Catadioptric telescopes are combination telescopes, which use a mirror as an objective and a large correcting "lens" in front of it. The purpose of this plate is to correct the defects in the curvature of the mirror. Hence these instruments are coma free. In these telescopes, the light passes through the corrector, and strikes the objective mirror. The light is reflected up front to another mirror which reflects it back through a hole in the objective. Thus a long light path is folded into a short tube. Catadioptric telescopes or "cats" as they are called, are very compact and easy to use in the field, but they do not necessarily have superior optics. In fact, their image is likely to be slightly worse than for a Newtonian of equivalent size. This is simply because they must be built to more precise tolerances. Also, they tend to have slightly less image contrast than Newtonians because light tends to "leak out" of their more complex light path. Because the light path is long the image you get is highly magnified, but the F number is high, commonly about F 10. These telescopes are often used for photography, but the high F numbers may require long tedious exposures. Despite these disadvantages, cats are becoming very popular for several reasons. For one thing, they are easy to care for. Because their tubes are fully enclosed, their optics are protected from dust contamination. They do not generally lose their optical alignment, as Newtonians can. However, anyone can align Newtonian optics, but cats must be realigned at the factory. This is an expensive proposition. Primarily, their popularity results from their portability and ease of use. In this age of city light pollution, there is a premium placed on portability and the cats are unsurpassed in this department.
