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The Schiefspiegler (lit. oblique mirror in German) also called tilted-component telescopes (TCT) are a type of reflecting telescope featuring an off-axis secondary mirror, and therefore an obstruction-free light path. This is accomplished by tilting the primary mirror so that the secondary mirror does not block incoming light. William Herschel was one of the first to have tilted the mirror of his telescope in order to avoid light loss due to the low reflectivity of his speculum-metal mirror. Schiefspieglers offers a significant increase in contrast, which is useful for instance for planetary study.
Tilting the mirrors causes severe coma and astigmatism, however as Anton Kutter showed in the 1950s, by a suitable choice of radii these aberrations can be corrected to an acceptable level.