[PKay (Peter)]

Once again I am trying to put an understanding of difficult concepts into words.
Please feel free to contribute if I am in error.

I was reading PI documentation (in the Image Integration process) that is titled ‘How many images?’.
With regards to improved SNR.
It gives a graph that says 30 images is ideal, and beyond 50 there is little to be gained.

My understanding. There are two concepts involved here.
Firstly say a pixel can have a count from 1 to 10 (for simplicity).

Maximum signal:
Take any target for example an average star.
It is sending us photons at a certain rate (brightness) that are then captured in pixels.
Once a pixel is full (a value of 10) there is no more signal to be found.
For example a single image can capture a bright star. By taking another 20 images, no new detail will be revealed. After integration, the average or mean would will still be 10.

Atmospheric Noise:
Take the above example and call the ‘centre most’ pixel capturing the star Pixel01.
Move to the left (say 50 pixels) where there is no signal and call that Pixel50.
Pixel01 will capture photons on average faster than Pixel50.
(This is under the assumption that the target is bright enough to be above the average noise level).

Example: Take several exposures (say 100 sec).

Exposure 01:
Pixel01 = 10.
Pixel50 = 5.

Exposure 02:
Pixel01 = 10
Pixel50 = 5

If the same values were repeated for 10 or 20 or whatever images:
In this imaginary scenario, a single image would capture all the information that you can.
This is because the average or median values would be the same, for signal and noise.

Another example:
If pixel50 (the noisy one) varied about ie: 4,5,6,3,8,2 etc
It would still reach an average (or median) value very quickly, and the theory says that is after 30 images.

It can also be noted, at this stage, clever maths can recognise a pixel that varies greatly from the average (or median) value. This is how noise is isolated.

So in conclusion, what does this mean, and how do we use it?
I believe that once again, it is the choice of correct exposure time that rules, and that is more important than the number of subs going beyond 30 (50 tops).
Better to take 30 - 50 exposures at the correct exposure than 200 (at any setting).