catadioptric telescope

an optical instrument in which the image is constructed by a complex objective containing both mirrors and lenses. Correction lenses of comparatively small diameter are used in all modern reflectors to increase the effective field of view, but they are not classified as catadioptric telescopes. Only telescopes in which the lens elements are comparable in size to the primary mirror and are designed to correct the image (which is constructed by the primary mirror) may be said to be catadioptric. Among catadioptric telescopes are the Schmidt telescope (the Schmidt camera, 1931) and the Maksutov telescope (a meniscus telescope, 1941). In the Schmidt telescope aberrations of the spherical primary mirror are eliminated by means of a special correction plate of irregular configuration that is mounted in the entrance pupil. In the Maksutov telescope the aberrations of the primary spherical or elliptical mirror are corrected by a meniscus mounted in front of the mirror. Catadioptric telescopes of the super-Schmidt type (1947)—a combination of Schmidt systems with Maksutov meniscus systems—are used for observations of meteors and artificial earth satellites.

In reflectors of the Ritchey-Chrétien system, comparatively small lens correctors, which are installed in a converging beam in front of the focus of the telescope, are used. Such a corrector was first proposed in 1935 for the 5-m reflector of the Palomar Observatory (in the United States). However, in the narrow meaning of the term such systems are not catadioptric telescopes.