How many exposures are too many to reach a given signal level?


Assume you want to take N minutes total exposure time
How many exposures should you take to get N minutes total integration time?
One long exposure might be good except for saturation and blooming or excessive dark current and maybe aircraft/spacecraft trails or poor tracking
More exposures that are short may degrade the s/n due to cumulative read noise effects


Dark shot noise, dark fixed pattern noise, Light fixed pattern noise and sky background are all ignored
Dark shot and Dark fixed pattern noise addressed via cooling
Light fixed pattern noise addressed by flat-fielding
Background is suppressed by imaging in dark skies with emission line filters
All that is considered is Signal Shot noise and Read Noise
Noise = SQRT ( signal + read_noise^2)
S/N = signal / noise

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Part 2: Using multiple subexposures for attaining the same S/N as a single exposure 

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