The Airy Disk (or Airy Disc or Airy Pattern) is a pattern of light within a telescope image produced by light from a point source, the general case for stars. The pattern is specially an Airy Disk if the telescope has a circular Aperture. The Airy Disk pattern is a round spot surrounded by concentric circles. This is a Diffraction pattern due to the aperture size and shape of the telescope: the larger the telescope, the smaller the Airy disks that it produces, and is 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 first minimum (ring with less light) is at approximately:
λ sin θ ≈ — d
Point-Spread Function (PSF)