tri-Schiefspiegler

A reflecting telescope with three mutually tilted, curved mirrors arranged in such a way that the optical path is unobstructed and the aberrations introduced by tilting the mirrors are cancelled out. "Schiefspiegler" is German for "oblique reflector", and the "tri" indicates the number of mirrors. The name was coined by Anton Kutter, who was also responsible for many early designs, which he promoted in the 1950s. The modern version is the outcome of work by a Californian, Dick Buchroeder. In a typical system, the light falls first on to a concave primary mirror of relatively large focal ratio (f/12, for example) and near-spherical curvature. The optical axis is inclined at about 3° to the path of the incoming rays. The converging beam is intercepted by a spherical convex secondary with the same radius of curvature as the primary, positioned just outside the entrance aperture of the telescope. This mirror is also tilted with respect to the incoming light, and largely cancels out the astigmatism and coma caused by the tilt of the primary. This two-mirror combination is virtually optically perfect up to apertures of about 120 mm. However, coma exceeds acceptable limits at larger apertures, and a third optical component must then be added. The result is successful up to apertures of at least 300 mm, and the system is regarded as an ideal planetary telescope with the quality of definition associated with refracting telescopes but no chromatic aberration.