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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
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