|WFC3 Data Handbook v.4|
5.2 5.2.1 Bias CalibrationBias reference frames are acquired frequently for scientific calibration purposes and for monitoring the detector performance. Multiple bias frames are stacked into a reference superbias image, removing the cosmic rays accumulated during the readout time enhancing the signal-to-noise ratio of the final results. calwf3 performs the bias correction in two steps (see Section 3.2.5 and Section 3.2.6). The first consists in subtracting a global bias level, measured from the overscan regions, which provide a measure of the instrument bias level at the time when the images are taken. The location of the overscan regions in a raw image varies, depending upon the type of readout that is performed. This overscan-based bias level is subtracted from the raw image through the BLEVCORR step in the WFC3 calibration pipeline. In the second step, residual two-dimensional bias structure is removed via the superbias reference file correction applied via the BIASCORR step.When science data are obtained in subarray format, the requisite dark and flat-field corrections are obtained from the full-frame calibration files, extracted from the appropriate subregion. The superbias correction is also extracted from full-frame 4-amp readout superbias files (if the subarray resides entirely within a single detector quadrant). Tests have shown that this does not degrade the quality of the dark, flat-field or bias corrections as compared to full-frame data. For subarrays that span detector quadrants, special superbias files must be constructed from individual bias frames read out through the same amplifier as the subarray. For example, a single-chip readout (the largest possible subarray) read out through amp A must be calibrated with a superbias constructed from single-chip bias frames read out through amp A; similarly, science data read out through amp B must be calibrated with a superbias constructed from bias frames read out through amp B.