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Cassegrain Basics

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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 the Orion 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 primary.

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 FieldGeneral PurposeNarrow Field
15" f/4.5 DOB10" f/10 DOB12" SCT
12" f/4.5 DOB10" f/6 DOB6" Refractor
6" f/5 Newt8" f/10 SCT6" Maksutov
6" f/5 DOB6" f/8 Newt6" f/10 Newt
3.5" f/8 Refractor4" f/11 Refractor4" f/15 Refractor
4.5" f/4.5 DOB5" f/10 SCT5" f/15 Maksutov
2.4" Refractor4.5" f/10 Newt3.5" f/15 Refractor
Binoculars3" f/10 Refractor3.5" Maksutov

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Personal Notes

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.