This is a very pertinent discussion for me as well, as I have been thinking through these same sorts of ideas.
Quote:
Originally Posted by Terry B
Sorry but that is silly. You are saying that the limiting magnitude is the same for 2 scopes of equal focal length but different aperture as you just take a longer exposure for the smaller scope. Hence the limiting magnitude is infinite for any scope as long as you can take an infinitely long exposure.
You have to be realistic and use a standard length exposure to compare scopes.
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Question is what do you regard as a "standard" exposure length? If FL determines limiting magnitude, the isn't this where f ratio, QE and image scale now start to come into the equation, if the intention is to compare different setups?
