Bias Frames
The WFPC2 bias correction is performed in the pipeline in two steps: a pedestal level is removed and a bias image subtracted. The pedestal level is determined from the overscan columns in each science image; the specific values subtracted are documented in the bias-even / bias-odd science image header keywords. However, the value of the pedestal can also vary with position across the chip. Therefore, after the pedestal correction is performed, the pipeline removes any position-dependent bias pattern by subtracting a bias reference file. This reference file is typically generated from a stack of 120 bias frames (CCD readouts without an exposure); new bias reference files are usually installed in the pipeline about once a year.
