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Hi
The way I look at it and its probably wrong is as follows.
A longer FL instrument has a smaller field of view (appears more magnified for a given ep/camera fl) therefore while the actual image you are looking at appears as bright as in a shorter FL instrument (the portion actually visible in the ep/camera) you are actually gathering light from a smaller portion of the sky and so overall less light per unit area of the CCD and so the image takes longer to form on the CCD for a longer FL instrument.
Paul
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