The Cassegrain Telescope Design
Catadioptric telescopes, like the many Schmidt Cassegrain and Maksutov
telescope design sold by Meade, Celestron, Orion, and others, use a concave
mirror as the primary objective, similar to the Newtonian reflector. However,
the popular amateur models usually use a short focus (f/2 or so) spherical
mirror. They are exceedingly compact and portable, as you can see by examining
StarMax 90mm TableTop Maksutov-Cassegrain Telescope. Near long focus refractor-like quality images in a
telescope that compact. Now wonder they're so popular.
Since a spherical mirror doesn't bring light rays to a proper focus, the
catadioptrics use a lens at the front of the telescope to correct for the
spherical aberration of the main mirror. Since the lens provides correction
only and is not the prime imaging component, the color problems normally
associated with lenses are avoided.
The two common versions of catadioptric instruments used by amateurs
are the Schmidt Cassegrain and the Maksutov.
The Maksutov Cassegrain Telescope (MCT)
The Maksutov Cassegrain, shown in the diagram above, uses a thick meniscus
lens for correction. On the back of the lens is a silvered spot that reflects
the converging light back through a hole in the concave mirror. The silvered
spot also acts like a focal length magnifier. The ray-trace gif illustrates
the path of light through the telescope.
The Maksutov design embraces the spherical surface concept. Not only is
the primary mirror spherical, but so are both surfaces of the corrector
plate. The advantage of all the spherical surfaces is that they are easy
to make to very high precision.
Above is the business end of my Meade
Instruments ETX90. You can see the thick meniscus lens and the
silvered spot that acts as the secondary. A neat system that with care is quite
maintenance free. That is, there is no alignment adjustment for the secondary,
nor is alignment by the user generally necessary.
The Maksutovs are available in the 3.5 inch (90mm) to about the 7 inch size.
They aren't generally made in larger sizes because the deep curve and thickness
of the corrector plate becomes unmanageable. The Maksutov is a great
instrument for lunar and planetary observing, rivaling the refractor in image
quality. Since the Maksutov design has around an f/12 or higher focal ratio,
it isn't as good a telescope for general observing. Wide star fields will
extend beyond the typical Maksutov field of view.
The picture accompanying this description is of an Orion StarMax 90mm Tabletop Telescope
a 90mm Maksutov. It's about the least expensive way to obtain a new Maksutov. The low price is made possible by the clever yet simple altazimuth table-top mount.
The Schmidt Cassegrain Telescope (SCT)
The Schmidt Cassegrain, or SCT, is a bit stubbier (shorter) for a given
aperture, as illustrated above. It uses a somewhat complex curve on a nearly
flat corrector plate to compensate for the spherical primary. The aspheric
curvature of the corrector plate is exaggerated in this diagram. The corrector
plate has a convex secondary mounted to its back, which reflects the converging
light from the primary back through a hole in the center of the primary.
Above you see the business end of a Celestron
NexStar 5 SE Telescope. The corrector lens here almost looks like a flat
plate of glass. The secondary is evident, and because of the smaller f ratio
generally of SCTs, tends to be larger with respect to the mirror size than that
of a Maksutov.
There are usually adjustment screws on the SCT secondary, as it
needs occasional collimation. This particular one has had the stock screws
replaced with handier Bobs
Knobs Collimation Knobs, which allow alignment without the clumsiness of a
The Schmidt and Maksutov telescopes resemble refractors
in that their focusers are mounted at the rear of the instruments.
The Schmidt Cassegrain, with a focal ratio of about f/10, is a much better
general purpose instrument. Amateurs commonly use 5 inch to even 14 inch SCT
telescopes. They give images with typically a little less contrast than a
similar sized Maksutov, but have fields of view more appropriate for most star
gazing. Both Cassegrain types typically have built in clock drives, and now
commonly have the computerized drives with built in star almanacs for easy
location of targets.
