Hi,
I did an Excel sheet about Signal / Noise relation to do some test. I have also implemented how optic vignetting lower the S/N in the corners.
It's very simplyfied but still it give you a good idea how readout, sky background, object strenght, calibration nosie works together.
You can download the Excel sheet on the page:
http://astrofriend.eu/astronomy/tuto...ng.html#part06
/Lars