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Astronomy Hints



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 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)

SCT optical diagram
SCT optical diagram

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 screwdriver.

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 computerized goto.

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 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

Personal Notes

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 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. 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.