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Main reason Eric is that with longer exposures you can go deeper....get that faint detail that would not even appear on short exposures....but as you know the longer you go the better your mount has to perform...also sky glow comes into the factor, a bit of a balancing act.
I personaly think that 10 x 2 minute images is closer to 5 x 3 minute images but then again you want more then 5 images to stack to give you a higher signal/noise ratio.
I am talking more on behalf of a DSLR.
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