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, 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 Schmidt Cassegrain, or SCT, is a bit stubbier (shorter) for a
given aperture. It uses a somewhat complex curve on a nearly flat
corrector plate to compensate for the spherical primary. It has a convex
secondary mounted to the back of the corrector plate that reflects the
converging light from the primary back through a hole in the center of the
The Schmidt and Maksutov resemble refractors
in that the eyepiece is mounted at the rear of the instrument.
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 8 inch Schmidt Cassegrain. Like many of the
modern Cassegrains, most of these are fully computerized, so you spend your
time observing, not looking for objects.
The Maksutov Cassegrain, a similar design with a bit longer focal length,
and renowned for providing superb planetary images, might be more your focus.
If you want to learn more, read on. You can learn more in particular about
the Meade ETX 90 be reading my Meade ETX 90 Review. Keep in mind, mine is the older model,
not having the advantage of computerized goto.
The Maksutov And Schmidt Cassegrain Designs
The picture accompanying this description is of an Orion StarMax 90mm Tabletop Telescope
a 90mm Maksutov.
The Maksutov is a clever design that uses only spherical surfaces on all
internal components. The primary is a spherical mirror, the corrector plate is
a meniscus lens with spherical surfaces on both sides, and the secondary is
simply a silvered spot on back of the corrector.
The Maksutov is factory aligned, and is not generally ever aligned by the
owner. Maksutovs operate more in the f/15 focal range, making them better
suited for lunar and planetary work.
The Schmidt design generally gives a focal ratio of about f/10. This makes
the Schmidt Cassegrain a better telescope for general use, and the Schmidt
is commonly used as an all around work horse telescope.
The obvious advantage of the catadioptric is its 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? The main one is expense. While not nearly as
expensive as a comparable sized 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 Maksutov (such as the Meade ETX series shown in the picture) or the
Schmidt Cassegrain models.
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/15 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 Schmidt Cassegrain, with a focal ratio of about f/10, is a much better
general purpose instrument. It gives images with typically a little less
contrast than a similar sized Maksutov, but its field of view is more
appropriate for most star gazing. Both models typically have built in clock
drives, and now commonly have the computerized drives with built in
star almanacs for easy location of targets.
I've had little experience with SCTs, but I have, use, and love a
Meade ETX90 Maksutov.
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
Use this astro-customized search to browse some excellent telescope
I happen to own the 90mm Maksutov in the displayed image. 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 bucks.
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.
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.
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.