HST Data Handbook for WFPC2

4.1 Bias Subtraction Errors

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All WFPC2 data retrieved from the HST Archive since May 16, 2001 have been through On-The-Fly Reprocessing and are not affected by the bias subtraction errors discussed in this section.

Very early in the WFPC2 mission (January 1994), it was discovered that the first few columns of the overscan region of the WFPC2 CCDs can be positively offset if a strong signal is present in the image itself. These columns were used for the bias level correction (BLEVCORR); the result was an oversubtraction of the bias level, which caused the sky background level in calibrated images to be incorrect. In a few cases, a significant part of the image had negative pixel values. The improvement was implemented in calwp2 version 1.3.0.5 (March 1994) and is automatically performed on any image processed through OTFR.

Somewhat later in 1994, it was also realized that an improved bias level (BLEVCORR) subtraction could be obtained by using separate bias values for odd and even columns. WFPC2 data processed through the pipeline before May 4, 1994 used only a single average value for the bias level subtraction and as a result, a striated pattern with a typical amplitude of a few electrons (a fraction of a DN) remained. Observers working with versions of these images that were calibrated prior to May 1994 (e.g., from data tapes) should therefore consider re-requesting these data from the Archive. Alternatively, the images could be reprocessed manually using calwp2, version 1.3.0.6 or later, unless the signal of the observation is so large that the noise statistics are dominated by Poisson noise. A detailed study of the on-orbit characteristics of bias frames, the overscan region, and superbiases used in the pipeline can be found in WFPC2 ISR 97-04.