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A Catadioptric Telescope is one 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 two most popular catadioptric designs are Schmidt-Cassegrain (SCT) and Maksutov-Cassegrain.