catadioptric

Catadioptric refers to optical systems that use both reflective elements (mirrors) and refractive elements (lenses) to form an image. The term blends catoptrics (reflection) and dioptrics (refraction) and is applied to instruments that combine these principles for performance advantages.

By folding light paths with mirrors and correcting aberrations with lenses, catadioptric designs can achieve long effective focal lengths in compact packages or provide wide fields of view that would be difficult with lenses alone. This approach is common in astronomical telescopes and in some imaging lenses.

In astronomy, common catadioptric telescopes include Schmidt-Cassegrain and Maksutov-Cassegrain designs. A Schmidt-Cassegrain uses a thin Schmidt corrector plate at the entrance, a concave primary mirror, and a convex secondary mirror that folds light back through a hole in the primary. A Maksutov-Cassegrain employs a thick meniscus corrector with a similar folding arrangement. The Schmidt camera is a related imaging system optimized for wide fields.

In photography, catadioptric or mirror lenses pair a refractive lens group with a curved mirror to deliver long focal lengths in a compact package. They are valued for reach and light weight but typically produce distinctive bokeh, including circular or torus-shaped highlights when stopped down.

In machine vision and robotics, catadioptric cameras use curved mirrors to project scenes onto sensors, enabling ultra-wide or panoramic imaging and different projection geometries, from central to non-central. Advantages include compactness and aberration control; drawbacks include central obstruction, mechanical complexity, and potential vignetting.