Star Ware: The Amateur Astronomer's Guide to Choosing, Buying ...

So You Want to Buy a Telescope! 35
There are cases where no matter how well aligned the optics are, image
quality is still lacking. Here, the fault undoubtedly lies with one or both of the
mirrors themselves. As the saying goes, you get what you pay for, and that is as
true with telescopes as with anything else. Clearly, manufacturers of low-cost
models must cut their expenses somewhere in order to underbid their competition.
These cuts are usually found in the nominal-quality standard equipment
supplied with the instrument but sometimes may also affect optical
testing procedures and quality control.
Cassegrain reflectors. Though they have never attained the widespread following
among amateur astronomers that Newtonians continue to enjoy,
Cassegrain reflectors (Figure 3.4) have always been considered highly competent
instruments. Cassegrains are characterized by long focal lengths, making
them ideally suited for high-power, high-resolution applications such as solar,
lunar, and planetary studies. While Newtonians also may be constructed with
these focal ratios, observers would have to go to great lengths to reach their
eyepieces! Not so with the Cassegrain, where the eyepiece is conveniently
located along the optical axis behind the backside of the primary mirror.
The Cassegrain’s long focal length is created not by the primary mirror
(which typically ranges around f / 4) but rather by the convex, hyperbolic secondary
mirror. As it reflects the light from the primary back toward the eyepiece,
the convex secondary actually magnifies the image, thereby stretching
the telescope’s effective focal ratio to between f / 10 and f / 15. The net result is
a telescope that is much more compact and easier to manage than a Newtonian
of equivalent aperture and focal length.
Figure 3.4 The RC-10, 10-inch Ritchey-
Chrétien Cassegrain reflector. Photo courtesy
of Optical Guidance Systems.