Quote:
Originally Posted by toryglen-boy
Hi There
as stated, yes i know about the field changing size, and TBH i didnt want to be rude at all the replies, but i kinda had a rough idea about all that to, but what i asked hasnt really been tackled by any reply, so i will try to explain it more simply
say i am taking an image of the Eta Carina. Ragardless of image size, or any of that, i have 2 telescopes an 80mm refractor at F7.5 and a 200mm reflector at around F5. now then, purely based on telescope aperture and the telescopes light gathering power what would be the eqivelant exposure time for an image of equal brightness? thats equal brightness, not detail captured, or area captured, or size of image.
hope that explains it.
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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.