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The term “diffraction limited” originated in 19th century England with Lord Rayleigh, who stated that a telescope with a wavefront error of ¼ wave length of light would provide most of the performance that is possible when compared to a theoretically perfect optical system. That diffraction limited standard originally applied to unobstructed telescopes (refractors). In multi-mirror designs such as Newtonians and SCTs, wavefront errors in each optical element are cumulative in the telescope; thus better individual optics are needed in order to produce an overall diffraction limited system.

Practically speaking, the classical Rayleigh criterion defines a telescope system that is only minimally acceptable. A more stringent criterion for diffraction limited is that the telescope mirrors or lenses have such small wavefront errors that there is very little to be gained by improving the optics further. However, some manufacturers misuse the definition, so to advertise an instrument as “diffraction limited” probably tells you very little about its optical quality at the eyepiece.

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