This post is an initial assessment of a number of different CMOS cameras in which flat fielding is a particular issue.
Some months ago, a friend returned a Kepler 4040 camera, which stubbornly refused to flat-frame effectively. After some head scratching, the proposed solution was to ensure the flat frames had the same duration as the light frames…which was not met with enthusiasm.
Rolling on to 2021 - I have a new CMOS 4/3-inch camera with an IMX492. It is a much lesser sensor than the 4040 but it too refuses to flat frame effectively.
To my astonishment, I found a 1-second exposure with an EL display had a different flat-frame pattern to a 15-second one with a ND filter over the lens. There were dark vertical bands on either side and the width of these changed with the exposure duration. Both had median values about 2/3rds along the histogram. Using the 15-second flats to calibrate my 900-second NB exposures did a better job than the 1-second. Still not perfect, so I’m going to rig up a dimmer light source and see how far I can go. (I use darkflats in all cases, rather than bias and all traces of amp glow were removed… and everything at -15C)
So, - the dark frames themselves look evenly ‘illuminated’, so this is not an offset thing, it has to be a gain thing. From my electronics background, I can only think that the sensor amplifier elements are temperature senstive and the localized heating changes with exposure duration (as the readout speed in all cases would be the same). If true, and I cannot be sure if is a design thing, or due to manufacturing variation, - this is a real PITA for astrophotographers.
Has anyone had similar experiences and thoughts?