Telescopes for CCD Imaging

Practical Considerations
in Choosing and Using
Telescopes for CCD
Imaging
Introduction
„ This talk discusses the characteristics of different kinds
of telescopes when applied to CCD imaging. It includes
a discussion of the pros and cons of different designs in
imaging applications.
„ This is an introductory level discussion. It doesn’t delve
into optical theory, but focus on practical considerations
of using the kinds of scopes available to amateurs.
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Five Factors to Consider in evaluating
telescope used for CCD Imaging
1. Focal Length
2. Focal ratio
3. Aberrations
4. Optical Quality and Design
5. Size of Central Obstruction
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Focal Length
„
Determines Image scale and field of view.
„
Degree to which scope is affected by seeing.
„
Guiding accuracy
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CCD Pixel Size
Arc sec/pixel = Pixel size (microns)/focal
length(mm) * 206
This formula provides good approximations in
the focal length ranges typical of amateur
telescopes. It is not accurate at very short
focal lengths (e.g. 50mm).
„ Resolution is maximized, more or less, when
the smallest angular detail covers two pixels.
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Popular Amateur Telescope Types
1.
Refractors
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„
2.
Newtonian Reflectors.
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3.
4.
5.
6.
7.
Achromats
Apochromats
Modified Newtonians
Schmidt-Cassegrains
Maksutov Newtonians
Maksutov Cassegrains
Ritchey-Chretien
Other Cassegrains
„
„
Classical Cassegrains
Dall-Kirkham
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Achromatic Refractors
„ Run the gamut from cheap dept. store
models to well-made, high quality offerings.
„ Because of their small apertures and closed
tubes, they cool down to ambient temperature
relatively quickly. Problems with tube
currents are minimal.
„ The closed tube also means maintenance
issues are minimal.
„ Relatively inexpensive.
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Achromatic Refractors (continued)
The main problems with using achromats for CCD imaging is that
they suffer from chromatic aberration.
„ This occurs when different wavelengths of light reach focus at different
points. As a result, only some of the wavelengths of light are in focus.
Others are out of focus.
„ Chromatic aberration is minimal in good quality achromats that have
focal ratios of f/15 or larger. Even at f/10 chromatic aberration is often
not too objectionable in high quality models. However, these
aberrations are more evident in CCD images than in visual astronomy.
„ Brighter stars in images taken through achromats often have stars that
appear fuzzy or show false color.
8
Omega Centuri Orion ST80 f/5
Achromat–Jim Edlin Image
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Achromatic Refractors (continued)
You can overcome chromatic aberrations and still produce great
CCD images with fast achromats through the use of filters.
„ Filters allow a narrower band pass of light to pass to the CCD
chip. As a result, problems associated with bringing different
wavelengths of light at different points are lessened. Yellow and
green filters can help, especially in monochrome images. Red
filters can produce pleasing results on the right objects.
„ Really narrow band pass filters such as H Alpha filters can
produce results similar to those delivered by APO refractors
when used with the same filter.
10
Orion ST-80 80mm f/5 achromat. M8
with H alpha filter. Jim Edlin image.
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Apochromatic Refractors
„ Apochromats bring all wavelengths of lights to focus
at more or less the same point, by using special glass
elements, one or more of which may contain fluorite.
„ The chromatic aberration which plague achromats is
largely eliminated.
„ There are few competing designs that can match or
exceed their ability to produce such crisp, wide fields
of view, at any price. For this reason, my personal
bias is to try to get the fastest f ratio APO I can get at
a given aperture. This gives the largest FOV.
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Apochromatic Refractors (continued)
„
Apochromats are among the easiest of telescopes
to use for CCD imaging.
1.
2.
3.
4.
5.
„
They are easy to focus.
Seldom affected by tube currents.
Aren’t limited by seeing to the same extent as other
longer focal length designs.
Collimation is seldom a problem.
The relatively short focal lengths of these scopes
makes them relatively easy to guide.
These qualities would make them an ideal scope for
CCD imaging beginners except they are very
expensive.
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Apochromatic Refractors (continued)
„ There are a number of excellent offerings out
there including, but not limited to those from
Astro-physics, Televue, Takshashi, TMB, and
TEC.
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Apochromatic Refractors (continued)
Limitations of Apochromats
ƒ They are pricey
ƒ Some may have insufficient “in focus” to accommodate all the
accessories that you may want to place in the optical train.
