telescope. Since the main and accessory telescopes are used
together they should be fairly close to each other. If the finder or guide telescope is positioned too
far down the tube it becomes difficult to shift observer position from one to the other eyepiece.
Locating the accessory telescope too far around the circumference of the tube can cause problems.
While you can rotate the main tube for good viewing, this may also put the accessory telescope in a
difficult to view position. However this problem can be greatly reduced by using a star diagonal on
the accessory telescope. A star diagonal can be rotated to change the viewing position of the
eyepiece. Most guide telescopes are supplied with star diagonals to increase viewing ease but
finders usually must be selected and purchased with a star diagonal if desired because often it is
difficult to add one if the finder is not designed for it. The usefulness of a rotating star diagonal will
depend on how far around the circumference of the tube the accessory telescope is mounted from
the main telescope focusing mount.
If the upper end of the accessory telescope is mounted past the focusing mount of the main
telescope and mounted too close together it can be a problem reaching the focusing mount knobs
over the accessory telescope. Generally the accessory telescope will be at least 45 degrees around
the circumference of the main tube to avoid this problem. The spacing between the focusing mount
of the main telescope and the accessory telescope will depend on the height of the accessory
telescope. Guide telescopes are usually used with star diagonals making it possible to mount them
close to the main tube. Finders without star diagonals must use ring mounts high enough to allow
your head to be positioned along side the main telescope to permit your eye to look into the finder
eyepiece. This is not a problem with commercial finder ring mounts but can be with homemade
designs. A star diagonal on a finder is not necessary but it will make finder use more comfortable. It
is best to set up the telescope and hold the accessory telescope in position by hand to try out spacing
before holes are drilled in the tube. Test the position while the telescope is pointed near the East,
South, and West horizons.
Two common problems in mounting accessory telescopes are changes in position of the
accessory telescope after adjustment and damage to the painted finish of the accessory telescope by
the adjusting screws of the ring mounts. Both problems are related to each other. Adjustment screw
tips will dig into the accessory telescope to grip it firmly enough to prevent changes in position after
adjustment. The heavier the accessory telescope the more firmly the grip must be. On smaller
accessory telescopes and ring mounts a narrow strip of springy metal is sometimes used to reduce
damage by the adjustment screws. This can reduce the grip so can cause problems on larger units.
Some smaller finders have U grooves machined into the tube wall for a positive, firm grip but this
requires a heavy tube wall and this adds weight and cost.
Several other solutions are easily used on most accessory telescopes. First: position the
mounting rings not more than an inch or two from the focusing mount base and lens cell on refractor
types. Most refractors have thin tube walls which often are difficult to hold and sometimes dented in
by adjustment screws too far from internal support. Guide telescopes larger than 4 inches are
usually reflectors although reflectors down to 3 inch aperture are often used. On reflector guide
telescopes the ring mounts should be positioned within several inches of the spider and mirror mount
unless the wall thickness and stiffness of the tube material are enough to support properly anywhere
on the tube.
Second: assuring good gripping of the ring mount adjustment screws while protecting the
surface takes a little extra work. Metal tape made of aluminium or stainless steel coated with an
adhesive is available in large hardware stores and auto supply stores. If not available locally some
mail order hardware and auto supply firms carry metal tape. Aluminium tape is more readily
available locally. Stainless steel tape available from U.S. General Supply Corp., 100 General Place,
Jericho, NY 11753;Brookstone Co., Brookstone Bldg., Peterborough, NH 03458;Warshawsky & Co.,
1900-24 S. State St., Box 8440A, Chicago, IL 60680. The most common width is 2 inches so you
can cut two or three narrower strips to wrap around the accessory telescope at the position of the ring
mounts. The tips of the ring mount adjustment screws seat against the metal tape which protects the
surface of the accessory telescope. The adhesive of the metal tape firmly grips the tube. Since the
metal tape is quite thin, several layers should be used. The strips should be at least 5/8 inch wide
48
Newtonian Notes
and perhaps wider if the rings are wider. Metal tape with a paper backing covering the adhesive is
easier to handle but not all brands come with paper backing.
Once you know where to position the ring mounts the holes are laid out for drilling. In most
cases it is best to layout a centre line on the tube. If the accessory telescope is not parallel to the
optic axis of the main telescope the error can be adjusted out with the adjustment screws. But in
some cases the adjustment range is not enough and in any case it is a waste of adjustment range.
The longer the accessory telescope is the more important parallel layout is. A simple way of laying
out a long parallel line on a tube is to lay the tube on a flat floor or table or if necessary on two
boards. Use wood blocks or books to keep the tube from rolling and use a ruler, or better, a square
to measure up from the floor, table, or boards about ½ the tube diameter. Rotate the tube to the
centre line location desired. Repeat some distance down the tube using the same measurement as
before but do not rotate the tube. After marking a straight edge can be used to mark a centre line.
Now mark the centre line of the ring mount base with chalk, a corner of masking tape, or
adhesive tape. If the mounting screw holes are drilled through it is sometimes possible to hold the
ring centred on the tube as a pencil is inserted in the hole to mark the tube. If the pencil is too short,
try a scriber or awl. You can also use a sheet of carbon paper facing the tube with the ring positioned
on top. A round toothpick inserted into the mounting screw hole can be wiggled around to mark the
tube. If the ring mount does not have mounting screw holes drilled through then center the ring
mount on the centre line. Without lifting the ring mount, tilt it at an angle toward the centre line far
enough so you can see the mounting hole. Insert a pencil between the ring mount base and the tube
then mark the tube just under the screw holes.
After marking the tube the holes should be drilled with a drill bit about 1/16 inch larger than the
mounting screw diameter. Center punch the screw location to make starting the drill easier. Be
careful not to let the drill bit drift away from the right spot. A sharp drill bit is important and moderate
pressure on the bit is recommended.
