General Data Analysis Facilities

The Guide Star Catalog and Plate Solutions

One remarkable product of the Space Telescope project is the Guide Star Catalog (GSC). The GSC contains positions for some 20 million celestial objects, accurate to a few tenths of an arcsecond and about half a magnitude. This catalog is published on two compact disks, and can be read with software in the gasp package in STSDAS.

The STSDAS gasp package is a subset of the GASP (Guide Star Selection System Astrometric Support Package) software available at STScI. To use the gasp package, you need to have a CD-ROM drive available and a copy of the GSC. Because CD-ROMs are not quite standard devices on all computer systems, we only support access to the CD-ROM device on Sun systems at this time. Certain tasks in the gasp package do not use the GSC CD-ROM directly, and can be used on any system.

Tasks in gasp can be characterized as either low or high level. That is, their function is either specialized (low level) or several such specialized tasks are brought together in a higher level task. For example, sgscind searches an index table of the coordinates of the guide star regions, listing those regions that overlap the user's field, extgst extracts only the stars actually within the user's field from a list of regions, and fttostt converts one or more FITS format tables to STSDAS format tables. The higher level tasks are regions, which creates a set of STSDAS tables containing the guide star regions that overlap the user's field, and targets, which includes the functions of regions, and adds the functionality of intrep, which allows the user to determine a plate solution and obtain guide star coordinates for any arbitrary position on an image.

The images and the catalog coordinates interact with each other in this package. In some tasks, it is necessary to know the transformation between the world (celestial) and pixel coordinates.

To understand what the gasp tasks do, it is useful to know a bit about the catalog. The Guide Star Catalog is distributed as a set of two compact disks. The catalog is subdivided into 9537 regions, bounded by circles of right ascension and declination. All regions subtend 7.5 degrees of declination and varying widths in right ascension. Data for each region are stored as separate files organized into subdirectories. Each subdirectory contains all of the regions for one 7.5 degree wide zone of declination. The region files are distributed between the two disks, divided at a declination of -7.5 degrees. The description files and supporting tables are duplicated on both disks. All region files are in FITS table format. One of the supporting files is a FITS table giving the boundary coordinates of each of the regions. Searching this table allows a particular region number to be determined.

The gasp package includes two general purpose tasks, pltsol and intrep. These provide a general-purpose facility for determining plate solutions and transforming between plate and sky coordinates.

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