ƒ Chromatic aberration is never completely eliminated. Some of it
is typically present at the fringes of images, especially on those
taken through some of the newer wide field CCD chips.
Personally, I don’t find the limited chromatic aberration to be
objectionable.
ƒ 6-7” aperture is a practical upper limit for most people. Larger
aperture scopes are expensive and have such long tubes that
most people prefer competing telescope designs.
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M31 with Tak FSQ 106 4” f/5 APO
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Schmidt-Cassegrains
„ These are probably the most popular type of
amateur telescope.
„ More people begin imaging with SCT’s than
with any other telescope design.
„ They provide relatively large apertures, in a
very compact tube, at relatively affordable
prices.
„ These generally provide slow focal ratios
(f/10 or more) and longer focal lengths
(2000mm +) unless used with focal reducers.
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Advantages of Schmidt-Cassegrains
„ Because they are widely used, myriad accessories have been
developed for SCT’s including those that aid in imaging. This
include zero-image shift focusers, flip mirror systems, focal
reducers, and numerous accessories for coupling CCD cameras
to the scope.
„ SCT’s have more “in-focus” than most other competing designs.
This means that you can add a number of accessories into the
optical path of an SCT. For example, you can add adaptive
optics accessories such as SBIG’s A0-7 along with a filter wheel
and focal reducer and still bring an image to focus. This isn’t the
case with most competing designs.
„ Focal reducers are readily available for SCT’s that allow you to
reduce the focal length by factors of .63, ½, or 1/3 depending on
the reducer. These enhance the versatility of SCT’s by allowing
change the image scale and field of view.
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Disadvantages of SCT’s
„ SCT’s suffers from significant coma and field curvature.
„ SCT’s use a moving primary for focusing. Because of the mechanical
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„
„
„
design, the mirror tends to shift or tilt slightly during focusing. It also
sometimes shifts when you point to different positions in the sky.
SCT’s are difficult to focus. It is difficult to tell when you are in focus. A
separate electronic focuser for close focusing, that does not move the
primary, such as the JMI NGF-S or Optec TCF is highly desirable in
this context. So is software such as Focus max which can
automatically focus an SCT in conjunction with one of these focusers.
SCT’s are highly sensitive to collimation and lose collimation easily.
Mirror shift can contribute to this problem. SCT’s aren’t very easy to
collimate. “Bob’s knobs” can help make collimation easier.
The combination of features described above make the SCT one of the
more difficult telescope designs to use in CCD imaging.
Quality control in SCT manufacture has been a problem in the past with
the result that some of these scopes are significantly better than others.
In recent years, quality control appears to have improved.
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M27 taken with a C-11 at f/6.3
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Newtonian Reflectors
„
Classical Newtonian reflectors are well suited for
CCD imaging.
1.
2.
3.
4.
5.
6.
Newtonian secondary obstructions tend to be smaller
than those of SCT’s resulting in higher contrast.
They tend to hold collimation quite well.
Focusing is fairly easy.
Many are available with fast f ratios in the f/4 – f/6
range.
The primary off axis aberration of Newtonian’s is
coma. This problem increases inversely with focal
ratio.
Diffraction spikes
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Newtonian Reflectors (continued)
„ In order to correct for coma in Newtonian reflectors, a variety of
correctors have been developed over the years.
„ An example of this is a two-element corrector offered in
Takahashi’s MT line of Newtonian’s which significantly reduces
off-axis aberrations. Televue’s Paracorr is another example.
„ Takahashi Epsilon line of astrographs is worthy of note as
instruments especially made for imaging. These utilize special
hyperbolic primary mirrors and a four element field corrector
flattener that results in ultra fast f/3 Newtonian systems. These
are capable of delivering pinpoint star images across a 35 mm
film sized field. This is perhaps the closest thing to a Schmidt
Camera available for amateur CCD imaging. It produces
stunning images that compare favorably with the best offerings
from APO refractors.
22
Brian Lula NGC 6992 Ha/HaLRGB image
with 20” f/5 Homebuilt Newtonian
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Maksutov Newtonians
„ These employ a sharply curved meniscus corrector
that lies inside the field of curvature of the primary.
„ These designs yield reduced off-axis aberrations
compared with a classical Newtonian. They typically
have very small secondary obstructions which
enhances contrast. These are good planetary
scopes.
„ F ratios are typically in the f/5 – f/8 range.
„ While aberrations aren’t eliminated entirely, MakNewts produce sharp images across a wide field that
rival APO refractors of similar aperture and at a much
lower cost.
