Can you explain the cause of the difference between one long exposure and stacking short exposures? I'd naively expect one 10s exposure to produce equivalent data to 10x1s exposures.
What I suspect nawitus is referring to involves the read noise of a CCD. There is always some low level noise involved in reading out a CCD. If this noise dominates, which may be the case for amateur astrophotography, it can explain the noise difference between a single exposure and stacking an equivalent number of shorter exposures.
If for the sake of illustration, the read noise is 10 counts and the signal for a 1 second observation is 10 counts, you will have a signal to noise of 1. Stacking 10 of these will give you a signal to noise of ~3 (square root of 10). Conversely, for the 10 second observation, you will have 100 counts for a signal to noise of 10.
However, there are other sources of noise, and instruments at major observatories are generally designed such that the read noise is not the dominant source of error either through more sensitive detectors or gathering more light (a bigger telescope or larger pixels) or some other means. If even a short exposure constitutes 1000000 counts, the difference between one 10s exposure and ten 1s exposures will be negligible.
Exactly the sort of thing I was looking for. Thanks for the explanation, it makes perfect sense. I hadn't realized that there would be sources of noise that hit you for each read.