Because of their extreme compactness, these telescopes have become the most
popular used by amateur astronomers, displacing the venerable Newtonian.
Perhaps the most popular is the rather incredibly priced Celestron
NexStar 8 SE Telescope. It, like many of the modern Cassegrains, is fully
computerized, so you spend your time observing instead of searching for
objects. It's hard to imagine a totally computerized 8 inch telescope for the
kind of prices available now-a-days.
If you want to learn more, read on. You can learn more in particular about
the Meade ETX 90 by reading my Meade ETX 90 Review
and more about my Nexstar 5SE at my NexStar 5SE Review.
Keep in mind, my ETX 90 is the older model, not having the advantage of
Schrodingers Cat Small Poster
What's Behind Catadioptrics Popularity?
The obvious advantage of the catadioptrics are their compact size. Yet while
being compact, the instruments have relative long focal lengths allowing them
to be effectively used at high power. The compactness makes mounting easier.
Fork mounts are often used, and motor driven mounts are common and effective.
For astro photography, the catadioptric offers the most in ease of use,
minimal vibration, and portability.
What's the compromise? One is expense. While not nearly as expensive as a
comparable sized high quality refractor, the catadioptric costs quite a bit
more than a similar sized Newtonian. And if star fields and deep space are
your interests, a much larger Newtonian with its shorter focal ratio will give
better star-field views. None-the-less, the catadioptric has become one of the
most popular designs for all the benefits it offers.
If compactness and fairly low maintenance are high on your criteria list,
a catadioptric might be your best choice. The common choices are between
the many Maksutov telescopes on the market, or the
Schmidt Cassegrain models.
If you're considering adding a Cassegrain of any variety to your
arsenal, check out the following chart to double check its applicability
to your anticipated observing expectations.
Telescope/Observing Preference Table
(Small Instruments At Table Bottom)
|Wide Field||General Purpose||Narrow Field
|15" f/4.5 DOB||10" f/10 DOB||12" SCT
|12" f/4.5 DOB||10" f/6 DOB||6" Refractor
|6" f/5 Newt||8" f/10 SCT||6" Maksutov
|6" f/5 DOB||6" f/8 Newt||6" f/10 Newt
|3.5" f/8 Refractor||4" f/11 Refractor||4" f/15 Refractor
|4.5" f/4.5 DOB||5" f/10 SCT||5" f/15 Maksutov
|2.4" Refractor||4.5" f/10 Newt||3.5" f/15 Refractor
|Binoculars||3" f/10 Refractor||3.5" Maksutov
I happen to own a 90mm Maksutov. It's a Meade telescope, modeled somewhat
after the long-time contender (and much more expensive) Questar. I used to
dream almost yearly of owning a Questar, but never quite came up with the
While the Meade version carries a lot of plastic where the Questar has
aluminum, Meade bragged that their optics where unbeaten by any similar sized
instrument. I've only glimpsed through a Questar once, but I can tell you
that the images through my ETX are very good. I was most startled with first
viewing Jupiter, and seeing the Galilean moons appear as tiny, sharply
defined discs. My poorly aligned Newtonian always showed them as uninteresting
splashes of light. To get an idea of the ETX 90 image quality, check out
the images at ETX 90 Astrophotos.
My ETX is an older one, made before the computerized version. I had to put
a drop of epoxy on the spur gear to keep the clock drive from slipping. I
made a mod to the circuit card to give me a fast/slow control, but that's
a topic for another page.
I also have a NexStar 5SE, and have slowly grown accustomed to using the
computer interface instead of manually moving the telescope. When used
efficiently (not slewing all over the place), the computer control can
assist to save a lot of time in finding targets, allowing a dozen or more
enjoyable sights in a single evening. I've dabbled in photography with the
NexStar, as you can see at NexStar 5SE Astrophotos.
Both Meade and Celestron make a considerable variety of Maksutov
and Schmidt Cassegrain telescopes. Bushnell has a few entries, but I have no
experience with them.