Additional references. All About Telescopes by Brown, Edmund Scientific 1967
Some New Illuminated Finders In Sky & Telescope Magazine March 1975 pages 183-7
The Finder scope by Mullaney in Astronomy Magazine August 1975, pages 30-33
Using the Main Telescope as a Guide Telescope
Using a square to lay out long lines on the tube parallel to
the tubes axis. The tube is kept from rolling with small
blocks. Any tube roll will cause errors. The square and the
tube are set on blocks if the floor is not flat. This method of
laying out long center lines is recommended for guide
telescopes and large piggyback cameras.
Locating the finder so the focuser can be
easily reached. Here a pair of shorter
ring mounts would make it easier to
reach the focuser knobs over the finder.
The knobs usually must be reached from
both sides of the tube.
49
Newtonian Notes
Several methods of photography use the main telescope as a guide telescope. Each has
advantages and disadvantages so we will look at each system to understand what to expect before
you spend time and money. Then select the system for your needs.
PIGGY-BACK 35mm CAMERA: One of the simple yet rewarding amateur photography
systems is to attach your camera, preferably a 35mm single lens reflex, to the tube of the main
telescope. By pointing the camera in the same direction as the telescope a picture is taken while the
main telescope is used to guide the camera during exposure. Interchanging lenses in the camera will
give the object size and field of view desired. This method is best for wide fields of view, more to
photograph entire constellations to larger deep sky objects rather than small fields of view needed
the moon and planets. Most cameras used for general photography have a small light gathering
power since they are designed for daylight to room light use. Using a long focal length lens makes
the image larger but the exposure is quite long
You can build a large camera using military surplus camera lenses of large aperture. The
lenses usually must be remounted in a light-tight box with the lens at the upper end and a plate film
holder at the bottom. Some complete cameras are available but the weight can be excessive. A
number of firms handle military surplus cameras. The largest selection is available from C & H
Sales, 2174 E. Colorado St. Pasadena, CA 91107. The completed camera can be piggy-back
mounted to the main telescope for guiding during exposure or the camera can be mounted in the
cradle in place of the main telescope and a guide telescope attached to the camera for guiding.
Without the single lens reflex feature it is more difficult to align the camera and guide telescope.
These cameras have good light gathering power and usually a wide field of view.
OFFSET GUIDER: Another method of using the main telescope as a guide telescope is with
an offset guider. The main telescope is used to collect the light for the camera attached to it but
some of the light is diverted to a guiding eyepiece. Two types are available; one that uses a partial
transmitting mirror to reflect 10 to 50 percent of the light beam to the guiding eyepiece while the
remainder of the light goes to the film and the other type uses a very small prism or mirror to divert
most of a small part of the light beam. Ideally the amount of light diverted is just enough to do the
guiding so as much light as possible reaches the film. A larger telescope may need only 5 or 10
percent of the light beam for guiding but on a small telescope considerably more may be needed.
The problem is that on small telescopes the film needs all the light you can get so diverting any light
for guiding means increased exposure time or reduced magnification. The offset guider has the
advantage of allowing you to see the same view as the camera if the focal planes are adjusted
exactly. They are simple to use by inserting between the camera and the focusing mount if the
focusing mount has enough "in" travel. Many existing telescopes do not have this provision.
Commercial offset guiders are available in both types. Consult the ads in the astronomy
magazines for suppliers. Some are made only for specific telescopes to match the light diverted to
the aperture size and other technical reasons. One variation not commercially available uses a
dichoric mirror-filter instead of the diverting mirror and the colored filters used most often in planetary
photography. Dichoric mirror-filters have a special coating to reflect some colours and transmit other
colours. By replacing the regular mirror in the offset guider and by selecting a dichoric mirror filter to
transmit the colours you want to photograph in, the light reflected to the guiding eyepiece will be in
the colours normally wasted by regular colour filters. Regular colour filters absorb as much as 95
percent of the light to allow photographs in a very narrow colour band. A suitable dichoric filter would
transmit a wider band which could be narrowed further with regular colour filters as desired. Good
quality dichoric mirror-filters are fairly expensive and would require changing to get separations in
red, green, and blue light. One dichoric filter would let you photograph in red light while guiding in
blue light. Another dichoric filter must be used to photograph in blue light while guiding in red light.
A simple and inexpensive method of using the main telescope as a guide telescope is by using
two focusing mounts on the main telescope. The main focusing mount carries the camera and uses
the main diagonal mirror. A second focuser is used for guiding and is also attached to the main
telescope. It usually uses a small mirror or prism attached to a single vane diagonal holder. The
diagonal size to use depends on the size of the telescope, what small sizes of small diagonals or
mirrors are available, and on trial and error experience. They generally run 3/8 to 3/4 inch minor axis
size. By drilling and tapping the mounting screws on the focusing mount the entire unit can be
removed during visual use. One variation is to use a small mirror or diagonal cemented to the large
diagonal holder. But this adds diffraction during visual use. Another variation is to use the small
mirror or diagonal attached to a clip or clamp to be attached to the main diagonal holder or to the
spider vane. In any case the diagonal must be larger than in an offset guider because the mirror or
prism diagonal is much farther from the eyepiece. The extra diagonal and the single vane holder
50
Newtonian Notes
adds diffraction which is often undesirable especially in photographs of bright stars and to get
planetary detail. It is more difficult to get both focusers in focus at the same time. Once both are
focused you can tighten the focusing tension on the guiding focuser so it is not changed accidentally.
The layout of the second focuser must be carefully made to avoid interference with the main
diagonal or spider.
