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I just ran the numbers on some red data for NGC1097 taken under darkish skies. I was using 15 minute subs which were still nowhere near sky limited (I'd need to do about 1 hour subs to get all the way there...)
Comparing the SNR for a 15 min sub vs a stack of 3 x 5 mins the SNR is 4% better for the bright galaxy core, 20% better for a bright/non-core galaxy area and 34% better for a dimmer area on the outskirts of the galaxy.
To get the same SNR as a 15 minute exposure in the dim part of the galaxy requires stacking more than 5 x 5 minute subs. For the bright/non-core area you need to stack more than 4 x 5 minute subs to get the same SNR. For the galaxy core, a stack of 4 x 5 minute subs has better SNR than the 15 minute sub.
Just a random example, but I think it shows that read noise can have a very significant effect on SNR. It also demonstrates nicely that this effect is much greater for dim targets.
Cheers,
Rick.
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