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. This occurs specifically if the telescope has a circular aperture. The Airy disk pattern is a round spot surrounded by concentric circles (and without the circular aperture, there would be 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 stars in the image, so the aperture size 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 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 that angle away from the first, then it would be at this ring.