Quote:
Originally Posted by Kal
T
For objects like stars which are pinpoint, focal ratio is completely irrelivent because it doesn't matter if you are shooting at F2 or F10 the stars are still a pinpoint of light, so aperture is the only determination with regards to how deep you can image.
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In a sense....
Limiting stellar magnitude is a function of sky fog limit. The longer the f ratio the longer you can expose and the fainter your limiting magnitude ( kind of the opposite of imaging extended objects).