A telescope that uses a combination of mirrors and lenses to increase
the effective focal length of the telescope while allowing it to be
folded into a more convenient and compact size. The use of a
full-aperture correcting lens in these scopes virtually eliminates
spherical aberration, chromatic aberration, and coma. The word
catadioptric is derived by combining the term for an optical system that
forms images by using mirrors (catoptric) with the one for a system
that uses lenses (dioptric). The most popular catadioptric designs are
the Schmidt-Cassegrain and Maksutov-Cassegrain.