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Old 10-12-2008, 07:44 AM
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Amongst the stars

Garyh is offline
Join Date: Jun 2006
Location: Glen Innes, N.S.W.
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Originally Posted by bojan View Post
Paul, I have a small correction to the above..
That depends on how f/ratio was increased, and what objects are in question.
The increase of f/ratio with barlow will have the effect you are mentioning (longer exposure time required), but on "spread" objects only, like nebulae (which have surface brightness) .. Stars, however, being spot-like light sources, will leave roughly the same number of photons per pixel (unless FL is increased such that star images become bigger than pixels, in which case they will start to behave like nebulae as well).

Increasing of f/ratio while keeping the same FL means actually smaller aperture. Then everything fades exactly as you described
Bogan is right on the money there! Any object that is extended shall react on how fast the lens system is (f/5 is 4 times faster than a f/10 system) but when we look at aperture and the airy disk in the same system we would have the same brightness (same aperture f/5- f/10 scope for instance) the airy disk will be 4 times brighter in say a 12" f/10 than a 6" f/5 and as long as the ccd is matched well to the system as well.....

The 6" f/5 shall need 1/4 the exposure of say the Horsehead neb (not including stars) as a 12" f/10 while if you imaged Omega Cent the 12" f/10 will be 4 times quicker in recording all those stars..
cheers Gary

Last edited by Garyh; 10-12-2008 at 07:56 AM.
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