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抯-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.
6tZI["\ UMi~14& ; The ratio of the focal length (F) of a mirror or lens to its diameter (D) expressed as a number, f/# = F/D. Borrowing the language of photography, small focal ratios, below about f/6, are said to be fast and result in a brighter image for a given aperture. Large focal ratios, equal to or above about f/8, are said to be slow.
+
