Catadioptrics

Catadioptrics refers to optical systems that combine refractive (lens-based) and reflective (mirror-based) elements to form images. By using both lenses and mirrors, catadioptric designs can correct aberrations more effectively and, in many cases, achieve long focal lengths in compact packages. The approach leverages reflective surfaces to fold light paths and lenses to manage chromatic and optical aberrations, enabling fast optics and useful control over the field of view.

Common catadioptric telescope designs include Schmidt-Cassegrain and Maksutov-Cassegrain configurations. The Schmidt-Cassegrain uses a thin aspheric corrector plate at the front, a spherical primary mirror, and a secondary mirror that folds the light back through a hole in the primary to the eyepiece or camera. The Maksutov-Cassegrain employs a meniscus corrector lens and a solid spherical primary with a reflective secondary, typically formed by a silvered spot on the corrector. Earlier Schmidt cameras, which use a corrector plate to correct spherical aberration in a fast, wide-field optical system, are considered precursors to these modern designs. These catadioptric configurations are widely used in both amateur and professional astronomy for their compactness and long effective focal lengths.

Applications extend beyond astronomy to photography and imaging systems requiring long focal lengths in small form factors, including certain camera lenses and compact endoscopic or industrial inspection equipment. Limitations include alignment sensitivity, central obstruction reducing contrast, and the complexity and cost of manufacturing the corrector elements.