Schiefspiegler

[1] 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.

[2] The obstructions in telescope tubes, such as secondary mirrors and their mechanical supports, cut off the intensity of captured light and cause diffraction.

The diffraction causes artifacts such as the radial spikes that project from images of bright stars, and it also reduces the contrast of fine details.

[3]: 102  Schiefspieglers offer a significant increase in contrast, which is useful, for instance, for lunar and planetary study.

[4] 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.