Airy disk

The Airy disk (or Airy disc or Airy pattern) is a type of diffraction-generated circular pattern of light (or any type of EMR) within a telescope image produced by light from a point source, the normal case for stars. The Airy-disk pattern occurs specifically when the telescope has a circular aperture; it consists of a round spot surrounded by concentric circles (and without the circular aperture, a point source would produce some other non-Airy-disk pattern). The smaller the aperture, the larger the Airy disks that it produces and too much overlap of two Airy disks make it hard to distinguish the sources of the disks, e.g., hard to distinguish two near-to-each-other stars in the image, making the aperture size a limiting factor in a telescope's angular resolution. A telescope that solves all the other problems that affect angular resolution (e.g., incorporating an adaptive optics system) is described as diffraction limited. The Airy disk's inner-most minimum (ring with less light) is at approximately:

        λ
sin θ ≈ —
        d

• λ - wavelength.
• d - diameter of the aperture.
• θ - angle "across the sky", i.e., if another light source was at that angular distance from the source of interest, then it would be coincident with this ring.