Using the Overscan Bias Correction

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The overscan bias correction provides a highly accurate way to correct the bias signature of a CCD image. The value of using the overscan method is that it corrects the bias in each row of the image at the time it was read from the CCD camera. To estimate the electronic bias of the CCD, it may be "overscanned" by adding dummy reads of the electronics without the presence of physical pixels. Repeating the dummy reads for the entire chip adds 1 or more lines of data which might be generally called overscan lines. Overscanning the horizontal register adds columns to the image that may be used to perform a column bias correction. Reading dummy rows before or after the physical CCD can be used to perform a row bias correction. Since the CCD can be considered a horizontal state machine, significant bias structure usually appears along columns, as a function of row number. For this reason, the column bias method is most widely used and it is described below.