The smallest image a particular telescope can make of a point source of light, such as a star. Diffraction as the light passes through the telescope aperture causes the image of even a point source to have a finite size. The diameter of the Airy disc is smaller for larger apertures. In practice, it is rarely possible to achieve images as small as the Airy disc because of turbulence in the Earth’s atmosphere, which distorts and enlarges the perceived image.