A star appears in a telescope as a small Airy disk surrounded by faint diffraction rings. A telescope is said to be "diffraction limited" if its optics are made with enough accuracy so that all the light rays from a star fall within that star's Airy disk and diffraction rings, with no excess light being scattered out of the disc and rings by defects in the mirrors. Optics that bring all light rays to a focus within 1/4th of a wavelength of light of each other at the final focus are considered to be diffraction limited. Technically, a telescope is diffraction limited if it meets the Rayleigh limit - which specifies the separation in arc seconds of two equally-bright binary stars which appear to be just touching as being equal to 140 divided by the aperture in mm. The Rayleigh limit, which deals with a telescope's ability to separate closely-spaced stars, should not be confused with the Rayleigh criterion, which deals with how accurately an optical system is made. Note that the visual Rayleigh limit for an 8" (203mm) aperture telescope is 0.69 arc seconds (140 / 203), a less-stringent specification than the Dawes' limit of 0.57 arc seconds. Telescopes meeting either limit can resolve more detail than the Earth's atmosphere will allow us to see under average seeing conditions, as our atmosphere typically limits the seeing to no better than one arc second resolution (the resolution of a 6" scope) on even a very good night. Five arc second resolution or worse is more typical of an average night.

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