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It all works on f number. Two scopes with the same f number need the same exposure for extended objects. As an example take an 8" f5 (40" focal length) and a 4" f5 (20" focal length). The 8" gathers 4 times as much light as the 4", but because the image scale is doubled this light is spread over 4 times the area, so the extra light is exactly cancelled by the extra area over which it is spread.
The general result is this: If you take an exposure of length t1 with a scope of focal ratio f1 and then you go to a scope of focal ratio f2, you will need an exposure t2 given by
t2=t1*(f2/f1)^2
So just work out the focal ratios of your two scopes and plug into this result
Last edited by Geoff45; 23-06-2009 at 06:50 PM.
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