The central spurious disk of the image of a star formed by a telescope. Because of diffraction, even with perfect optics a star's image is never point-like, but consists of a central disk, the Airy disk, surrounded by several fine diffraction rings. All telescopes of given size have the same size of Airy disk, which gets smaller with increasing aperture. The size of the Airy disk is given approximately in radians by 1.22λ times the f/number, where λ is the wavelength of the light. The size of the Airy disk limits the resolving power of a telescope, although in apertures larger than about 100 mm the Airy disk is often smaller than the false disk caused by seeing. It is named after G. B. Airy.
Subjects: Astronomy and Astrophysics.