Catadioptric telescope. Computer illustration of a catadioptric telescope, which combines a Cassegrain reflector optical system with a Schmidt corrector plate used to correct spherical aberration. The Cassegrain optical configuration was first defined in 1672 by Laurent Cassegrain, a French Catholic priest who used a combination of a primary concave mirror and a secondary convex mirror to fold a long focal length into a short and light optical tube.