Data Formats

STSDAS Images

All HST data sets are stored in i Generic Edited Information Set (GEIS) format. Like IRAF format, images are stored in two files:

An ASCII header file with default file extensions of .hhh (although other extensions are used).
A binary data file with default extension .hhd.

The two biggest differences between OIF and GEIS files, from the user's point of view, are:

GEIS format can accommodate multiple images in a single file (each of these images is called a group)
Some image descriptors are found in the binary data file, in an area called the Group Parameter Block, rather than in the header file.

Data Structure Overview

GEIS data files have a structure that is based on, but significantly different from, the FITS group format.*1 The GEIS group format was designed to accommodate data such as time-resolved spectroscopy, where many small data arrays share common header information, but also have a certain amount of array-specific information. For example, the FOS and HRS instruments aboard HST have observing modes for time-resolved spectroscopy which can produce hundreds of spectra, all of the same size (in pixels), and all for the same observation set. If these were each stored as separate files, the overhead in file size and management would be horrendous. These data are, therefore, combined into single observation sets in GEIS group format. In this case, the descriptors that are common for all the spectra (such as the size of each spectrum, the instrument configuration, etc.) are stored in the header file, but the group-specific descriptors (e.g., the offsets for the GIMP correction) would be stored with the binary data.

Parameters that are specific to a subset of the data are called group parameters, and they are stored in a reserved area of the binary data file called the group parameter block. Each group*2 of binary data has a reserved area for the group parameter block, allowing each group to have different values for group-dependent parameters. A representation of the structure is shown in Figure 3.3.

Note that in GEIS format the group parameter block follows each group of binary data, and that the group parameters can be any supported data type. But as with FITS groups, each image must be of the same data type.

Structure of a STSDAS Group Format File

Image Descriptors --Header Keywords

Header files are composed solely of ASCII text, and can be printed using standard text-handling facilities (type, page, etc.). However, the lines in GEIS headers are always 80-character, fixed-length ASCII records. Any change you make in the text must not change the length of a line from 80 characters. The images.hedit task will preserve the 80-character line width. There are also tasks such as chcalpar (in the ctools package) which will change the calibration switches if you wish to recalibrate a dataset.

Always edit headers using tasks like hedit. Editing headers with a text editor may corrupt the files by creating incorrect line lengths.

Group Descriptors--Group Parameter Block

Because the actual values of group parameters are stored in the group parameter block with the binary data, you must use the tasks images.hedit or toolbox.headers.groupmod. Similarly, you cannot view the values of group parameters simply by looking at the ASCII text header. Rather, you must use a header listing tool such as images.imheader, which will list the full set of header parameters, including both the main header and the group parameter block. For example, the first several lines of the file hrs.hhh might appear as shown in Figure 3.4.