The ability of an optical system to distinguish detail. Theoretically, the resolution is limited by the size of the aperture, through the effects of diffraction. Because of diffraction, the image of a point source is actually a disc surrounded by a number of rings. The diameter of the disc, known as the Airy disc, is given in radians by the formula 1.1λ/Δ resolving power. In practice, however, the resolving power of a large ground-based optical telescope is limited by the quality of seeing rather than the aperture.