Quote:
Originally Posted by Satchmo
Generally light increases by the ratio of surface areas, or vary simply by the square of the ratio of aperture size. An 8" scope then shows objects 4 times brighter than a 4" . The ratio of sky background brightness to object indeed stays the same - those are properties of the sky not the telescope. The situation with CCD detectors is somewhat more complex and involves pixel size and readout noise .
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Thanks, yes this is what I would expect but I'm pretty sure if I remember my previous discussions here (which I can't find) about such things others might popup and say differently ...
Quote:
Originally Posted by Satchmo
The f ratio of the telescope is completely irrelevant to the discussion on brightness of objects between different telescopes . Only the aperture and exit pupil used are relevant.
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Hmm, I'm not knowledgable on exit pupil stuff, but wouldn't the size of the exit pupil change based on focal ratio?
Quote:
Originally Posted by Satchmo
Perceived brightness of point sources may not be as consistent as large apertures will often have their light spread out into a larger disc by the seeing and therefore not show as intense a point source as they are capable of.
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... their light spreads out according to focal length, I would expect, which is related to focal ratio. So while in principal I understand what you're saying aren't you really also saying the light would be spread into a larger disk by a larger aperture only because larger apertures tend to have longer focal lengths?
Quote:
Originally Posted by Satchmo
If you spread the light of star into a disc twice the size then it becomes somewhat fainter - the star image is usually some kind of Gaussian curve so its not that simple , but its been obvious to me many times that smaller high quality optics can reach a similar magnitude limit visually as poor quality larger ones, although the smaller scope may not have the same ability on extended objects.
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Agree.