I mentioned yesterday that I'd seen something odd when reviewing the histograms of some of my master bias frames. While each individual bias frame shows a normal distribution, the integrated frames are bimodal, with a dip between the two modes that deepens with the integration of larger numbers of frames while the second of the two peaks gradually drops.
I thought I'd share the data and see whether anyone else has any ideas on what might be the source of this phenomenon. I'm not trying to solve any particular problem here. I'm just curious as to whether anyone has seen this before or has any thoughts on it.
These bias frames were shot in SGP with a Zwo ASI1600MM-C at 198 gain, 50 offset, -20degC, for exposure times of 0.5 sec (I shot another set at "0.0" which shows the same characteristics). Masters were created from individual frames in PixInsight with the following settings (followwwing David Ault's recommendations:
Here are the statistics from a single frame (original histogram at the bottom, stretched at the top):
10 integrated frames:
20 integrated frames:
40 integrated frames:
80 integrated frames:
160 integrated frames:
300 integrated frames: