HST Data Handbook for WFPC2

Chapter 4:
WFPC2 Error Sources

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4.1 Bias Subtraction Errors
4.2 Flatfield Correction Errors
4.3 Dark Current Subtraction Errors
    4.3.1 Electronic Dark Current
    4.3.2 Dark Glow
    4.3.3 Pointing Information in the Image Headers
4.4 Image Anomalies
    4.4.1 Bias Jumps
    4.4.2 Residual Images
    4.4.3 PC1 Stray Light
    4.4.4 Other Anomalies

This chapter discusses some of the error sources associated with WFPC2 data. Most of the chapter focuses on specific calibration errors, complementing and partially overlapping the discussion given in "Improving the Pipeline Calibration in section 3.5. Other subtle errors and problems which appear in WFPC2 data, but which are not directly related to data calibration, are discussed at the end of this chapter and in chapter 5.

Note that many of the error sources affecting calibrated WFPC2 data can be reduced by either recalibration with newer reference files or post-pipeline improvements, such as correction flatfields (see section 3.5.2). Recalibration can easily be accomplished by simply re-requesting the data from the Archive through OTFR (On-The-Fly Reprocessing, see section 3.4. An alternative is to use StarView to compare the recommended best reference files with the reference file names recorded in the calibrated science headers; if the names differ, it may be worthwhile to re-retrieve the data. However, finding that a reference file has changed since the data were last calibrated doesn't always mean that recalibration is necessary. The decision to recalibrate depends very much on which reference file or table has changed, and whether that kind of correction may affect the analysis. Section 3.4 gives more details on the required procedures.