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I would disagree Peter.
Having used many scopes the few that were way up there in their Strehl ratio and that were way beyond diffraction limited way out performed lesser figured scopes.
Diffraction as a barrier is only part of the picture. Light scatter, throughput, the extent of the spectrum that is in focus at the same point, anti reflection coatings reducing ghosting, increasing micro contrast etc all play a part. Even focusing a high end instrument is a lot easier as there is more of a snap to focus effect. Star sizes off axis, correction for the 6 optical aberrations all play a large part in making a high quality image.
I think diffraction limited has been pushed so hard so as to justify not being willing to or able to go the extra mile so they make out that there really is no need anyway. Its a marketing justification for an inferior optical system.
Greg.
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