GSC (A0): Guide Star Catalog field boundaries (19930522)
FILES:
gsc.a0 (9537 small region entries)
gscl.a0 (732 large region entries)
gsc.ls (position-list for locating small GSC regions)
CONTENTS: Hubble Space Telescope Guide Star Catalog Region Boundaries
DATASET: These files come in HyperSky dataset A-2, version 19930522 Availability on HyperSky CD-ROM: Vol. 1 (1993)
DESCRIPTION: These datafiles contain the boundaries for the regions defined by the Guide Star Catalog, originally developed for use with the Hubble Space Telescope. Guide Star Catalog regions are defined by a two-tier system which uses both large and small regions. File GSC.A0 contains the 9537 small region boundaries, while file GSCL.A0 contains the 732 less commonly used large region boundaries.
The Guide Star Catalog itself is organized according to the small regions; the large regions are of secondary importance, serving mainly as a grid for organizing the small regions. Both types of regions cover the entire sky. The large regions are each 7.5 degrees high and organized in bands of Right Ascension extending from pole to pole. Each large region is composed of either 4, 9 or 16 small regions, a number which depends on the number of objects in the local area: the more objects, the more small regions per large region.
A detailed description of the two-tier system may be found in a paper entitled "THE GUIDE STAR CATALOG. III. PRODUCTION, DATABASE ORGANIZATION, AND POPULATION STATISTICS" which appears in THE ASTRONOMICAL JOURNAL, volume 99, number 6 (June 1990).
DATA:
File GSC.A0 displays the number of the small region containing the current position. File GSCL.A0 displays not only the large region number, but also information about the small regions it contains.
As an example the center of the default HyperSky field (in Orion) is identified by file GSCL.A0 as "12 (16: 114)". This identifies large region 12, which contains 16 small regions, the first of which is number 114. Large region 12 thus contains small regions 114-129. GSC.A0 identifies this same position as small region 114. The position of nearby ALPHA Orionis (Betelguese) is the same according to GSCL.A0: "12 (16: 114)", while GSC.A0 identifies the small region as 129.
Comparing the above two example positions on the HyperSky field will give some sense of the size of the large and small GSC regions.
Accuracy considerations: HyperSky area datafiles are capable of storing boundaries to 1-second accuracy in Right Ascension and Declination. GSC regions are, however, specified in the above paper to accuracies of fractions of seconds. Thus, HyperSky region identifications within 1 second of RA or Dec boundaries are subject to error. This should not be problem in general, however, since the portion of the sky within 1 second of a GSC region boundary is a very very tiny fraction of the sky as a whole.
GSC.LS:
GSC.LS is a position-list also provided with set A-2 that lets you set your HyperSky field at the center of any small GSC region just by selecting the region number from the list. See manual page GSC.MLS for more detail on this file.
MAKEGSC:
MAKEGSC is an auxilliary program provided with set A-2 which lets you actually build HyperSky datafiles for GSC small regions if you have access to a GSC 1.0 or 1.1 cd-rom set. If you installed MAKEGSC, an online manual chapter, MAKEGSC.MAN, has also been installed which provides detailed information on using MAKEGSC and its output files.
POSITION-LIST:
Position-list GLS.LS contains the equinox 2000 positions of the field centers of all 9537 small regions in the Hubble Space Telescope (HST) Guide Star Catalog (GSC). Because of the number of regions in the GSC, the list has three levels: a thousands menu, a hundreds menu, and finally individual lists of regions. For example, to set your HyperSky field at the center of GSC region 1234, you would select group "1000 - 1999" from the toplevel menu, then select group 1200 from the second level menu, and finally select 1234 from the resulting GSC region-list.
Remember that this list is only a position-list; it does not display or identify the GSC region at the position you select. A region will be identified at the HyperSky field position only if you have also loaded area boundary file GSC.A0, also supplied in dataset A-2.