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Fred,
In the worst case s/n ratio increases as a square root of increase in QE. The worst case being where the greatest contribution to image noise is from the sky background.
Here is a question for the forum: I want to photograph the horsehead nebula over an area of 1 degree wide field. Therefore I've determined I need 500mm focal length with the CCD I am using. Which of the following - well corrected optically - instruments would you choose?
a. 500mm focal length f10
b. 500mm focal length f2
Answer that and tell me whether you think focal ratio is not important..
T.
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