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Rick is correct that there are a lot of myths out there. For instance, I recently read the book "Astrophotography with Mark Thompson". It's a nice introduction and generally provides excellent information, but you can tell that the author really doesn't understand the science of image calibration. For instance, he writes that bias and dark calibration are used to subtract noise from images. It might be simply a bad choice of words, but as written this is incorrect - these actually inject noise into the calibrated images, but they are effective at subtracting offset/current. It is actually impossible to subtract noise via image calibration, because white noise is, by definition, random (otherwise it would be signal). One can only to minimise it compared to signal.
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