Figure 16, Guiding
PIGGY-BACK SCHMIDT CAMERAS: Schmidt cameras operate much like a rich field
Newtonian telescope if you can think of a film holder in the place of a diagonal. In other words, a
Newtonian telescope uses a diagonal to bring the light beam out of the tube for visual use or for use
with a camera attached to the outside of the tube. The Schmidt camera is made only for
photography and avoids the extra optics with unavoidable light loss by suspending the film holder on
a spider in the middle of the tube. No camera body is used. Film is loaded into the camera by
reaching inside the tube and placing film in the holder on the spider while the upper end of the
camera is covered to prevent exposure. Schmidt cameras are often mounted piggy-back on another
telescope to guide the camera but sometimes the camera replaces a telescope in its cradle and a
guide telescope attached to the camera is used for guiding. Schmidt cameras have wide fields of
view, relatively light weight for their aperture size, exceptional optical correction (close to perfect
correction compared to the coma of most reflectors or compared to the limited colour correction of
refractor lenses including those of cameras), but have a curved focal plane. To help get around this
one problem the film holder is curved to match the curve in the focal plane.
The major problems with Schmidt cameras are the high cost of commercial units, the difficulty
in making optics and curved film holders for do-it-yourselfers, the need to handle raw unexposed film
(cutting to fit the film holder and loading into the camera), usually the need to process your own film,
and the fact that the Schmidt camera can't be used for visual use. (Somehow people accept the fact
that a 35mm camera is good only to take pictures but an exceptionally good telephoto camera like
the Schmidt camera is for some reason considered defect if it can't be used like a telescope.)
Schmidt cameras are available from Celestron International, 2835 Columbia St., Torrance, CA
90503. Beginners are especially uneasy about handling film without a camera body both in the dark
room and at the telescope camera. Some people have the same feeling about handling frogs or
snakes or even mushrooms. It may take some getting used to. Once you get used to processing
your own film it takes little additional skill to handle a Schmidt camera OK.
Most amateur astronomers think of Schmidt cameras and Schmidt telescopes with the
corrector lens over the upper end of the tube. This lens is difficult to make but for Schmidt cameras
is not needed according to one of Bernard Schmidt’s original designs. This modified and simplified
51
Newtonian Notes
design uses only one optical element - a spherical primary-with the upper end of the tube open as it
would be with most reflectors. The upper end of the tube does have an aperture stop where the
corrector lens might be expected. Without the corrector lens but with the aperture stop this lensless
Schmidt camera performs very well. Several lenseless Schmidt cameras have been described in
Sky &: Telescope magazine that were built by amateurs. Many amateurs were interested in making
similar Schmidt cameras but the low f-ratio mirror and the curved film holder are difficult to make.
Coulter Optical Co. now makes a lenseless Schmidt camera kit which includes an 8" f2 mirror,
curved film holder, and film cutting template. This kit with other commonly available telescope parts
can be used to assemble a complete camera at a quite reasonable price, typically under $150 with a
commercial telescope tube, less with a cardboard or plywood tube. Other needed parts are fairly
simple to make yourself.
52
Newtonian Notes
Construction of a Lenseless Schmidt Camera
A number of particular Schmidt camera designs can be used. The following is one design that
uses as many commercial parts as possible. If you want you can substitute other parts as desired. It
does start with Coulters lenseless Schmidt camera kit. Thus a regular 8" Newtonian mirror mount is
used to hold the mirror. An 8" spider is used to support the Coulter curved film holder. Since this
holder takes a 3/8"-16 bolt the spider must accept this size (Telescopics #90038, Novak Research
Quality or simular). To improve the image quality the mirror is stopped down to a small aperture.
The smaller the stop the smaller the light gathering power will be but the better the image quality will
be. The maximum recommended aperture for the 8" f2 is 5 inches. This gives an effective f-ratio of
f3.2 and small well corrected star images over the entire field. Stopping down to 4 inches aperture
gives an effective f4 and improved star images.
The camera must be light tight so a snug fit is required where the stop is attached to the tube.
To make the stops easy to make light tight, use telescope tubing with end rings that wrap around the
tube wall to the tube end (Meade Instruments or similar). Cut the outside diameter of the stop the
same size as the outside diameter of the tubing. Slip the stop in the end ring then slip the end ring
over the end of the tubing. No bolts, glue, or sealant is required. Cut the hole in the center of the
stop to the aperture you want. Since this mounting method is so simple many amateurs prefer to
make both a 4 inch and 5 inch clear aperture stop to at least try both to compare results. The lower
end of the tube is closed to exclude light with another disc without a hole. You have to reach the
collimation screws on the primary mirror mount but since the end ring holding the disc is easy to
remove this is no problem. The stops and the disc can be made of Masonite, cardboard, metal, or
opaque plastic. Note that the stop is mounted at twice the focal length of the mirror. This is the
position necessary for the camera to work properly. Do not try to make the camera more compact by
moving the stop closer to the spider.
Figure 17, Lensless Schmidt camera
The film holder is mounted on the spider so the film surface is at the focal plane which is one
focal length from the mirror. If you make your own mirror the spacing dimensions shown in the
drawing must be changed to match the focal length. The film is focused at the spider by adjusting
the film holder toward or away from the mirror. Allow some adjustment in positioning the spider,
perhaps ±¼ inch or so.
To load film in the camera and to collimate and focus the camera you must reach inside the
tubing. A large hole is cut in the side of the tubing to reach the spider and film holder. You can use
a key hole saw or sabre saw after first making a starting hole to insert the blade. The hole should be
large enough to pass your hand with film comfortably. To keep the camera light tight the hole is
53
Newtonian Notes
covered when not in use. A cloth sleeve made of two layers of black cloth and mounted to the side
of the tube can be used. A door mounted with hinges can be made of metal or an extra piece of tube
material with a size about ¾ inch larger than the hole to allow for overlap. Sliding doors can be used
by making door tracks of layers of strip material (wood, plastic or metal). Some even use cardboard
held in place with masking tape.
Consult Coulters instruction sheet for details of collimation, focusing, and cutting film to fit the
film holder. A simple piece of black cloth draped over the upper end of the tube is used as a shutter.
Load film into the camera, point the camera using the guide telescope, remove the black cloth to
start the exposure, guide the camera with the guide telescope, and recover with the black cloth to
stop the exposure.
