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Depends on what you are trying to image or achieve. Night sky images have a huge dynamic range. A bright star will saturate the detector with less than a 1 sec exposure. A dim nebula will never saturate the detector. If both are in the same field then you have a problem. You use exposures that will give you a reasonable S/N ratio of the component you want to image.
My interest is photometry so I will use an exposure that keeps the maximum count below about 45000 for the stars I'm interested in. This is the linear range for my CCD.
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