Did some more testing with my ASI294‘s bias frames. After powering on the cam and doing bias frames with shortest possible exposure I get correct frames with a median value as expected (depending on the offset I chose). But once I do exposures > 1 s (all fine with them) and then do bias frames < 1 s again, these frames are all zero. The magic threshold here is 1 s; above it‘s OK, below it‘s all black. Until I powercycle the cam.
In light frames > 1s I notice an all zero pixel column to the very right of the image. Maybe they are opaque covered „dark pixels“ for doing an automatic dark correction? Some CMOS sensors have this feature. Maybe some sort of auto dark offset correction kicks in once the first exposure was taken > 1s? For astro purposes, I think this should be deactivated in the cam‘s firmware (or whatever code is responsible for that)...
Anyway, I now do only use frames > 1s exposure, be it light (of course
), dark, flat or flat-dark. And I don‘t use bias frames at all. The 294 has so high amp glow, it doesn‘t help being able to scale bias-corrected dark frames as PixInsight can (for example). And in the end, my images look quite OK, so no problem... But I would like to understand that behaviour.
CS, Bernd