[gregbradley]

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?

Greg.