A reflecting telescope in which a convex secondary mirror reflects light to a focus through a hole in the primary mirror; it is an adaptation of the Gregorian telescope. The convex secondary has a hyperboloidal cross-section, while the primary mirror is paraboloidal. The primary mirror has a central hole through which the light is brought to the Cassegrain focus. (The term Cassegrain focus is also applied to any focus similarly located behind a mirror, such as in a Ritchey–Chrétien telescope.) A Cassegrain reflector is more compact than a Newtonian of the same focal length. It is also easier to mount heavy or bulky equipment at the Cassegrain focus than at the prime focus or Newtonian focus. The design was devised in the 17th century by Laurent Cassegrain (c.1629–1693), a French priest and teacher. See also newtonian–cassegrain telescope; schmidt–cassegrain telescope.
Subjects: Astronomy and Astrophysics.