Reflecting telescope

A reflecting telescope (reflector) is an optical telescope which uses mirrors to reflect light, rather than lenses to pass light. The British scientist Sir Isaac Newton designed the first reflector circa 1670. He designed the reflector in order to solve the problem of chromatic aberration, a serious degradation in all refracting telescopes before the perfection of achromatic lenses.

Technical considerations

The primary mirror is the reflector telescope's basic optical element and creates an image at the focal plane. The distance from the mirror to the focal plane is called the focal length. Film or a digital sensor may be located here to record the image, or an eyepiece for visual observation.

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Reflector mirrors eliminate chromatic aberration but still contain other types of aberrations. Expensive telescopes will have additional optical elements to correct some of these aberrations;

Related Topics:
Chromatic aberration - Aberrations

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• spherical aberration when a non-parabolic mirror is used (the image plane is not flat)
• coma
• distortion over the field of view

Nearly all large research-grade astronomical telescopes are reflectors. There are several reasons for this:

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• 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 wavelengths travels through a medium other than vacuum at different speeds. This causes chromatic aberration in uncorrected lenses and creating an aberration-free large lens is a costly process. A mirror can eliminate this problem entirely.
• There are structural problems involved in manufacturing and manipulating large-aperture lenses. A lens can only be held by its edge, which means that the sag due to gravity can be sufficient to distort the image. In contrast, a mirror can be supported by the whole side opposite its reflecting face .