This package provides access to the HST Guide Star Catalog on compact disk (CD-ROM). To do so, it requires special hardware, software, and account privileges. However, many of the tasks do not depend explicitly on the catalog and may be used regardless of the resources and system configuration. Currently, the only configuration supported is via the Sun CD-ROM hardware and drivers under Sun-OS 4. This permits the disks to be mounted as part of the normal Unix file system (NFS). If you don't know how to make the CD part of the file system, see the file gasp$README_CD.
You need not know the file names of the actual catalog files, but you do need to know the mount point of the disks. That is, the path name to the top level on the disks. By default these are assumed to be /data/gsc1 and /data/gsc2. These need not be mounted on the same host on which you are running IRAF, but may be accessed through IRAF networking.
To use tasks that access the CD, you must first make known to gasp the mount point (root path name) of the catalog. The two parameters cdrom_north and cdrom_south for the task sgscind specify the paths to the two volumes of the GSC. If the disks are not mounted on the local host, they may still be available through IRAF networking if they are mounted on a host accessible via IRAF networking. To specify a remote host, prepend the host name on which the disks are mounted to the mount point. Note that you still must have account access to the host on which the disks are mounted.
The tasks in the package may be characterized as either low or high level. That is, their function is either specialized (low level), or they act indirectly, calling other tasks (high level). Low-level tasks include sgscind, which searches an index table of the coordinates of the guide star regions, listing those regions that overlap the user's field; extgst, which extracts only the stars actually within the user's field from a list of regions; and copyftt, which copy one or more FITS format tables to local FITS files with correct extensions.
High-level tasks are regions, which produces a set of STSDAS tables containing the guide star regions overlapping 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.
This package relies on the interaction between images and catalog coordinates. In some tasks it is, therefore, necessary to know the transformation between the world (celestial) and pixel coordinates.
The Guide Star Catalog is distributed as a set of two compact disks (CD-ROMs). The catalog is divided into 9,537 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 every region 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 Northern regions on Volume 1 and the Southern regions on Volume 2. The description files and supporting tables are duplicated on both disks. All region files are in Flexible Image Transport System (FITS) table format. One of the supporting files is a FITS table giving the bounding coordinates of each region. Searching this table allows a particular region number to be determined.
The gasp package includes two general purpose tasks: pltsol and intrep. These tasks provide a complete, general-purpose facility to determine plate solutions and transform between plate and sky coordinates.