The Schmidt camera is without the convenient camera body to hold the film and control the
shutter. But with careful work the results can be the best in photographing wide fields of 4 to 6
degrees with excellent optical correction. Those that have Schmidt cameras have been rewarded
with one of the best astro-photo systems available. Those with a home assembled lenseless Schmidt
camera have this fine performance at a quite low cost, often less than a good 35mm. astro camera
alone. You must, however, overcome the fear of handling naked film without benefit of those little
cassette magazines.
Lenseless Schmidt Camera, Sky & Telescope magazine, October 1959, construction design
Lenseless Schmidt Camera, Sky & Telescope magazine, May 1974, construction design
Schmidt Camera, Amateur Telescope Making, book two, Scientific American, 1965, theory,
construction design, optical making and testing of mirror and corrector.
Schmidt Camera, Amateur Telescope Making-book three, Scientific American 1964, theory,
construction design, optical making and testing of mirror and corrector.
Schmidt Camera, Outer Space Photography by Henry Paul, Amphoto 1967, discussion and photos of schmidt cameras
(details by D-r. Paul are given in ATM book three above)
54
Newtonian Notes
Importance of Balancing
One aspect of telescope performance is not so much related to design as it is to operation of
the completed telescope. This is balancing the telescope so it can move smoothly. Most amateurs
are familiar with the counterweight on the declination axis of the most common German equatorial
telescope mount. The weight is adjusted to counterbalance the tube assembly so with the shaft
clamping bolts free, the polar axis does not turn unless you turn the telescope. It will be free to turn
but does not turn no matter where the telescope is pointed. Similarly the tube assembly is moved in
the tube cradle so with the shaft clamping bolts free, the declination axis does not turn unless you
turn the telescope. In both cases the heavy side will rotate down due to gravity, pulling the light side
higher.
Another type of balancing is not always done-tube balancing of off-axis weight. With a small
focuser- and small finder the off-axis weight is small and need not be balanced. So usually you will
see these telescopes without tube counterweights. However with large finders, guide telescopes,
piggy-back telescopes, large tall focusers, cameras, and other accessories mounted off-axis that
weigh over a pound it may improve performance by balancing the off-axis tube weight with
counterweights. They may not actually be required but without them the telescope will be irregularly
balanced. In other words, the telescope may seem balanced when pointed at one part of the sky and
seem unbalanced when pointed at another part of the sky. If the telescope can't handle the
imbalance it makes it difficult to track objects in the sky, especially during photography. The amount
of power needed to turn the telescope shifts from light to heavy simply by pointing to different parts
of the sky. This is part of the reason tube counterweights are used most often in photo set-ups when
tracking is most important.
Clock drives will run at a constant speed only if the motor is not overloaded. Drive correctors
do little to help imbalance or overload problems. Because the clock drives use motors that are small
and not very powerful it is easy to overload the clock drive. For awhile the motor will run irregularly if
overloaded. Frequent overloading will burn out the motor or the drive corrector and/or blow out the
fuse in the corrector if it has a well selected, sensitive fuse. Most often the drive corrector will be
fused to protect the corrector and not the motor. If your drive motor burns out in less than 5 years it
was probably overloaded. Frequent and heavy motor overloads may shorten motor life down to a
few weeks or months. In most cases you will not be able to tell for sure the motor is overloaded
unless it is so overloaded it won't run.
To avoid tracking and motor burn out problems use a good reference book on telescope
operation such as "ALL ABOUT TELESCOPES" in the sections "How to Use an Equatorial Mount'
and "Balancing a Telescope". There is no good reason why a telescope can not be very well
balanced for on-axis weight. It is difficult to tell how much imbalance the drive motor can handle.
Motor power and the amount of power needed to overcome friction in the telescope vary widely.
Only a very small amount of power is needed to drive most amateur telescopes with the rest of the
power used to overcome imbalancing.
Off-axis weight depends on both the weight that is off-axis times its distance from the center of
the tube. If you have a 2" focuser 4½" tall with the center of gravity 2¼” above the tube, with the
tube being 7" in diameter or 3½" in radius then the center of gravity of the focuser on the tube is 2¼"
plus 3½" = 5¾" from the center of the tube. Multiplied by the weight of the focuser at 1½lbs you get
8 5/8 inch-lbs. of off-axis weight. Will the large focuser overload the drive motor, say of 6 watts?
Probably not by rule of thumb. As a rule of thumb you can allow one inch-lb. of off-axis imbalance
for each watt of motor power to be quite safe in not overloading the motor. Thus a 6 watt motor
could probably take 6 inch-lbs. imbalance with very little risk of overloading. Also by rule of thumb
you can allow two inch-lbs. of off-axis imbalance for each watt of motor power to be fairly safe in not
overloading the motor or 12 inch-lbs. for this example. If on-axis weight is well balanced as it should
be, the two inch-lbs. per watt goes from fairly safe to quite safe. Please note that any small binding
in the telescope shafts or drive mechanism is not considered in this rule of thumb. Cameras
frequently overload the drive, especially in eyepiece projection. The focuser height, camera
adaptors, and extension tubes puts the camera weight very far off-axis.
55
Newtonian Notes
Telescope Design Example
Now let's try a telescope design to be sure you can understand and can follow all the details.
The first step is to decide what you want the telescope to do for you. In this sample case the desired
specifications are more difficult to get in a telescope and nearly impossible to design using most
books on telescope design. In this book the emphasis is on outlining your possible range of choice,
noting practical limitations, then giving specific formulae to get the performance you want.
This telescope is being designed for comets, wide views of the Milky Way, large nebulae and
galaxies. A very wide field of view is important. But this telescope must also be rather small and
lightweight to be very portable. A rich field Newtonian in a fairly small aperture is the natural starting
point. For any given f-ratio, the larger the aperture the smaller the field of view. And the larger the
aperture the longer the tube needs to be and the greater the weight. To design to a specific field of
view you need to know something about your wide field eyepiece. In this case it is a 35mm Erfle with
a 60° apparent field of view. It takes a 1.44" linear image size to give the maximum field of view.
