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That is how I figure it as well MrB - for extended objects like nebula you increase S/N ratio (or decrease exposure times) with shorter focal ratio as the same amount of photons are collected onto fewer CCD detectors.
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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