[Shiraz (Ray)]

Probably normal Paul.

The sub signal can only have values in steps of 16, so eg in your bias, the values can only be .....288,304,320,336.... etc. At gain139, these 16ADU steps are equivalent to single electron changes in the signal, so there is not any point in getting finer resolution. Shot and read noise, will show as a random smattering of noticeably lighter and darker pixels - all of the noise affected pixels can have only a limited number of values about the signal mean, so you see a few distinctly lighter and darker pixels and nothing with in-between values.

Stacking enough subs fills in the intermediate values in the final image, but the data distribution in the individual subs is very sparse at high gain (eg 139). Of course that means that standard non-linear rejection methods have to be used carefully. Sigma rejection in particular can throw away lots of data and leave distinct holes in the resulting stack histogram by rejecting a quite large number of pixels that have the same value (eg, if you ask it to reject 5% outliers, it may not be able to do so if the steps in the sub exposure histograms only give the option of rejecting say 1%,3% or 10% - it will reject 10%, which may make a noticeable difference to the data). I have found that min/max rejection works best on dithered short subs - throwing away a fixed percentage of the data at the top and bottom gets rid of almost all cosmic ray events and most hot pixel leakage without disturbing the real data distribution too much.

Interesting chip and, at high gain, completely different to a normal CCD in many ways. cheers Ray