Nearly 20 pages of Astrophotography for the Amateur were devoted to exposure calculations and tables. Guess what? We don't need to calculate exposures any more. With a DSLR or any other digital imaging device, it's easy to determine exposures by trial and error.
For a bright object that fills the field, such as the Moon, the Sun, or an eclipse, auto exposure can be useful. The rest of the time, what you should do is make a trial exposure and look at the results. For deep-sky work, I almost always start with 3 minutes at ISO 400. One reason for standardizing on 3 minutes is that I happen to have built a 3-minute timer. The other is that if all the exposures are the same length, dark-frame subtraction is easier; dark frames must match the exposure time and ISO setting of the image to which they are being applied.
If the 3-minute, ISO 400 image is underexposed, I switch to ISO 800 and 6 minutes, then take multiple exposures and stack them. If, on the other hand, the image contains a bright object, I reduce the ISO speed (to gain dynamic
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