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My guess is that sky background darkness is a function baffling and F# alone and that smooth optics will mean that precious energy isn't scattered from diffuse objects . However I'm not sure what the relationship between spherical aberration and contrast of diffuse objects. I'd say it is much less important a consideration than optical smoothness for imaging faint diffuse objects. Users who report seeing `darker' sky background in premium reflecting optics I think are having themselves on. Perhaps the coatings are cleaner.
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