[Merlin66 (Ken)]

It's interesting to see this discussion on theoretical SNR....
I doubt many AP imagers actually measure the SNR in their images.

In spectroscopy we must measure the actual SNR in our data to comply to the ProAm campaign requirements.
We usually have to target SNR>200

The shot noise contribution is not IMHO affected by the use and summing of subs. The total signal determines the shot noise.
Data of 100,000 photons will have a shot SNR of 316 whether it came from 1 or ten subs.
Also remember that ANY manipulation of the data will add noise. This goes for darks, flats, colour balance corrections etc. etc. etc.

In the SimSpec spreadsheet we use to analyse the performance of the spectrograph Christian Buil presented a method of calculating the anticipated SNR which includes all the factors discussed.

(See also: Howell's "Handbook of CCD Astronomy", Sect. 4.4, p73 gives the "CCD Equation", Budding & Demircan's "Introduction to Astronomical Photometry, Sect 5.2.6 - Noise in photometry, p188, Appenzeller's "Introduction to Astronomical Spectroscopy", Sect. 3.4.2, p78)


(The SNR comments above are based on the original post:
3) All else being equal, then for the very faintest targets, the snr is proportional to exposure time T, but proportional only to the square root of the number of subs N. That means that for very faint targets, it will be hugely better to do ten 3600 second subs than 36,000 one-second subs. It will be better by a ratio of 3600 root(10) to 1 root(36000), or a ratio of six to one. Long subs rule.)