This is either measured on the eyepiece field stop or looked up in "All About Telescopes" in the chart
on page 173. Looking at the Coma Limits chart on page Error: Reference source not found for 4¼"
and 6" aperture, f4 through f6 under image quality grade C (maximum field of good image quality)
the available linear and angular image sizes can be compared. To find the maximum field of view
for this eyepiece and any focal length telescope we can use formula (3); A=3438 L/F. For a 4¼"f5
the focal length is 21.25" so A=244 minutes or just over a 4 degree field of view. A 32mm Erfle with
a 65 degree apparent field would also give a 4 degree real field. Since the 35mm eyepiece gives the
minimum magnification for 4¼" telescopes it will also give the maximum field of view that will fit in
the normal eye (page 15).
The 4¼"f5 has good image quality (grade C) over a 1" linear image size and 162 minute
angular image size. In other words 70% of the linear field used by the eyepiece will be of good
image quality covering a 2 2/3rds degree field of view . Out to a 4 degree field the image has some
noticeable coma on bright objects but is still useful at this very low magnification. Why not a 4¼"f4
instead? Good image quality (grade C) is over a .64" linear image size and 129 minute angular
image size. Using formula (3) again we find the maximum field of view with the same eyepiece is
now a whopping 4.85 degrees! But the good image quality now only covers 44% of the linear field of
the eyepiece for a little over 2 degree angular field. The overall field of view is larger but the image
quality is, less. Now look at 4¼"f6. The good image quality and maximum field of view are now both
3¼ degrees. The maximum field of view is getting smaller, the field with good image quality is
getting larger, and the tube is getting rather long. Good image quality now fills the 35mm eyepiece.
However the large size of good image quality can not be used on shorter focal length eyepieces. A
25mm Ortho can only use the 7/8" center of the 1 3/8" size of good image quality. As a personal
choice I would rather have a shorter tube and wider overall field of view which I am willing to trade
for a smaller image size of good quality since shorter focal length eyepieces will also be used. The
4¼"f6 is a good telescope but not for the things I want. This narrows the choice down to an f4 to f5
telescope. Looking in the same way at the 6" apertures for f4 to f5 you will find the same problems of
the longer tube with smaller overall field of view. Since this telescope is to be small and compact
with a giant field of view the choice is very limited.
Now look at the diagonal sizes required for the 4¼"f4 to 4¼"f5. It will depend on the height of
the focuser. Because of the wide field of view desired it will have to be a 2" focuser. To keep the
light loss obstructed by the diagonal as low as possible the focuser should be a short model. The
image size of 100% illumination is selected to be 3/8". As discussed in the Limits of Performance
this will give at least 75% of full illumination over the entire field of view. For small aperture
telescopes a very large percentage of the focal length is between the diagonal and the eyepiece.
This tends to force the diagonal to be comparatively large. Too large an image size of 100%
illumination and tall focusers will unnecessarily make things worse. Since no Barlow will be used and
prime focus photography will not be done the "in" travel needed is zero. See Selecting and Using
Focusing Mounts. The 2" focuser is 3¼" high with l¼" adaptor. To find the diagonal size use formula
(8) where D = ½ T + H + b = 2½” + 3¼” + 0 = 5¾" and N = MD + IF - ID/F. For 4¼"f4, N = 4¼ x 5¾
+ 3/8 x 17 - 3/8 X 5¾ / 17 = 24.44 + 6.38 - 2.16 = 1.69”. For 4¼”f5, N = 4¼ x 5¾ + 3/8 x 5¾ / 21¼ =
1.42". This is the minimum size diagonal for the parts used to get the 3/8" size of 100% illumination.
As a personal choice I do not want to use a diagonal larger than 1.52" minor axis size in order to
keep light loss down. A 1.52" diagonal does not seem large but in a 4¼" telescope it is proportional
56
Newtonian Notes
to using a 3.50" diagonal in a 10" telescope. This eliminates the 4¼"f4 because it requires a larger
diagonal. I do not want to trade the larger light loss for the larger overall field of view, especially
since the field of good image quality in the f4 is smaller than in the f5. The f5 will give a larger field
of good image quality with more light reaching the eyepiece because of the smaller diagonal, while
the tube length is 4¼" longer. If a 2" focuser of regular height were used (only l¼" taller) the
minimum diagonal size for the 4¼"f5 would be 1.65" and for the 4¼"f4 it would be a very large 1.97".
Focuser height is not as critical for many telescopes. Rich field telescopes especially those of small
aperture are most sensitive of focuser height.
Now referring to page 59 for telescope layout we find most dimensions are a matter of choice
or simple measurement. A = ½"; B = 2 3/8"; D = 3"; E = 1"; H = 3¼"; "in" = 0";and T = 5". The
measured focal length of the primary mirror turns out to be 21½". Please note that the exact, real
focal length has some variation from one mirror to another. Just because a 4¼" mirror is supposed
to be f5 and the f-ratio multiplied by the mirror diameter is supposed to be the focal length that does
not mean the focal length is exactly 21¼". In laying out the telescope you should use the measured
focal length of the mirror. A 3% tolerance in focal length can cause several inches of variation which
becomes layout error unless you use the real focal length. Layout errors of more than ¼ inch can be
serious. To find distance C we must take the focal length, 21½" and subtract "in", H, and ½T or 21½"
- 0 - 3¼" -2½” = 15¾". The tube length required is the sum of B + C + D + E = 2 3/8 + 15¾" + 3 + 1
= 22 1/8". If this tube length is a bit too long you may choose to select a 4¼" f4 instead which would
reduce the tube length by about 4".
For a lower cost telescope you can use a 1¼" focuser. Commercial 4¼” rich field telescopes
are available ready-made with l¼" focusers but the field of view is limited to about 3 degrees. For
the 4¼" f5 designed here with a 4 degree field the area of sky is about 80% larger so much more sky
is visible using a 2" focuser and wide field eyepiece. For a 4¼" f4 designed here with a 4.85 degree
field the area of sky is about 160% larger, with the same image quality as the ready-made telescope.
