Space Telescope Science Institute
STIS Data Handbook 2011
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STIS Data Handbook > Chapter 2: STIS Data Structure > 2.4 Headers, Keywords, and Relationship to Phase II

As with previous HST instruments, the FITS header keywords in STIS data files store important information characterizing the observations and telemetry received during the observations, and describe the post-observation processing of your dataset. Each keyword follows FITS conventions and is no longer than eight characters. Values of keywords can be integer, real (floating-point), or character string. Many are HST and STIS specific. Knowledge of the keywords and where to find them is an important first step in understanding your data. By examining your file headers, using either infostis, catfits, imhead, hselect or hedit, you will find detailed information about your data including:
Exposure-specific information such as more detailed timing, world coordinate system information, and fine guidance sensor identification.
Calibration information such as the calibration switches and reference files used by the pipeline and parameters derived from the calibration, such as image statistics and wavelength shifts.
The easiest way to quickly identify the observational parameters of a given dataset is to run the task infostis (see Figure 2.5), which prints selected header information for STIS FITS images. Wildcard characters (*raw.fits) or a file list (@fitslist) may be used for input.
Figure 2.5: Using infostis to Display Header Keywords
cl> infostis o3xi03a1q_raw.fits

Rootname: O3XI03A1Q Detector: CCD
Proposal ID: 7071 Obs Type: IMAGING
Exposure ID: 3.031 Obs Mode: ACQ
Lamp: NONE
Target Name: GD153-1 Aperture: F28X50LP
Right Ascension: 12:57:02.3 Filter: Long_Pass
Declination: +22:01:53.2 Opt Element: MIRVIS
Equinox: 2000.0 CCD amp: D
Gain: 4
Axis 1 binning: 1 CR-split: 1
Axis 2 binning: 1
Subarray: yes

Total Exp. Time: 0.7 sec
Number of imsets: 3
STIS takes CCD and MAMA spectroscopic and imaging data, as well as acquisitions and ACQ/peakups. The keywords relevant for one of these data types will not necessarily be relevant to another. Accordingly, you will find that the header on your particular file contains a unique combination of keywords appropriate for your type of observation. Long definitions for the keywords can also be accessed from the following Web page, which provides detailed explanations of the contents and algorithm for populating the keywords. This site also provides sample headers for different STIS file types:
Keywords that deal with a particular topic, such as the instrument configuration, are grouped together logically throughout the headers. Table 2.5 lists a useful subset of these groups of keywords, indicates the name of the grouping, and where applicable, shows their relationship to the corresponding information from the Phase II proposal.
Table 2.6 summarizes the possible calibration switch keywords, and indicates whether they are present for a particular observation; it also indicates the reference file keyword corresponding to the particular calibration step. A calibration switch keyword is populated with values of OMIT, COMPLETE, or PERFORM. Similarly, Table 2.7 summarizes the reference file group of keywords that identify the files used by the pipeline during calibration (see Chapter 3 for a detailed description of pipeline processing).
Table 2.5: Selected Header Keywords and Relationship to Phase II Parameters
Right ascension of the target (deg) (J2000).
Declination of the target (deg) (J2000).
Indicates the visit and exposure number from the Phase II
proposal: Visit_Number.Exposure_Number.
Number_of_Iterations Time_per_Exposure
Target_Name = WAVELINE or
Target_Name = WAVEHITM
Turn off automated wavecal exposures.
Note: This is not a default or recommended setting and is listed here for informational purposes only.
Time_per_Exposure/CR-SPLIT or
Time_per_Exposure (if not CR-SPLIT)
Exposure time of an individual exposure in a CR-SPLIT or REPEATOBS series, in seconds (a real number).
Table 2.6: Calibration Switch Keywords
Table 2.7: Reference File Keywords
Header Keyword

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