Reflector telescope

A reflector telescope is a telescope, with which the substantial part of the optics consists of reflecting elements. Optical telescopes use highly exact polished concave mirrors as primary mirrors. The shape of the mirrors can be spherical, parabolic or hyperbolic. The surface must be worked on with an accuracy from 100 to 20nm. Mirror sizes to 8.6m were realized.

Nearly all large research-grade astronomical telescopes are reflectors. This is due to several reasons:

• In a lens the entire volume of material has to be free of imperfection and inhomogeneities, whereas in a mirror, only one surface has to be perfectly polished.
• Light of different colors travels through a medium other than vacuum at different speeds. This causes chromatic aberration.
• There are technical difficulties involved in manufacturing and manipulating large-aperture lenses. One of them is that a lens can only be held by its perimeter. A mirror, on the other hand, can be supported by the whole side opposite to its reflecting face.

In radio telescopes metal surfaces, which collect the radio waves in the actual antenna, work as mirrors. These are used as parabolic reflectors. The largest single piece antenna is the Arecibo radio telescope in Puerto Rico.

According to their geometry one differentiates different kinds of reflector telescopes:

• (classical) Cassegrain telescope
• Maksutov-Cassegrain telescope
• Newtonian telescope
• Ritchey-Chrétien telescope
• Schiefspiegler telescope
• Schmidt-Cassegrain telescope
• Schmidt camera