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Yes mean is the average of a sequence. So its kind of a pretty rough approximation of what your average pixel should be. If you have lots of hot pixels it will throw the calculation off quite a bit. I suppose that is why you run min/max first so the mean becomes more accurate.
But min/max probably only throws out values above/below a certain range. Again that is crude as any image has bright and dark areas and they are not artifacts.
Sigma reject is more complex computation that is pinpointing more accurately the pixels that are quite clearly an artifact and outside what is statistically expected of pixel values taking into account the whole image.
An ideal computation would detect every artifact yet leave intact bright and dim areas without change. That is the problem the mathematicians are trying to solve with these various combine methods. It is my understanding (please correct me if I am wrong) that sigma reject does this the best of the current methods. Although I see CCDstack defaults to min/max. I should look into how that is calculated.
Greg.
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