The other two basic types are purely for photographic purposes. The main difference between the two of them is the shape of the large lens element in the front of the tube. Each is really a telescopic camera that is never used for direct viewing. Both the basic Schmidt and basic Maksutov have no secondary mirror; instead, a photographic plate sits inside the tube where the image cast by the objective is focused. Both of these telescopes allow very wide panoramic fields of stars to be photographed. The extra wide field of view is primarily the result of the reflecting surface of the objective mirror being very deeply curved. As a rule, the more steep the objective’s surface is, the more it costs because more glass has to be removed when the mirror is ground and polished.
The transparent front element of the Schmidt is called a corrector plate. Its purpose is to allow the telescope to produce an excellent image with a less than perfect objective mirror. The corrector plate cancels out several defects in the mirror which are: spherical aberration, coma, and astigmatism. Since the corrector plate cancels these defects, the telescope will function as if the objective was perfect. For a scientific explanation of the three defects, see Understanding Optical Defects.
The Schmidt’s advantage to the manufacturer is that it takes less time and is less costly to produce a steeply curved objective mirror with the three defects than to produce a perfect steeply curved mirror. This advantage still doesn’t completely offset the extra cost incurred from having a steeply curved objective surface. Thus, even though nearly perfect shaped mirrors are required for basic Newtonian and Cassegrainian reflectors (since they have no corrector plate) they are cheaper because the curve of their objectives is typically not as greatly curved.
The Maksutov’s front element is called a meniscus. It is a lens which is convex on one side and concave on the other. Like the Schmidt’s corrector plate, the Maksutov’s meniscus compensates for an objective mirror’s spherical aberration, coma, and astigmatism; however, it does one extra thing that the Schmidt’s corrector plate does not. It causes the cone of light reflected by the objective mirror to be longer. The importance of this fact will be revealed when we discuss various telescopes you can buy which are hybrids (i.e., mixtures) of the Maksutov, Schmidt and Cassegrainian types. Such mixture instruments are know as catadiotropic telescopes. By the way, if you see a telescope referred to as a Maksutov, it probably will not be the pure Maksutov shown in Figure 4, but the catadiotropic Maksutov-Cassegrainian depicted in Figure 5.
Perhaps the most popular telescope of serious amateur astronomers is the Schmidt-Cassegrainian reflector. By marrying the Cassegrainian optical system with the corrector plate of the Schmidt astrocamera, you get a very compact telescope with a fairly large objective at a relatively reasonable price. Again, this reasonable cost comes from the fact that the objective mirror does not have to have a maximumly corrected surface because the corrector plate will cancel out the optical imperfections of the objective mirror to give you the best possible view. Having a relatively large objective and a body with little bulk and low weight, Schmidt-Cassegrainians are especially useful to someone who transports his/her telescope to reach a place adequate for observing. Should compactness and lighter weight not be an issue for you, you will usually get more objective for your dollar with a pure Newtonian (for reasons explained on the previous page).
copyright 2004 Singularity Scientific