[toryglen-boy (Duncan)]

Originally Posted by ghsmith45

The 80mm scope will require 2.25 times the exposure to give the same brightness for the nebula, which you get by calculating (7.5/5)^2.


The stars, being point sources follow a different rule. They will be brighter in the 200mm pic (by a factor of a little less than 3) if the 80mm exposure is 2.25 times longer than the 200mm exposure.

So in fact there has to be a compromise. Using two different aperture scopes you can arrange for the stars to be the same brightness with different exposures, or you can have the neb the same brightness in different exposures, but you can't have the stars and the neb the same brightness in different exposures.

Thanks Geoff, thats just what i was looking for, so i could take an exposure that would be shorter, and have less noise in a bigger scope?

now you can see my reason for asking...