Quote:
Originally Posted by cometcatcher
That doesn't sound right. f10 is 2/3 of an f stop dimmer than f8. A full f stop would require double the exposure, so 2/3 f stop make 66% more exposure needed?
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Teach me to write a 10 second reply... As I walked out the door I realised the error in my post.
So for clarity;
For (under-sampled) star images the exposure times will be approximately proportional to the aperture ratio squared;
ie) the 12" will collect 44% more light so the 10" will take 56% longer for the same depth of exposure. The unspoken assumption here is that a combination of background sky brightness and detector noise will determine the absolute magnitude limit. There is quite a difference between sensors (as much as 2 magnitudes) The Sony cmos chips being probably best in class within the amateur budget.
For extended objects the exposure times are proportional to the ratios of the f-numbers squared.
In this instance it will be the same as the numbers above (derived from relative aperture) but this isn't always the case so it is probably worth noting the distinction.
c