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25.1 Data Files and Suffixes

 HST data files are generally distributed in FITS format, either via tape or electronically from the Archive, and need to be converted into the so-called GEIS format using the STSDAS task strfits. Users not familiar with GEIS format or the strfits task should consult Chapter 2. After conversion, a directory listing (type dir within IRAF) will show that the files all have a nine-character rootname and a three-character suffix (formerly called the "extension"). For each instrument, this suffix uniquely identifies the file contents. The WFPC2 suffixes are listed in Table 25.1.

Files whose suffix ends with the letter "h" (e.g., u2850303p.c1h) are ASCII header files. The header files contain keywords that describe the parameters used to take the observation, the processing of the data, and the properties of the image. Files whose suffix ends in the letter "d" (e.g., u2850303p.c1d) are binary data files; these files contain the data as well as the group keywords. A single GEIS image is composed of a header and data pair (e.g., the files u2850303p.c1h and u2850303p.c1d together represent a single image). Note that, in FITS format, header and data are combined into a single file. For network transfers, the name of the FITS file is composed as follows: rootname_suffix.fits, with the letter "f" replacing "h" or "d" in the suffix (e.g., u2850303p_c1f.fits).


WFPC2 Dataset Suffixes and File Sizes

Suffix

File Contents

Approximate Size
Raw Data Files

.d0h/.d0d

 Raw science data

 5 MB

.q0h/.q0d

 Data quality for raw science data

 5 MB

.x0h/.x0d

 Extracted engineering data

 100 kB

.q1h/.q1d

 Data quality for extracted engineering data

 100 kB

.shh/.shd

 Standard header packet containing observation parameters

 30 kB

.pdq

 Text file containing jitter statistics and other data quality information

 20 kB

Calibrated Data Files

.c0h/.c0d

 Calibrated science data

 10 MB

.c1h/.c1d

 Data quality for calibrated science data

 5 MB

.c2h/.c2d

 Histogram of science data pixel values

 220 kB

.trl

 Trailer file

 20 kB

A single WFPC2 exposure is obtained as four images (one image for each CCD chip). GEIS files use group format to keep all of the data from a given HST exposure together in a single image file (see Chapter 2). The data corresponding to each sub-image for the WFPC2 are stored sequentially in the groups of a single GEIS image. The header file for an image contains the information that applies to the observation as a whole (i.e., to all the groups in the image), viewable by paging the header. The group-specific (that is, chip-specific) keyword information is stored with the group data itself in the binary data file; group keywords are only accessible via specialized software, such as the STSDAS tasks hedit or imhead.

WFPC2 images are normally four-group images: the first group is used for the planetary camera and groups 2, 3, and 4 are used for the wide field cameras. If only a subset of chips are read out, only a subset of groups will be present. The group keyword DETECTOR lists the detector used for that group (1 through 4).


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