21.1 Pipeline Processing Overview

Conceptually, calstis is several pipelines in one, reflecting the complexity and diversity of STIS observing modes. Your STIS data will have been calibrated to different levels, depending on their nature:

• ACQs, ACQ/PEAKs, and all available-mode data are not calibrated by calstis; you will get only the raw data from observations taken in these modes.
• All other science data are processed through basic two-dimensional image reduction (basic2d), which includes such things as bias subtraction, dark subtraction, flatfielding, and linearity correction. In the case of CCD CR-split or repeatobs data, your data will also be passed through cosmic ray rejection.
• All spectroscopic data (exclusive of slitless spectra or long slit echelle data) are then passed through spectroscopic reduction, to produce flux and wavelength calibrated science data. In the case of long slit data, a two-dimensional rectified image is produced. In the case of echelle data, a one-dimensional background subtracted aperture extracted spectrum is produced.
• Data taken in TIMETAG mode are output both as a raw uncalibrated event stream (to a FITS binary table), and as an ACCUM mode image which then passes through standard calibration.
• For MAMA data, the input raw data format is 2048 x 2048 (so called "highres" pixels), while the calibrated data are binned by the pipeline to 1024 x 1024 native format pixels (see "LORSCORR" on page 21-22). See Chapter 20 for the naming conventions of the various input, intermediate, and output calibrated files.

  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 21-30 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.