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After all the physical pixels have been read out from each row of each quadrant, the electronics continue to cycle. These cycles create additional overclock `pixels' which are simply reading out the electronics' noise+bias values. These overclock pixels are virtual pixels in that they do not really exist but are recorded in the raw FITS image in the same way as real pixels are. THE FITS keywords IxMINCOL, IxMAXCOL, IxMINROW, IxMAXROW (where x is A, B, C, or D) tell us where the real pixels are located in the image.
The number of overclock pixels can be changed by adjusting the electronic sequencers. The number of overclock pixels per row for ACIS-2C is currently set at 17.
The overclock pixels are used to make adjustments to the bias level on small temporal scales. Thus if the bias level changes uniformly across the entire CCD (eg if the CCD temperature fluctuates), we use the average overclock level to adjust for this without having to capture a new bias image.