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Originally Posted by bratislav
Again, brightness doesn't factor in untill you start to see the diffraction structure as extended image. That is pretty much at the same point (well below 5mm eyepieces for f/4 mirror). Extended comatic blur (that subtends say 5 arc minutes) will have same flux (# photons per retinal area) so it will look the same regardless of eyepiece's focal length (as long as diffraction structure isn't resolved - say 1mm exit pupil or thereabouts).
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I see your point. I have no direct experience of how that looks in practice. I certainly cannot see anything wrong with star images at the edge of a 70 degree FOV of 10mm XW eyepiece at f/5, or even at f/4. Looking at Al Nagler's graphs (above), I should. I remain curious to try a Paracorr.
Let's leave that out of it. Beginning optics courses concern themselves with ideal lenses and mirrors with perfect focus for all colours, and perfectly flat infinite focal planes. Only if real-world optics worked like that...