ABSTRACT
In order to produce a wide-field camera for astronomical survey use, with a very large aperture, while using the cheap, easily fabricated spherical mirror, the pure mirror design was modified by using dioptric elements, i.e. refracting lenses, to one of catadioptric form. The clear area of the primary mirror may be ‘figured’ so as to flatten the image field. Unlike catoptric lenses, catadioptric systems may be unsuitable for ultra-violet and infra-red photography because of their spectrally selective absorption and reduced chromatic correction. Detailed accounts of the numerous variants of the several corrected forms of catadioptric lenses have been given by C. Villa and D. Gavrilov. Catadioptric lenses may be focused in various ways, including axial movement of either the whole lens, the primary mirror, or the secondary mirror, and by internal floating elements. Mirror lenses are available in focal lengths from 250 to 2000 mm or more, as their characteristics are best suited to long focus, narrow-field designs.
