Bob,

Lots of this traditional logic on flats and flat darks and constructing optimized darks from a master dark and a master bias scaled to exposure time. comes from years of experience from CCD cameras. My camera is CMOS and the fact that every pixel has an amplifier makes bias frames at very short exposures very nonlinear and very unrepeatable....an even worse problem. I have a couple of data sets acquired with the skyflat script with apparently excellent looking sky flats, that unfortunately these under correct and introduce artifacts from the sensor into my final image because I use an averaged dark that only matches the median exposure time of a set. And if I attempt to acquire Bias frames so I can created "optimized" darks, it leaves these chip patterns in my lights that come from CMOS Bias frames. Lots of recent info is out there written about CMOS chips and Bias frames and it's pretty unanimous that bias frames with optimized darks are not good.

For example, on my last image, the skyflats had a total variation in exposure time of 30-60% time from the first in a group of 7 to the last. While that is only 2-3 seconds for most flats (except the one taken in very dark skies, at t total exposure of 4 seconds this is a huge relative variation in the stacked flat, given that I only subtract one average dark frame close to the median exposure time. and given a flat is used multiplicatively to calibrate the light frame, this residual dark current which carries chip pattern noise amplifies the effect in the calibrated light frame. And I see that in every target. One "easy" solution is to do panel flats, but my observatory is not geometrically configured well for panel flats and good luck finding an illuminated panel that big for my CDK20.

A significant way to reduce that noise in sky flats is to have a dark for each flat that matches it's exposure time. Hence, the need to expose for a fixed time once the correct exposure range is found for a flat (almost like treating a sky flat like a panel flat). Over the time of acquiring a small set of flats for a filter, the sky variation will introduce much less noise to the flat and importantly no pattern noise from a mismatched dark. And even though the flats will have differing illumination, this is normalized out in the median stacking equation and carries no pattern noise. Bottom line: lower pattern noise to the final image if the dark and flat are exactly the same exposure time. The Old CCD Chips had very few patterns except the occasional linear defect. The New CMOS chips have extensive patterns that look like checkerboards.

I've acquired many sets of ACP sky flats in groups of 7. Once a filter/rotator position starts acquiring, the time increase/decrease from the first to the last flat in the set is 30-50%. I am sure that of I started acquiring a group of flats at 50% ADU, the final flat in that group would be 25% ADU or greater. Hence the question whether the logic to find the starting ADU for a group is coupled to the logic to change the exposure time. For instance, if I set the ADUAcceleration_AM and ADUAcceleration_PM factors to1.0, what happens?

I can show you more of the effect of what's happening. For now I am looking generate a test set of flat data directly in Maxim to prove out my observations. But ultimately I'll need a solution because I can't process any images due to the flats being off for my equipment combination.