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As with the calibration pipelines for the other HST instruments, the specific operations that are performed during calibrations are controlled by calibration switches, which are stored in the image headers as keyword=value pairs. Any given step in the calibration process may require the application of zero, one, or more calibration reference files, the names of which are also found in the image header. The names of the keywords containing the switches and reference file names were introduced in the previous chapter, and the section "Data Flow Through calstis" on page 21-9 will discuss them in detail. It is important to realize that only the calibration-related keywords that are relevant to the particular observation mode will appear in any given data header. Likewise, the path your data files take through the pipeline is determined by the calibration switches set in the header of the raw data, which in turn depends directly on the type of data you have.
A few other general comments are in order. STIS differs from earlier HST instruments in that some of the calibration reference data are obtained contemporaneously with the science observations. These data may be used to refine the calibration process (as with the automatic wavecals), or may require you to replace a default calibration reference file with a contemporaneously obtained one, as in the case of a CCD near infrared (NIR) fringe flat. The details of how these contemporaneous calibration files are used in calstis can be found in "Descriptions of Calibration Steps" on page 21-15. The STIS (and NICMOS) pipelines are also unusual in that they are re-entrant. That is, a user running calstis off-line may elect to reprocess STIS data partially, performing one or more of the intermediate steps without re-exercising the complete calstis pipeline, for instance to perform cosmic-ray rejection, or one dimensional spectral extraction. Refer to "Recalibration of STIS Data" on page 21-29 for the mechanics (and restrictions) of this kind of processing. Finally, as with other HST pipelines, calstis propagates statistical errors and tracks data quality flags throughout the calibration process.