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WFC3 Data Handbook V. 4.0 |
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5.2 5.2.1 Bias Calibration IssuesBias reference frames are acquired daily for scientific calibration purposes and for monitoring the detector performance. Multiple bias frames are combined together on roughly a monthly basis (120 frames) into a reference superbias image. The combination removes the cosmic rays accumulated during the readout time and enhances the signal-to-noise ratio of the final results. calwf3 performs the bias correction in two steps (see Section 3.2.4): BLEVCORR subtracts a global bias level as measured from the overscan region and BIASCORR subtracts a reference bias image. The location of the overscan regions in a raw image varies, depending upon the type of readout that is performed. The overscan regions are used to monitor the instrument as well as provide a measure of the bias level at the time of the image. This overscan-based bias level is subtracted from the raw image, normally through the BLEVCORR step in the WFC3 calibration pipeline. Residual two-dimensional bias structure is removed via the superbias reference file correction applied via the BIASCORR step.