Quote:
Originally Posted by gregbradley
Overscan is an area of a CCD not used in the image but records some pixel information. I think its used for internal settings for the CCD. When you see a sensor that has say 15mp of pixels and 14.8 effective the difference is in the overscan area. Its blacked out and the pixels don't receive light.
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A sensor will have active pixels which are exposed to light, some rows and/or columns which are masked so that light doesn't reach them and also some rows and/or columns which are clocked out and read but don't correspond to actual physical pixels. This group of virtual pixels is the overscan region (some of the more sophisticated CCDs in professional use may have multiple overscan regions.)
The data read from the pixels of the overscan region is an overscan value (a component of bias) plus read noise. There's no per pixel bias, no dark current/dark noise and no light signal. The overscan data can be used in the calibration process and this has benefits if you have a camera which shows variation in overall bias over time - it corrects for this variation.
I use overscan calibration with my Apogee U16M which shows a fair amount of bias drift. I think this drift is fairly common but most people don't notice or choose to ignore it. I've seen it on SBIG and SX cameras as well. The difference is typically only a few e-, but if you're going for very dim targets that might be as much as your signal
The only processing packages that I've come across that support overscan calibration are PixInsight and Mira.
Cheers,
Rick.