We thought it was time to do some actual calculations with a real galaxy, (a) using a luminance filter, and (b) using an H-alpha filter.
The finished images (not the raw data) can be seen on Ice In Space
here.
The results are extremely illuminating.
For a luminance shot, the sky glow is the overwhelming contributor to the noise, far exceeding dark current or in particular read noise. Consequently, 130 six-minute subs with our gear would be almost as good as 13 one-hour subs.
For a 3nM h-alpha filter on this faint target, the results were completely different. The snr for 130 six-minute shots was 1.2, and leaped to 2.6 with 13 one-hour subs. That vindicates our practice of using long subs with faint narrowband objects.
We examined theoretically what would happen with a zero readout noise camera. It would produce almost no improvement with a luminance filter, because of the overwhelming effect of sky noise. Conversely, with an h-alpha filter, it would produce about a 30% improvement for a 1-hour sub, (not enough to justify our throwing out our 16803) and a 280% improvement for a 6 minute sub, pretty enticing for those who need short subs for practical reasons.
Our grand conclusion is that one has to actually make the measurements and do the arithmetic for the specific case (equipment, location, subject) in mind.
A first draft of the detailed methods, results, and discussion paper is attached. There will undoubtedly be mistakes, even howlers. The most recent version can be seen at DropBox
here.