Guide Star Catalog Background Information

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Introduction

The Catalogs and Surveys Branch of the Space Telescope Science Institute has been digitizing the photographic Sky survey plates from the Palomar and UK Schmidt telescopes to produce the GSC and DSS. These catalogs support ground and space-based telescope operations and provide a valuable scientific resource to the astronomical community.

GSC-I was constructed primarily to support the operational requirement of the Hubble Space Telescope for off-axis guide stars. HST achieves its pointing and stabilization by the use of pairs of guide stars in the range 9.0 - 14.5 mag. The criteria for selection of guide stars are set, in part, by the small field of view of HST's Fine Guidance Sensors, by the sizes of science instrument apertures, and by the precision of the inertial system provided by the gyroscopes.

The initial strategy (c. 1979) was to create a production astrometry system driven by the HST observing schedule. Two microdensitometers would be used to scan the specific fields of photographic plates for which guide stars were needed, and subsequent reductions would provide a list of possible guide stars. These plans would have required a three shift per day operation over the operational life of HST, resulting in marginally adequate throughput for normal observing and slowing response time to targets of opportunity.

To overcome the problems associated with this initial strategy, we adopted the approach of scanning all the plates once at the beginning of the program, archiving the digital images, building a star catalog, selecting possible guide stars from the catalog, and developing utilities to solve operational problems such as identification of faint targets. To save time, this plan was based on scanning the photographic plates with
50um (3.4") samples. This would have required additional fine scans of guide star candidates and targets at 25um sampling to determine more accurate positions for observations requiring better astrometry.

Optimization of the microdensitometers for full plate scans, combined with a delayed HST launch, led to the final plan to digitize all the plates at 25um sampling, eliminating the need for partial rescans of many fields and leading to the construction of GSC-I with its ultimate form and precision. Early design studies also featured measuring at least two plates per field to provide proper motions and colors. The final design minimized proper motion errors by acquiring recent-epoch plates and minimized magnitude errors by using passbands closer to that of the HST Fine Guidance Sensors.

Production scanning began in 1985 and continued through 1988. Catalog construction was performed in parallel with the scanning. It proceeded in stages as production software became available, first for sky-following and object inventory, and later for classification plus photometric and astrometric calibrations.

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GSC 2

The CASB group at STScI and the Astrometry Group of the Osservatorio Astronomico di Torino , in collaboration with the Astrophysics division of ESA, the GEMINI project and the ST-ECF are working on a new version of the GSC. This will use both epochs and multiple bandpasses of the survey plates in order to obtain proper motion and color information in addition to accurate coordinates, magnitudes and classifications for all objects down to at least 18th magnitude. A database called COMPASS is being constructed to hold the GSC-II data using Objectivity, a commercial Object-Oriented database system.
The astrometric reference catalogs are the ACT and TYCHO2, and photometry will be based upon CCD sequences obtained for each plate. This project is expected to be completed by 2002.

GSC 2.0/GSC2.1
Preliminary catalog release of the GSC 2 to GEMINI and Astrophysics division of ESA.

GSC 2.2
First public release of the all-sky GSC 2 catalog. This preliminary catalog contains positions, classifications and at least 2 magnitudes for all object brighter than 18.5 in photographic F and 19.5 in photograhic J. Since bright objects are heavily overexposed on Schmidt plates, we have replaced the bright stars with those from the Tycho-2 catalog. The only proper motions provided are those from the Tycho-2 catalog. .

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GSC 1.0

The original version of this catalog was created to support the identification and use of Guide Stars for the pointing of the HST. It is based on the Palomar Quick-V survey in the northern hemisphere and the SERC-J survey in the south. This catalog contains objects in the magnitude range 7-16 and the classification was biased to prevent the use of a non-stellar object as a guide star. The photometry is based on a photoelectric sequence (9-15th mag) near the center of each Schmidt plate. Only stellar photometry was performed so magnitudes of galaxies in this catalog are systematically in error. Astrometry was determined using the AGK3, SAOC or CPC stars depending on plate declination. Although the relative astrometry (required for HST) is about 0.3 arcsec, there are known systematic errors near the plate edges of about 1-2 arcsec.

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GSC 1.1

A revised version of the GSC was created with the cooperation of the HIPPARCOS/TYCHO consortia. As part of the preparation for this mission, an input catalog containing the best available data for all stars to at least 11th magnitude was created. By merging these data with the GSC, we were able to add the missing bright star data to provide a complete all-sky catalogue down to the GSC limiting magnitude. In addition, many of the false objects due to artifacts around the halos and diffraction spikes of the bright stars were identified and corrected as well as a number of reported errors. A number of astrographic plates centered on southern hemisphere bright stars (m<3) were also processed and added to the catalog.
This is the version currently used by HST operations and published to the community.

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GSC 1.2

In collaboration with the Astronomisches Rechen-Institut in Heidelberg, a new astrometric reduction of the GSC has been completed. This new version, GSC 1.2 has dramatically reduced the plate-based position-dependant and magnitude-dependant systematics present in GSC 1.1
The PPM and AC reference catalogues were used and absolute position errors have been reduced to about 0.3-0.4 arcsec.  This version is currently only available to the community via the web interface whilst a number of corrections are still being applied. For the journal article see Morrison, et al. AJ 121,pp 1752-1763.
This version should NOT be used for HST phase 2 coordinates.

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Availability

Yellow Bullet The Astronomical Society of the Pacific publishes the GSC 1.1 to the community on CDROM.
Yellow BulletOn-line QUERY of the operational GSC 1.1
Yellow BulletOn-line QUERY of the preliminary GSC 1.2

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Software

Yellow BulletSTScI has written a task as part of the SDAS software package under IRAF to retrieve data from the CDROM sets.
Yellow BulletASP provides PC and MAC software with their CDROM sets.
Yellow BulletSunOS users may find it necessary to apply a patch to your operating system to get around a bug in the handling of ISO9660 standard CDROMs.
Yellow BulletThe freely available X-windows astronomy program xephem also supports the GSC CDROM set. Platforms supported include SunOS, Solaris, Linux, Digital UNIX, HP-UX, and SGI-IRIX.

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Additional Information

Yellow BulletREADME file for the published GSC 1.0

Yellow BulletREADME file for the published GSC 1.1

Yellow Bullet References to the description of the catalog in the astronomical literature.

Yellow BulletDescription of the Catalog Codes used for various fields.

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Last Updated Jan 2001
Copyright 1996 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.