Quote:
Originally Posted by Starkler
Although deviating from the OP's question I have a question relating to the thread title of "coma"... nobody ever mentions it in relation to refractors...
Is it the same thing or no?
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Coma is easily created in a refracting system (hence the Paracorr, which introduces coma of equal magnitude but opposite sign to that of the parabolic primary its designed for). There are some design principles which can be followed for refractor objectives which suggest that in certain cases - not all, and only to first order - the coma should be either zero (for geometric reasons) or insignificant compared to other aberrations over a sizable field but these are just approximations.
In practice if you're seeing "winged hairy shapes" from a lens it's almost certainly off-axis astigmatism mainly, not coma.