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There's no effective difference between summing and taking the average (mean) so long as you do it with reasonable numeric precision (like most decent image processing packages will). You can just multiply an averaged stack by the number of subs to recover the sum.
There is a difference between averaging and doing a median combine. A median combine will reject things like satellite trails better but will give you a poorer signal to noise ratio (roughly 20% worse).
Cheers,
Rick.
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