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Theoretically each sub should have a dark and a flat taken at the same time, added to each before stacking. This will usually ensure that the camera and chip errors can be cancelled out as accurately as possible leaving only the picture and the noise in the sub. In practice this is usually not necessary as has already been said and described.
Then as the noise is random stacking will increase the picture level much more than the noise thus improving the S/N ratio and consequently allow more latitude in processing the final picture.
Barry
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