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Old 09-12-2008, 07:57 AM
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bojan is offline
Join Date: Jul 2006
Location: Mt Waverley, VIC
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Originally Posted by [1ponders] View Post
The other thing you need to remeber too Kirk is that if you double your f/ratio (f/5 > f/10) Then you increase the time required to achieve the same level of brightness by a factor of 4. Now this may not be a problem with widefield, but for narrow field work increasing an exposure from 5 min to 20 min puts a lot more demand on your mount's quality and autoguiding abilities.
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

Last edited by bojan; 09-12-2008 at 08:11 AM.
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