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Airy disk

Airy disk
An Airy disk is the central spot in the diffraction pattern of the image of a star at the focus of a telescope, named after George Airy. It is surrounded by several fine diffraction rings like the rings around the bull's-eye of a target. The size of the Airy disk, given approximately in radians by 1.22 times the wavelength of light times the focal ratio, is the same for all telescopes of a given size and is less in instruments of larger aperture. (It is one of the quirks of astronomy that bigger telescopes produce smaller images of stars.) In practice, however, atmospheric turbulence, unless compensated for, results in a false disk that is larger than the Airy disk.

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