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Calibration using overscan regions
I get a significant amount of variation in the mean value of bias frames from my Apogee U16M camera. I've seen as much as 16 ADU difference between frames with an average value in the low 1200s, so this variation is nearly 1.5%. It may not seem like much, but that corresponds to a difference of 40 electrons. Compared to read noise of around 8 electrons that's a big deal!
Looking at the overscan region in the frames, the median value appears match the variations in average bias fairly closely, which is what you'd hope to happen. So, I think that using the overscan region to calibrate all my subs is going to be worth doing.
Does anybody else use the overscan region for calibration? I'd be interested in comparing notes...
BTW, I've noticed similar variations in average bias with my Starlight Xpress H-18 camera in the past. Unfortunately, the driver for that camera doesn't provide access to the overscan region, so I just had to live with it.
Thanks,
Rick.
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