There is another issue to consider, that I think
Luc Coffier explains well:
Quote:
Are 100 x 1 minute and 10x10 minutes giving the same result?
Yes when considering the SNR but definitely No when considering the final result.
The difference between a 10 minutes exposure and a 1 minute exposure is that the SNR in the 10 minutes exposure is 3.16 higher than in 1 minute exposure.
Thus you will get the same SNR if you combine 10 light frames of 10 minutes or 100 light frames of 1 minute. However you will probably not have the same signal (the interesting part). Simply put you will only get a signal if your exposure is long enough to catch some photons on most of the light frames so that the signal is not considered as noise.
For example for a very faint nebula you might get a few photons every 10 minutes. If you are using 10 minutes exposures, you will have captured photons on each of your light frames and when combined the signal will be strong.
If you are using 1 minute exposures you will capture photons only for some of your light frames and when combined the photons will be considered as noise since they are not in most of the light frames.
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So although you capture the same number of those precious rare photons in each case, unless you are careful, they can be averaged (kappa sigma clipped, etc.) out, as noise. So exposure depth is definitely required. As with most things in AP, there is not a firm rule!
Also, obviously, the read noise in the multiple shorter frames is a larger fraction of the total noise, but I think that is already assumed knowledge in this discussion.