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I'm always amazed that the Marechal's limit of 0.07 RMS ( equivalent to Lord Rayleigh's 1/4 wave P-V for primary spherical aberration) has been claimed by the commercial industry to mean `diffraction limited' which implies a standard at which not much improvement can be gained and suitable limit for commercial mirrors. These limits produce a Strehl ratio of around 0,8. As Lord Rayleigh found , at 1/4 wave or (0.07 RMS) primary spherical the effect on planetary detail and contrast was already `decidedly prejudicial'
I believe that Diffraction Limited means a wavefront that is only perturbed by the laws of diffraction that cannot be controlled ie the destructive effects of the circular aperture and the central obstruction. A wavefront that genuinely puts 96% or more of its available light into the Airy Disc will have a contrast transfer function that is nearly perfect and for me satisfies this definition.
Unfortunately while Strehl ratio's in the mid '90's are common place on documentation these days very few optics truly meet this criteria due to creative computing on the part of the manufacturers.
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