The classic Cassegrain telescope is a reflector type that has a concave (or parabolic) primary mirror and a convex (or hyperbolic) secondary mirror. With this classic type, the light enters and is reflected by the primary mirror onto the secondary mirror, which then reflects it back through an opening in the primary.
Two specialized reflector Cassegrains are the Ritchey-Chrétien and the Dall-Kirkham. The Ritchey-Chrétien (RCT) differs in that it has two hyperbolic mirrors but no parabolic. The Dall-Kirkham differs in that the concave primary mirror is elliptical and the convex secondary is spherical.
Catadioptric Cassegrain telescopes are different again in that they incorporate mirrors and lenses (as in refractor telescopes). Two more common ones are the Schmidt-Cassegrain and the Maksutov-Cassegrain, often written as SCT and MAK, respectively.
As the name suggests, the Schmidt-Cassegrain telescope combines a Cassegrain reflector mirror with a Schmidt corrector. The design can be compact or non-compact.
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