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Marc,
Seems to me that dithering will naturally eliminate any pix not eliminated by dark frame subtraction (which I never find to be perfect). During registration the stars are aligned which now means that the remaining hot pix are now not aligned. The sigma reject process thus easily picks out those pixels. If you have not dithered then aligning the stars also aligns the remaining hot pix and the sigma reject will not see those pix as statistical outliers. Anyway, that is how I think it works.
Peter
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