Pipeline Processing Overview

Science data taken by STIS are received from the Space Telescope Data Capture Facility and sent to the STScI pipeline, where the data are unpacked, keywords extracted from the telemetry stream, and the science data reformatted and repackaged into raw (uncalibrated) FITS1 files by the generic conversion process. All STIS data products are FITS files. The vast majority of the STIS data products are two-dimensional images that are stored in FITS image extension files as triplets of science, error, and data quality arrays. FITS image extensions offer great flexibility in data storage and allow us to package related science data that are processed through calibration as a single unit together into one file. The uncalibrated FITS files are passed through calstis, the software code that calibrates the data, producing calibrated FITS files. For more details on STIS data structure and the naming conventions for the uncalibrated and calibrated data products, see Chapter 20 of the HST Data Handbook.

Calstis performs the following basic science data calibrations:

• Basic, two-dimensional image reduction producing a flat-fielded output image (rootname_flt.fits), which, depending on whether the data are from the MAMA or the CCD and whether they are imaging or spectroscopic data, includes the following: data quality initialization, dark subtraction, bias subtraction, non-linearity flagging, flat fielding, and photometric calibration.
• Two-dimensional spectral extraction producing a flux-calibrated, rectified spectroscopic image (rootname_x2d.fits for MAMA data, _sx2.fits for CCD) with distance along the slit running linearly along the Y axis and wavelength running linearly along the X axis, for spectroscopic first-order mode data.
• One-dimensional spectral extraction producing a one-dimensional spectrum of flux versus wavelength (rootname_x1d.fits for MAMA data, _sx1.fits for CCD), uninterpolated in wavelength space, but integrated across an extraction aperture in the spatial direction, for first-order and echelle spectroscopic data.
• Data taken in TIME-TAG mode are corrected for the Doppler shift from the spacecraft motion and output as an uncalibrated event stream by the pipeline in a FITS binary table (rootname_tag.fits). The time-tag data stream is also integrated in time to produce an uncalibrated ACCUM mode image (rootname_raw.fits) which is then passed through standard calibration. The odelaytime task in STSDAS can be used off-line to correct the TIME-TAG spacecraft times to heliocentric times.

In addition, calstis performs two types of contemporaneous calibrations:

• For CCD exposures which have been CR-split or when multiple exposures have been taken, calstis combines the exposures, producing a cosmic-ray rejected image (rootname_crj.fits) which is then passed through subsequent calibration (e.g., spectral extraction).
• For spectroscopic exposures, calstis processes the associated wavecal exposure (see Routine Wavecals) to determine the zero point offset of the wavelength and spatial scales in the science image, thereby correcting for thermal drifts and the lack of repeatability of the mode select mechanism. Whereas the uncalibrated science data are stored in the rootname_raw.fits file, the accompanying wavecal observations are stored in the file rootname_wav.fits.2

Figure 15.1 through Figure 15.3 show example output from the calstis pipeline. The calstis program propagates statistical errors and tracks data quality flags through the calibration process. calstis is available to users in STSDAS, so they can recalibrate their data as needed.3 Recalibration may be performed in its entirety in a manner identical to the pipeline calibration by using calstis, or modular components of calstis, such as basic two-dimensional image reduction (basic2d), two-dimensional spectral extraction (x2d), one-dimensional spectral extraction (x1d), or cosmic ray rejection (ocrreject). The calibration step that calstis performs on a given set of science observations depends on the nature of those observations.4

As of spring of 2001, calibrated data products for STIS are available through On-the-fly-reprocessing (OTFR), which replaces On-the-fly-calibration (OTFC). The OTFR systems starts with raw telemetry products and converts these to FITS files and adds the latest instrument calibrations. This change is transparent to the user.

Figure 15.1: Two-Dimensional Rectification

Observers can retrieve HST data by using StarView (see http://archive.stsci.edu/starview.html) or the HST archive WWW inter face (http://archive.stsci.edu/) to select datasets. One can choose where the data are written: on the archive computer staging disk for copy using anonymous FTP (HOST option), directly sent to a home computer (NET option), or for very large requests, sent on a magnetic tape (TAPE option). To be able to retrieve HST data, one must be a registered archive user (see http://stdatu.stsci.edu/registration.html).

Figure 15.2: Cosmic Ray Rejection

Figure 15.3: One-Dimensional Spectral Extraction