57
Newtonian Notes
Putting A Telescope Together and Adjusting
This chapter covers assembling the optics (primary mirror and diagonal) and the optical
supports (mirror mount, spider, diagonal holder, and focusing mount) in the telescope tube to form
the telescope tube assembly. It is intended for amateurs assembling a complete telescope from
parts. It can also be used by amateurs wanting to change a ready-made telescope. You may want to
change your telescope to give more "in" travel for prime focus photography, larger image size of
100% illumination, wider fields, large diameter or long focal length eyepieces, etc. If you want to
replace or relocate the mirror mount, primary mirror, diagonal holder, diagonal mirror, spider, or
focusing mount then read this chapter to see which sections apply to the part being changed.
Changing one part can affect several other parts in position and sometimes size.
MIRROR MOUNT: Start putting the telescope tube assembly together by mounting the mirror
mount to the tube. Most mirror mounts have springs. At this point the springs should be fully
compressed. During collimation these springs will be released not more than ¼ inch and preferably
not more than 1/8 inch. Releasing these springs too much will cause the mirror position to shift from
side to side as the telescope is pointed to different parts of the sky. Set the mirror mount on a table
with the collimation adjustment screws pointed toward the table. Using a ruler, measure the distance
from the table to the center of the mounting screws. Call this distance A. Now unscrew the mounting
screws about half way and insert the mirror mount into the tube. This is best done with the tubing
standing on end. The mirror mount will slip into the tube until it rests on the mounting screws. Mark
the location of each mounting screw on the tube. Next, remove the mirror mount and extend the
lines along the tube by distance A using a square on the end of the tube. Drill holes at these points
using a drill about 1/16 inch larger than the screw size. Sometimes shims must be placed between
the mirror mount and the inside of the tube. A stack of steel washers from the hardware store make
good shims that are easy to use.
Now mount the primary mirror in the mirror cell. Be careful to follow the instructions supplied
with the mirror mount for specific details related to the design of the cell. The back of the mirror
should be firmly supported to keep the mirror in the same position even though the weight of the
mirror shifts position during normal telescope use. Next, the edge supports should be adjusted to
hold the mirror edge firmly. Reasonable pressure should not strain the mirror. If the mirror is not
firmly held it will shift from side to side during telescope use. The mirror clips are adjusted so they
rest against the mirror face. Pressure from the mirror clip must be very light. It is very easy for the
mirror clip to cause strain and optical distortion in the mirror. Mirror clips hold the mirror in place only
during the times the telescope is pointed below the horizon - usually only In storage and handling.
Many experienced amateurs adjust the mirror clips with a sheet of paper between the mirror clip and
the mirror. After adjustment the paper is removed to leave a very small gap.
Set the mirror mount on the table with the mirror facing up. Measure from the table to the face
of the mirror. Call this distance B. Mark distance B on the tubing by measuring from the end of the
tube. Put away the mirror mounted in the mirror mount while the other holes are being located and
drilled.
SEPARATION DISTANCE: Next, the distance from the primary mirror to the center of the
diagonal is figured out. This is also the distance from the primary mirror (marked on the tube as
distance B) to the center of the hole for the focusing mount. We will call this distance C but it is also
called the separation distance. To find separation distance C start with the focal length of the
primary mirror. Subtract ½ the tubing diameter. Then subtract the height of the focusing mount.
Then subtract the "in" travel you want for the focusing mount. Be careful. Read the chapter on
focusing mounts to be sure you get the "in" travel you want and need. Also cheek the focal length of
the primary mirror. A 6"f4 will have a focal length usually between 23 to 25 inches. An 8"f7 could
have a focal length between 54 to 58 inches. The f-ratio number is not usually exact. Measuring the
focal length is more exact to make sure the focusing mount travel is used for the "in" travel and the
"out" travel you want. The focal length minus the three subtractions is the C distance.
58
Newtonian Notes
Figure 18, Assembly
Next, the distance from the center of the diagonal to the centre line of the spider mounting
bolts is figured. Simply measure the distance shown in the drawing then add ½ the minor axis size of
the diagonal. Call this distance D. This distance depends on how much adjustment you allow
between the spider and the diagonal. The smaller this distance is the greater the stability and
vibration resistance will be. Some diagonal holders are designed to be rather bulky in this dimension
so you can't adjust to keep the distance as small as possible. With careful layout you can keep the
adjustment distance for moving the diagonal toward or away from the primary mirror rather small
(about ±1/8 inch). Before you measure the distance shown in the drawing you should adjust the
diagonal holder so the adjustment space is in place. Add distance D to distance C marked on the
tube. To mount the spider the line must be extended around the circumference of the tube. A strip
of paper wrapped around the tube can be used to extend the mark with very accurate results. The
strip should be wider than the diameter of the tubing and longer than the circumference of the tube.
It is also important to have the edge of the paper straight. Wrap the paper around the tube and line
up the edge of the strip at the overlap and slip the paper up to the mark. Extend the mark around the
tube. Next, mark the strip of paper at the edge of the overlap to find the circumference. Fold the
paper in half to find half the circumference. Fold again in half and the circumference will be divided
into quarters (which is suitable for most spiders since they are usually with four vanes). Use a pencil
to mark the folds and rewrap around the tube. Extend the pencil marks on the paper strip to cross
mark the circumference line. Now you know where to drill the spider mounting holes.
The focusing mount is placed at distance C. The large center hole is made first to allow the
focusing tube to pass through. For many amateurs this is the most troublesome part of assembling a
telescope from parts. It need not be because there are many ways of making the large hole required.
