Schmidt-Cassegrain and Maksutov-Cassegrain. What's the Difference?

Jack Kramer

What the heck is the difference between these two telescope designs? On the surface, they both look pretty similar. Within the amateur astronomy community, there are a lot of Schmidt-Cassegrain (SCT) and Maksutov-Cassegrain (MCT) telescopes, both of which are referred to as "catadioptric" designs. The SCT is more prevalent, being one of the most popular telescopes. As an example In the Meade line, the LX200 is an SCT and the ETX series is an MCT. Never having owned either, I turned for information on them to Rod Mollise, author of the book Choosing and Using a Schmidt-Cassegrain Telescope and Grant Gussie, a Canadian astrophysicist and amateur astronomer.

These telescopes are focused by moving the primary mirror. Doesn't a moving primary create a problem for collimation? Why not use a rack-and-pinion?

The moving mirror design gives the telescope a large focusing range - enough to accommodate virtually any combination of sensors, from cameras to CCD detectors to spectrographs. And you can still reach focus with devices like flip mirrors and off axis guiders. Also, this provides a stable, non-moving mount for a camera or other device. You may have heard of other scopes (generally Newtonians) that are "optimized" for visual or photographic use. This is really a recognition that the primary mirror needs to be installed differently in order for a camera to reach focus or to use certain eyepieces. That problem doesn't exist for catadioptrics.

Older SCTs had severe problems of mirror flop and focus shift, but modern versions have a relatively small focus shift and the new Meade primary mirror lock eliminates flop. It is true that a moving mirror scope is in perfect collimation at only one focus position, but the effects are not serious. At least one brand of MCT has a stationary mirror with a rack-and-pinion focuser as an option, but some eyepiece, Barlow, or binocular viewer combinations won't come to focus on that scope.

How does the corrector plate work and what's the difference between the SCT and MCT corrector?

The SCT corrector is a shallow, complex curve; the MCT corrector is a deep-dish meniscus. They "correct" (remove) the spherical aberration inherent in the spherical short focal length primary mirrors that these telescopes use. The secondary mirrors of both scopes are also spherical. Spherical aberration occurs when the telescope is focused at infinity. Light from infinity arrives in parallel rays, but the parallel light rays from the edge of the mirror come to a focus at a different distance in front of the mirror. However, a spherical mirror does focus all light rays correctly provided they are from a source at a distance of twice the focal length from the center of the mirror, so that the rays diverge. What correctors do is to cause a mild degree of divergence in the light rays so parallel rays diverge as if they were coming from a distance of 2F, rather than from infinity. This allows the mirror to then focus the rays correctly.

The advantage of using spherical mirrors, rather than a parabolic primary and hyperbolic secondary as in a classical cassegrain, is that the spherical mirrors can be made cheaply at much shorter focal ratios and remain free of coma. Nonetheless, some MCT designs do use an aspheric (parabolic) primary mirror.

An MCT corrector is a spherical meniscus lens, meaning that both the front side and the back side are curved in the same direction, resulting in a concave front and a convex back. In order to properly correct for spherical aberration, such a lens has to be relatively thick, about 1/2-inch for an 8-inch telescope. On the other hand, SCT correctors have a relatively complex shape created by deforming a thin (~1/4") plate glass over a vacuum and polishing it. This sounds complicated, but they are actually cheaper to mass-produce in larger sizes. An MCT corrector in small sizes is easier to make accurately than an SCT corrector, but above 7 to 8 inches, an MCT corrector becomes rather expensive because of the deep meniscus curve generated.

You might be interested in the following comments by Paul Laufer, who was Celestron's Schmidt corrector plate maker for many years. Here's what he says about how the correctors are mated with the rest of the optics at Celestron: "When I started at Celestron I was figuring C-90 correctors. Both sides. After a while I moved to the matching room, where the final hand figuring is done on Schmidt-Cassegrain scopes. I did 8's for a while and then 11's and an occasional 14. Then we added C-5's and C-9.25's. Basically you take the three optical components and put them into a test bed, which mimics a telescope tube, but allows you to quickly replace any of the three pieces (primary/secondary/corrector). The device has a flat mirror in place of the sky. So anything you shine INTO the eyepiece gets sent back OUT the eyepiece. By using a special 'pellicle' eyepiece holder and a laser you can simulate looking at a star. You then use a combination of things to determine how good the system is - the knife edge test, a Ronchi eyepiece, and a very high power eyepiece (for the airy disk). When you determine what the system needs you remove the secondary mirror (it isn't aluminized yet) and run it on a lap that is custom pressed each time you run it for that specific system."

Why do MCTs have a smaller secondary mirror?

The secondary mirror on an MCT can be smaller because the primary mirror has a higher focal ratio than an SCT, so the cone of light is smaller where it meets the secondary. MCT primaries generally are about f/3+ while SCTs are usually close to f/2. The final focal ratio of the MCT is also higher at about f/12 to f/15; most SCTs are at f/10. Thus the larger SCT secondaries create central obstructions of 30% or greater, which does adversely affect the sharpness of the images. The size of the central obstruction is dependent on the focal ratio of the system, which means that the central obstruction would be about the same size for an SCT and an MCT that had the same focal ratio.

The placement of the secondary mirror is also different in the two designs. The secondary mirror on an SCT is mounted in a baffle that protrudes through the corrector plate. Collimation is accomplished by adjusting the tilt of the secondary mirror. In an MCT, the secondary has the same radius of curvature as the corrector, so it is simply an aluminized circle on the back surface of the corrector. Collimation of an MCT is something best left to the manufacturer.

What are the advantages or disadvantages of an MCT versus an SCT? Don't MCTs have a reputation for superior images?

Since all the optics in an MCT are spherical (the primary, the secondary, and both sides of the corrector), and since making and testing spherical optics is easier, it takes less work to shape and polish MCT optics to a higher precision. In comparison, SCT corrector plates, with their weird curve, are difficult to shape correctly and next to impossible to polish to maximum smoothness. So it's not really that the MCT design gives superior images, it's just that MCT optics tend to be of higher overall quality than an SCT of the same size.

Most MCT's have a proportionally longer tube than a comparable SCT owing to the longer focal ratio of the primary mirror. Thus in the larger sizes, an MCT will end up being bulkier, and it will be more expensive (due to the cost of the corrector, mentioned previously). Also, due largely to the thickness of the meniscus corrector, the time required for an MCT to acclimate to the outdoor temperature will be greater than for an SCT. In the plus column is the fact that the image sharpness in a well-made MCT is generally better than in any SCT.

So there you have it ... as always, there are tradeoffs.