Often scopes are referred to as "diffraction limited" meaning that they can focus the rays of light to within 1/4 of a wave of light and therefore capable of more resolution than the Earth's atmospheric seeing will permit.
However why then do some APO refractors have .99 (essentially perfect) lenses and they produce better images than a refractor with say .96 strehl when both no doubt are well beyond diffraction limited accuracy?
Or does this theory apply more to mirrored scopes than refractors?
The strehl ratio is just one measure of accuracy.
You need also to consider the image size in all wavelengths of visible light. The diffraction limited label also only applies to stars perfectly on-axis again for a "good/ great" telescope or optics the whole field of view has to be considered; field curvature, coma and other aberrations.
The Apo has more aberrations corrected than say a "diffraction limited" achromatic.
HTH
However why then do some APO refractors have .99 (essentially perfect) lenses and they produce better images than a refractor with say .96 strehl when both no doubt are well beyond diffraction limited accuracy?
