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The stacking method also will depend on what the final aim of the image is.
If you are measuring photometry rather than just pretty pics then the only acceptable stacking methods are sum or average. Any sort of weighted stacking will bring artifacts into the counts.
As others have stated, long exposures will have less noise than shorter ones due to readout noise etc. Three are other factors that limit the lenght of exposures though. Thes include trailing due to flexure, background sky noise, blooming, artifacts like cosmic ray hits, satelite trails, planes etc.
I limit exposures to 10mins as it is pretty frustrating to lose a longer exposure due to a plane trail or other artifact.
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