Space Telescope Science Institute
WFC3 Data Handbook 2.1 May 2011
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WFC3 Data Handbook > Chapter 5: WFC3-UVIS Error Sources > 5.2 Bias Subtraction

Bias 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 the bias level 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.
When science data are obtained in subarray format, the requisite dark and flat-field corrections will be obtained from the full-frame calibration files, extracted from the appropriate subregion. The superbias correction will also be extracted from full-frame 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.

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