Chapter 27
WFPC2
Error Sources

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 "Post-Pipeline Calibration" on page 26-15. 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 28.

Keep in mind that many of the error sources affecting calibrated WFPC2 data can be greatly reduced, or even corrected entirely, by either recalibration with newer calibration reference files or post-pipeline calibration. The routine pipeline calibration initially performed on a dataset before it is deposited in the Archive and delivered to the observer uses the most current, WFPC2-specific calibration reference files available at the time of processing. If there appear to be unusual features in the data, or if the analysis requires a high level of accuracy, it is important to determine if a better set of later calibration reference files can be used to recalibrate the data. Finding that a calibration reference file has changed since the data were calibrated doesn't always mean that recalibration is necessary. The decision to recalibrate depends very much on which calibration image or table changed, and whether that kind of correction is likely to affect the analysis. Chapter 26 gives more details on the required procedures.