If you were to look at a star with a perfect optical telescope under high magnification, it's image would consist of a bright central disk, surrounded by concentric rings whose brightness diminished from one ring to the one next most distant from the center. In such a perfect system, the angular diameter of the central disk, defined by the diameter of the 'first null' separating the disk from the first ring outwards is given by 1.22 x W/D x 206265 in seconds of arc, where W is the wavelength of light and D is the diameter of the telescope mirror or lens in the same linear units as the wavelength. This expresses the maximum resolving 'power' of the telescope under perfect conditions in a vacuum. The atmosphere, however, limits this in practice to about 1 second of arc, although moments of clear seeing near the 'diffraction limit of the telescope' can be achieved.