With power tools a hole saw is commonly used to cut large holes, most often with a drill press but
can also be used in a large electric drill. Hole saws look like a short length of tubing with saw teeth
on one end and a drill shank on the other. A regular drill bit is in the center to act as a pilot in guiding
the saw. Hole saws can be divided into two groups by quality. The more expensive models are
made for durability in cutting wood, steel, aluminium, fibreglass, etc. The less expensive are
commonly available in hardware ;stores but are best limited to cutting wood, fibreglass, cardboard,
59
Newtonian Notes
and phenolic tubing although with some care and low speeds aluminium can be cut. Hole saws have
been driven by hand with a brace(normally used to drive auger bits). Simular to hole saws are
several types of fly cutters which also use a small center bit but have only one, two, or three cutting
teeth. Flycutters are not recommended because they easily "grab" when cutting holes through the
curved surface of the tube wall.
If you have or can borrow a sabre saw it can be used to cut large holes in tubing of any
material. For all materials except wood it is best to use a narrow, fine tooth metal cutting blade. A
starting hole must be drilled large enough to let the blade pass through to start the cut. Cover the
tubing with masking tape to reduce blade chipping and scratches from the bottom of the sabre saw.
Use slow speed if possible.
Several choices are available for using hand tools only. A chassis punch is most commonly
used in electronics to make large holes in metal panels but they are used in other fields. They work
with a center hole to allow a bolt to pass through with a punch on one side of the material and a die
on the other. Tightening the bolt drives the punch and the die together - cutting the material in the
process with a neat, uniform hole. Chassis punches are fairly expensive for making just one hole.
Some astronomy clubs buy one for the use of their members. For long life chassis punches are rated
for 1/16 inch thick aluminium and 3/32 inch thick phenolic or fibreglass. But for telescope building
they are rarely used enough to wear out so are often used on 3/32 inch thick aluminium or 3/16 inch
thick phenolic, fibreglass, or cardboard tubing.
There are two types of hand saws available to cut focusing mount holes that cost only a couple
dollars. One is a keyhole saw. It requires a starting hole to allow the cutting tip to pass through. It is
not recommended for metal. It is a little more difficult to cut focusing mount holes because only the
narrow lip can be used for cutting small diameters. The other saw is called a coping saw. It also
requires a starting hole. The blade is inserted through the starting hole then attached to the saw
frame. Very narrow blades, are available to cut most materials but a fine tooth metal cutting blade is
recommended for all materials except wood. The only problem is the saw frame must reach around
the end of the tube. This means the center of the focusing mount hole must be 6 inches or less from
the end of the tube for most coping saws. Some saws have a special deep throat and some have a
shallow throat so it is possible to be limited to more or less distance. With the correct blade and a
little careful work the results can be quite professional.
For most 1¼ inch focusing mounts make a 1½ inch hole. Some 1¼ inch focusing mounts
have a 1½ inch outside diameter focusing tube to allow increased field of view. For this size make a
1 5/8 or 1 3/4 inch hole. For 2 inch focusing mounts the hole is about 2½ inches. When the large
focusing mount hole is completed, set the focusing mount on the tube racked "in" all the way. Use a
pencil to mark the location of one of the mounting holes. Drill one hole, insert the screw to hold the
focuser in place, then drill the remaining holes. At this point you can locate and mount the
accessories. Then mount the mirror mount with mirror and the spider and diagonal holder with
diagonal.
Now the telescope is collimated. A number of references describe collimating so if you have
trouble with one reference, try another. Start collimating with nothing in the focusing mount. Simply
look in the open eyepiece holder. In the center will be a reflection of your eye from the diagonal.
Around the image of the diagonal is light from the primary mirror. In most cases collimation is
performed while the telescope is pointed at a well illuminated wall or ceiling so you should see a lot
of light. Note the mirror clips. Around the reflection of the primary mirror is the real edge of the
diagonal. With your eye close to the eyepiece holder it is hard to tell the real edge of the diagonal
from the light off the primary mirror. Back your eye away from the focusing mount about 6 inches to
see the edge of the diagonal flooded with light from the primary. Since both the inside of the tubing
wall and the inside of the focusing tube are usually both black it is hard to tell where one ends and
the other begins. It doesn't matter anyhow. What you do see is a black ring around the primary
mirror reflection.
Good collimation requires a collimation device or sighting ring to be used in the eyepiece
holder. You can buy them or make your own using aluminium foil. Wrap the foil over the eyepiece
holder and use a toothpick to make a 1/16 inch to 1/8 inch hole in the center. For starting coarse
collimation it is sometimes better for beginners to not use the sighting ring. It is easier to see and
understand what is happening. It also helps to put a small tab of masking tape on the diagonal
holder at the edge of the diagonal closest to the focusing mount. The loose, curled end of the tape
will show up on the image of the diagonal. This will help you to recognise what you are looking at.
60
Newtonian Notes
Adjust the diagonal first. Look at the real edge of the diagonal and the image of the primary
mirror only and try to ignore everything else. If the diagonal needs rotation, loosen the screws and/or
nuts that lock rotation, then turn the diagonal holder by hand and re-tighten. If the diagonal needs
angle adjustment you change the three or four collimation screws.
Now check to see if the diagonal is the correct distance from the primary mirror. Look at the
real edge of the diagonal and the dark circle around it which is the inside wall of the tubing
surrounded by the inside of the focusing tube. If both are black they will blend together, making it
difficult to tell one dark circle from the other. The important thing to look for is a dark circle around
the real edge of the diagonal. The dark circle should be equally wide all around the edge of the
diagonal so it looks like a "bullseye". This should be checked with the sight ring in the eyepiece
holder and checked with the focusing mount fully racked "in" and again with the focusing mount fully
racked "Out". The width of the dark circle will change when the focusing mount is racked "in" or "out"
but in both positions the width of the dark circle should be uniform around the bright diagonal. If the
dark circle is uniform with the focusing mount racked all the way in one direction but irregular with the
focusing mount racked all the way in the other direction, then you have a combined separation and
angle adjustment error. This error is fairly common if the diagonal adjustment is not checked with the
focusing mount in both positions of fully racked "in" and "out". it can be observed in telescope use by
getting a sharp focus at one edge of the field while the opposite edge is slightly out of focus. It is
easiest to observe in prime focus photography when you have a large print to look at. The problem
is that if the separation distance from primary to secondary mirror is wrong it can be made to look OK
by setting the collimation angle wrong if you use only one focusing mount position. If you find a
combined separation and angle adjustment error, readjust the angle and recheck with the focusing
mount both racked “in” and “out". The dark circle around the real edge of the diagonal will not be
uniform in width in either position. But try to adjust the angle so the narrow width is on the same side
in both focusing mount positions. Now readjust the axial adjustment screws and/or nuts so the real
edge of the diagonal is surrounded by a dark circle of uniform width. Recheck with the focusing
mount racked all the way in the other direction. If the width of the dark circle a-round the real edge of
the diagonal is uniform the adjustments of the diagonal, axial, rotation, and collimation are now
complete.
