Quote:
Originally Posted by Terry B
The focal ratio is less important for stars than for extended objects.
For the same exposure the 12" scope at f10 will show dimmer stars than the 8" scope at f10. For bright stars it makes less difference as they spread over more pixels.
If this wasn't the case there would be no point to the large professional scopes
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Well surely the point of a larger scope is you have higher resolution at a reasonable focal ratio - you have that longer focal length but at something useable rather than F/32 or such.
If a 12" at F/10 were to record brighter stars than a 8" at F/10 then why does a 200mm F/2.8 lens record brighter stars than a 200m F/4?
... edit, let me correct that to a better comparison where I'm not mixing aperture and focal length:
If a 12" at F/10 were to record brighter stars than a 8" at F/10 then why does a 60mm (aperture) lens @ F/2.8 record brighter stars than a 60m (aperture) lens @ F/4?
... edit(2) ... hmm, ok ... may have to answer my own question here. The lens would stop down the aperture, hence changing it from 60mm to something smaller (50mm say)
I'm going to have to think about this some more
