A telescope design that uses a primary mirror to reflect light towards a secondary mirror which then reflects the light back down the tube and through a small hole in the main mirror. The light is then brought to focus by the eyepiece, camera or light detector. Since the light is folded back on itself the design is compact when compared to a Newtonian reflector or a refractor telescope of similar or even smaller focal length.
French scientist Laurent Cassegrain proposed the design in 1672, which forms the basis of today's popular Schmidt-Cassegrain and Maksutov-Cassegrain telescopes (with the addition of a front corrector plate).