TheSchiefspiegler (lit.oblique reflector inGerman), also calledtilted-component telescopes (TCT) andoff-axis reflecting telescopes, are a type ofreflecting telescope featuring an off-axissecondary 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.[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 lowreflectivity of hisspeculum 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 severecoma andastigmatism, however asAnton Kutter showed in the 1950s, by a suitable choice of radii these aberrations can be corrected to an acceptable level.
The 1.6 meter New Solar Telescope at theBig Bear Solar Observatory, and the 4 meterDaniel K. Inouye Solar Telescope feature off-axis designs for sensitive observations of theSun.