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Shot noise refers to the random rate of influx of photons (e.g. from a dim star). The dimmer the star, the more the rate of incoming photons goes up and down due to random factors.
So for an exposure which nets you 10 photons, shot noise on one exposure might put that up to 12 the next time, or down to eight the time after, say a twenty percent change.
For an exposure which nets you 10,000 photons, shot noise will put that up to 10,002 ... which is close to zero percent.
To drown out shot noise, collect more photons.
Shot noise is averaged out across multiple exposures too. But then you introduce read noise, where each exposure, as it comes off the chip, has noise added by the CCD amplifier. So you have to balance the length of time for each sub (more = better), number of reads (less = better) with the quality of the mount and the guiding (longer subs require better mounts).
Hope this helps.
Regards,
Tony Barry
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