One Mirror Catadioptric

Consisting of a spherical or aspheric mirror with a corrector placed in front of it, this group provides good off-axis performane. The corrector closes the tube, protecting the mirror from the dust. These systems are normally used for photographic applications where the imade lies between the corrector and the mirror. When a much larger central obstruction, compared with the image diameter is allowed, it is possible to bring the image outside the tube by using diagonal mirror. In the compact configuration (Newtonian design), the spider can be omitted because the the secondary mirror can be attached to the corrector. In one-mirror catadioptrics, however, only concentric systems can be entirely free from astigmatism.

The best-known single-mirror catadioptric is the Schmidt camera. A concentric configuration with the corrector placed in the center of curvature of the spherical mirror, the Schmidt system is both aplanatic and anastigmatic. A derivative system is Schmidt-Newtonian, also has a spherical mirror. In it, the corrector is placed much closer to the mirror, resulting in a compact system. Because the principle of concentricity has been abandoned, the system suffers from both coma and astigmatism. For a compact Schmidt system to be free of coma, the mirror must be ellipsoidal, and the corrector must have considerably more power than in a non-compact system.

The Wright camera has its corrector at the paraxial focal plane, and offers the user a flat focal plane design. The next one-mirror catadioptric is the Houghton system; with the two-lens Houghton corrector, a compact system with all spherical surfaces is possible.

The last one-mirror catadioptric system is the Maksutov configuration. An aplanatic Maksutov system has a greater tube length than the Houghton system, but is shorter than the Schmidt camera. In its original form, the Maksutov camera is not compact in most cases abd the length of the tube depends on the thickness of the corrector.