These two commands from a shell created the following image.
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raspistill -t 10 -md 3 -bm -st -ex off -ag 16 -dg 1 -ss 20000000 -awb off -awbg 2.1,2.4 -drc med -o HQ1_200625_0315.jpg convert HQ1_200625_0315.jpg -level 0.5%,10%,1.2 HQ1l_200625_0315.jpg
Banding is especially visible in the middle right side of the image. Note that for -ag 16 banding is visible in other images even if none of ‘ -awb off -awbg 2.1,2.4 -drc med’ is specified.
I put the camera in the dark and made two images ~5 hours apart using
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raspistill -t 10 -md 3 -bm -st -ex off -ag 16 -dg 1 -ss 20000000 -o black.jpg
‘black’ jpgs in same environment 5h apart
I averaged over all pixels in each line to form simple line averages of each of RGB values in black.jpg. While no large amplitude single frequency seemed persistent top to bottom there is a very-regular 4-line frequency of brightness oscillation albiet with very low amplitude of <0.1 (of 0-255); e.g. average blue value for line numbers 1400-1600 is shown just below. (In images with ‘-drc med’ the amplitude of the 4-line oscillation looks about 4 times larger).
Sky photos are taken ~50 feet from house in a very rural setting with no outdoor lighting (nearest ‘street light’ is ~1/4 mile away) where it is powerd by 5v from a 16800mah battery pack. HQ and Zero are in PVC enclosure and Zero board LED is off.
Why does the banding occur?
Why does the low-amplitude 4-line brightness oscillation occur?
I need -ag 16 for short 20s exposures to have enough light to show the Milky Way: can I use the max analog gain (16) but still eliminate transient banding?
Increasing ‘black point’ post processing can eliminate banding but though discrete stars remain, the Milky Way mostly disappears.