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Mak-Newt 6” f/6: NGC 7293 – Helix Nebula
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Maksutov-Newtonians (continued)
„ The thick corrector plate and the primary
mirror can take cause the scope to take a
long time to reach thermal equilibrium with
the outside air. Cooling fans can help.
„ Caveat: Make sure the scope you want has
enough “in-focus” before buying. Some MakNewts have a problem in this area.
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Maksutov-Cassegrain
„ Employs a sharply curved meniscus lens to
offset spherical aberration in the primary.
„ Light is brought to a Cassegrain focus.
„ Very Compact design
„ Relatively easy to build
„ Many high quality scopes out there including
offerings from Intes, Intes-Micro, AP, and
TEC.
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Maksutov-Cassegrain continued
„ These tend to have long focal ratios (f/10 – f/15)
except for special versions made for imaging.
„ Secondary obstruction tends to be a little less than
Schmidt-Cassegrains, but larger than those found in
Mak-Newts.
„ There are few shorter focal ratio Mak-Cass’s on the
market because those scopes have large secondary
obstructions that limit their effectiveness in visual
astronomy, especially on the planets.
„ One exception is the Intes MK69 which is a relatively
inexpensive 6: f/6 Mak-Cass. This has a secondary
obstruction of more than 50% which is nonetheless
an excellent deep sky imager.
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Bob Holzer’s M81 with 6”TEC f/12
Mak-Cass imaging at f/8.14
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Ritchey-Chretiens
„ This is the “Cadillac” of scopes when it comes to
„
„
„
„
imaging at longer (1700mm and greater) focal
lengths.
This design virtually eliminates coma which makes
them ideal for imaging applications. You get nice
round stars to the edge of the field
The optical tube is more compact than Newtonians of
like aperture.
F ratios are moderately fast compared with SCT’s.
Open tube minimizes problems with dew.
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Ritchey-Chretiens - Disadvantages
ƒ Field curvature is an issue as stars in the center and
sides come to focus at different points.
ƒ The mirrors in a Ritchey-Chretien are highly aspheric
which makes them expensive to build.
ƒ Secondary obstruction tends to be quite large.
Around 40%. You might not choose this scope if
your primary interest is planetary imaging.
ƒ There is a certain optimum distance that must be
maintained between the secondary and primary
mirrors. If this spacing is allowed to deviate much
more than a few millimeters, significant deterioration
of performance will result due to spherical aberration.
This has implications when you enter new items in
the optical train such as focal reducers.
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Brian Lula M106 image with 20” f/8
RCOS
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Classical Cassegrains
„ These provide long focal lengths in a compact tube.
„ They exhibit coma similar to Newtonian’s of equal
focal length but have much more field curvature.
„ The offerings in this design are somewhat limited.
„ The nature of the Cassegrain design is such that it’s
fairly easy to build such a scope such that can be
used interchangeably as a Newtonian. This is
accomplished by swapping out the convex
secondary, replacing it with an elliptical diagonal
mirror, and installing a Newtonian focuser.
Takahashi’ CN-212 offers is an example of this
scope. It offers 212 mm aperture that is f/12 at the
Cassegrain focus and f/3.9 at the Newtonian focus.
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Dall-Kirkham Cassegrains
„
„
„
These are easy to manufacturer because of their elliptical primary’s
and spherical secondaries. This makes them relatively inexpensive
compared with other designs
They suffer much more from coma than Classical Cassegrains.
Takahashi’s Mewlon line consists of high quality Dall-Kirkham’s
where aberrations are minimized.
1.
Long focal length (f/12 or so) and compact tube.
2.
Aperture mask at the edge of the tube coupled with a slightly
oversized primary.
3.
They are well built and .
4.
Very sensitive to collimation errors, but easy to collimate & hold
collimation quite well.
5.
Smaller Mewlon’s focus with a moving primary and suffer from
mirror flop, but to a lesser extent than SCT’s. Larger Mewlons
focus by moving the secondary mirror and have no mirror flop.
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Lunar Southwestern Quadrant taken
with a Takahashi Mewlon 180mm f/12
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Parting thoughts
ƒ There is no best telescope design for CCD
imaging.
ƒ If you can reach focus, almost any scope can
deliver great CCD images if you work around
its particular limitations.
ƒ In general, the mount is more important than
the optics. Get a good quality tracking mount
whose carrying capacity is well in excess of
weight you intend to place on it. Overkill is
good.
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