Other Members of the CAT Tribe

Thus far, except for an occasional aside, the terms CAT and SCT have been used interchangeably. The SCT is far from being the only CAT in use by amateurs. A visit to any club observing field will also reveal SNTs (Schmidt Newtonian telescopes) MCTs (Maksutov Cassegrain telescopes), MNTs (Maksutov Newtonian telescopes), and maybe even KCTs (Klevtzov Cassegrain telescopes). All these variants are described in the next chapter, but a few words about the Maksutov family are in order here.

The Maksutov Cassegrain telescope, the MCT or "Mak," is without doubt the non-SCT CAT most beloved of amateurs. The SCT and MCT are such similar scopes that beginners often have a hard time telling one breed of CAT from the other. The principal difference, visible in Plate 12, is the MCT's corrector. Unlike the thin, complex-curved Schmidt Cassegrain lens, the Mak has a corrector plate that is thicker, deeper, and simpler. It is often called a "salad bowl" corrector because of its appearance. While it looks different, the Mak corrector's function is the same as that of the SCT corrector: remove spherical aberration. One other striking difference between these two types of CATs is that the MCT often does not have a separate secondary mirror. In Gregory design MCTs, the secondary is a silvered (aluminized) spot on the inside surface of the corrector plate.

Plate 12. (MCT Corrector) The deep-dish corrector plate of a Gregory-style Maksutov Cassegrain telescope, the Meade ETX125PE. Credit: Author.

Why would somebody want an MCT instead of an SCT? because—some amateurs think—MCTs have better optics. Although SCTs are good, the MCT does usually pull ahead of the SCT in image sharpness. This is due in part to the nature of the corrector. In the MCT, it is usually figured in an easy-to-make spherical shape, which is less demanding to make than the SCT's corrector, and for that reason is often of better quality. Also, the MCT's primary mirror (which is sometimes a sphere, just like an SCT mirror) usually has a higher focal ratio (and thus a shallower curve) than a comparable SCT (often f/3 instead of f/2). Higher focal ratio mirrors are usually more optically forgiving than lower focal ratio ones. The final focal ratio of the MCT's primary/secondary mirror combination is usually considerably higher than that of most SCTs (f/15 is common). The longer Mak focal length and longer tube that result from this design difference mean the MCT's secondary mirror can be smaller than that of an SCT, allowing the Mak to possibly deliver slightly higher contrast images.

Everything does not come up roses with the Mak, however. Problem number one is expense. Thick Maksutov correctors require expensive glass blanks that drive MCT prices up. The (usually) higher focal ratio of the MCT means there is lot of focal length, which delivers higher magnifications eyepiece for eyepiece and narrower fields of view. Do not buy an MCT to scan the vast star fields of Sagittarius. Buy one if you value optical quality above all else (and do not like superexpensive apochromatic refractors) and do not mind focusing on small and medium-size targets. If you are a planetary observer, a Mak may be just the CAT for you.

Our tour of a generic SCT is now at its end. I have already mentioned a few specifics of the two SCT makers' telescopes, but in the next chapter we get down to brass tacks and survey each company's scopes model by model and in detail. The MCTs, MNTs, and SNTs are also not ignored, and a few of the CAT zoo's even more exotic beasts are introduced.


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