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Originally Posted by janoskiss
But as you approach the point where the coma blur is comparable with size of the Airy disk, it becomes less obtrusive.
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Er, true. That is why I said 20mm and 40mm, not 2mm and 40mm
You aren't going to see diffraction structure in a typical f/4 mirror (which is where Paracorr becomes compulsory) until about 3 to 4mm f.l. eyepieces. For run off the mill deep sky selection oculars (9 to say 30mm) diffraction doesn't factor at all.
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Also as the brightness of the image is reduced at high powers (as inverse square of the magnification) I would expect that coma is more difficult to perceive
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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).
Let's have our 20mm and 40mm eyepieces again. Say right at the filed stop they show same size comatic blur - for example 5 arc minutes.
A given extended comatic blur (that subtends 5 arc minutes) will result in same same flux (# photons per retinal area) - so it will be the same brightness, regardless of eyepiece's focal length.
Again optics 101.