Nicely explained, Dave!
I was poking around my narrowband Helix data today and noticed that some of the faintest nebulosity that I'm able to see clearly in the stacked images corresponds to only 25e- in a 30 minute exposure. When you compare that to the 40e- bias drift I have seen from the camera I think it makes a convincing case for overscan calibration, especially when working with dim targets.
Cheers,
Rick.
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