5.2 Bias Subtraction

5.2.1 Bias Calibration

Bias 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.

5.2.2 Bias Correction for WFC3 Subarrays

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.