But at this point if you look at the image of the diagonal mirror compared to the image of the
primary mirror you will see they are not centred. Please note there is a difference between the real
edge and the image of the diagonal mirror. With the sight ring removed the image of the diagonal
mirror is the small circle with the spider vanes up to it and your eye about in the middle. In other
words, if you put your eye in the center of the diagonal mirror reflection it is not in the center of the
primary mirror reflection. This is what it looks like when the diagonal is all adjusted but the primary
mirror is not adjusted.
To adjust the primary mirror, look at the image of the primary mirror and the image of diagonal
mirror only and try to ignore all the other circles. For coarse primary mirror collimation don't use the
sight ring unless you are comfortable in telling the image of the diagonal mirror from the other circles.
With or without the sight ring the image of the diagonal mirror has the spider vanes up to its edge.
The primary mirror has the mirror clips which show up on the edge of the primary mirror image. Most
mirror mounts are placed in the tube so one primary collimation screw is on the top side of the tube
(the same side as the focusing mount is placed). The remaining two primary collimation screws are
called "right" and "left". Looking into the focusing mount you see the image of the primary mirror and
the image of the diagonal mirror. Now look for the curled edge of masking tape placed on the
diagonal holder. The masking tape tells you which side is the "top" side when you look through the
telescope. You can tell the "top" side of the telescope from the outside. So now you can look
through the telescope to tell what screw should be turned from the outside.
Look at the image of the diagonal. You can see which direction you want to move the image
of the diagonal to get it centred in the image of the primary mirror. Simply loosen the screw on the
side you want the image to move toward. If the image of the diagonal must be moved toward the
"top" side - loosen the top screw by turning counter-clockwise. During fine collimation use the sight
ring and turn the screws only a bit at a time. It is easy to over adjust and pass up the correct
position.
The last step is to check the optics for strain. Usually this is done using the telescope on a
bright star. Planets and the moon can't be used. The easiest methods use a Ronchi grating or
Coulter Optical Co’s Performance Test Kit (which also includes a sight ring). Both work the same
way. Rack the focusing mount "in" all the way with grating in the eyepiece holder. Then rack "out"
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Newtonian Notes
until four dark bands appear. A good mirror will show the four dark bands as straight parallel lines.
Strain will be seen as astigmatism where the bands are not straight and not all bands are the same
shape. Another method of testing for strain is to use an eyepiece and look at the diffraction rings
around the star. Details and drawings are given in the October 1976 issue of Astronomy magazine,
pages 27-29. However the diffraction rings are more difficult for beginners to judge. Test for strain
after the optics are mounted and again during temperature extremes, especially cold weather. Once
you know what to look for, the test for strain is simple to perform, taking only a minute or two. If you
find strain check the primary mirror mount and the diagonal holder to see that they are not too tight.
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Newtonian Notes
Figure 19, Collimation
Figure 20, Mirror mount set on the end of
the tube to mark mounting holes.
Figure 22, Hole
saw
Figure 21, Extending a line around the
circumference of the tube using a paper strip.
Figure 23,
Key hole saw
Figure 24, Cutting a focuser hole
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Newtonian Notes
Balancing the Tube of a Reflecting Telescope and Balancing a German Equatorial Mounting both by Robert E. Cox in
Telescopes-How to Make Them and Use Them,Macmillian Publishing co. 1966.
Making Your Own Telescope by Thompson, Sky Publishing Corp. 1973. Covers assembly of a telescope and adjusting.
All About Telescopes by Brown, Edmund Scientific 1967. Assembly of a telescope, adjusting.
Figure 25, Rectangular open tube with
diagonal braces.
Figure 26, Serrurier truss open tube.
Older style Rectangular open tube requires diagonal bracing for good stiffness. Maximum
stiffness is when diagonals are set at 45 degree angle to the rings and parallel tubes. A 60 degree
angle can be used to reduce the number of rings and diagonal braces.
The modern Serrurier Truss uses only two end rings and two cradle rings, usually coupled
together with a metal band or parallel tubes. Eight tubes couple the cradle ring to the end ring. Tube
ends are mounted in pairs with each pair 90 degrees apart around the rings. The small tubes form
triangles to produce maximum stiffness with minimum weight and parts. Sometimes the open
structural tube encloses a thin closed, non-structural tube to exclude stray light and tube currents
from outside the tube. These sources include the telescope mount, the observer, and the framework
itself. in large research telescopes the framework is temperature controlled to avoid this problem.
Near Full Moon, prime focus photography using High Contrast Copy film and yellow filter to bring out
details in rays from the craters. The diagonal is slightly tilted or the focal plane of the camera is tilted
so the entire field is not all in focus at once. Left: Top has good focus, bottom slightly out of focus.
Right: Top is slightly out of focus, bottom has good focus.
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Newtonian Notes
Near First Quarter, projection photography using Tri-X film. The diagonal is slightly tilted or the
camera is tilted so the entire field is not all in focus at once. Left: Low magnification puts a large
image over more of the film so the out of focus region at the top and bottom can be more easily
seen. Right: Higher magnification puts center of the image enlarged over the film so out of focus
region is less visible.
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