GSC2

An all-sky astrometric and photometric catalog to support the operation of the Hubble Space Telescope (Copyright (c) 1989, 1992, The Association of Universities for Research in Astronomy, Inc.) This set of two CD-ROMs in the ISO 9660 format contains the Guide Star Catalog - Version 1.1, with an issue date of 1 August 1992 The Guide Star Catalog (GSC) was prepared by the Space Telescope Science Institute (ST ScI), 3700 San Martin Drive, Baltimore, MD 21218, USA. ST ScI is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under contract with the National Aeronautics and Space Administration (NASA).

The Guide Star Catalog (GSC), which has been constructed to support the operational need of the Hubble Space Telescope (HST) for off-axis guide stars, contains nearly 19 million objects brighter than sixteenth magnitude, of which more than 15 million are classified as stars. The original version of this catalog, GSC 1.0, is described in a series of papers: Lasker et al. (1990); Russell et al. (1990); and Jenkner et al. (1990); hereafter referred to as Papers I, II, and III. Additions and corrections made in GSC 1.1 address incompleteness, misnomers, artifacts, and other errors due to the overexposure of the brighter stars on the Schmidt plates, - the identification of blends likely to have been incorrectly resolved,- the incorporation of errata reported by the user-community or identified by the analysis of HST operational problems.

Among the primary authors of the GSC 1.0 and the associated systems, the scientific responsibilities were divided as follows: Helmut Jenkner, system coordination and overall design; Barry M. Lasker, astrophysics and photometry; Brian J. McLean, algorithmic analysis and systems development; Jane L. Russell, astrometry; Michael M. Shara, system management; and Conrad R. Sturch, production management and quality control. GSC 1.1 analysis and production were performed primarily by Jesse B. Doggett, Daniel Egret, Brian J. McLean, and Conrad R. Sturch.

Helmut Jenkner is on assignment from the European Space Agency; Jane L. Russell is currently affiliated with the Applied Research Corporation, Landover, MD; and Conrad R. Sturch is with the Astronomy Programs, Computer Sciences Corporation at Space Telescope Science Institute. Daniel Egret is affiliated with Observatoire de Strasbourg, France.

As described in Paper I, the GSC is primarily based on an all-sky, single epoch, single passband collection of Schmidt plates. For centers at +6 degrees and north, a 1982 epoch "Quick V" survey was obtained by the Palomar Observatory, while for southern fields, materials from the UK SERC J survey (epoch approximately 1975) and its equatorial extension (epoch approximately 1982) were used. In addition, over 100 short-exposure plates were taken with the Palomar Oschin and UK Schmidt telescopes to cover complex regions including the southern Milky Way, the Magellanic Clouds, and M31. These northern, southern, and supplemental plates hereafter are referred to as N, S, and X plates, respectively. The plates were digitized into 14000-square rasters at 25 um sample intervals using modified PDS microdensitometers. The sky-background was modeled with a bi-dimensional cubic spline approximation to the modal level. Then an object finder, based on locating connected pixels at a certain threshold above the background, was used to obtain, for each plate, a list of positions, sizes, intensities, and related descriptive parameters. Images with multiple peaks were deblended by an algorithm based on correlations against a library of stellar images The identified objects were classified as stars or non-stars by an interactive multivariate Bayesian classifier that used image features from the object-detection steps and was started from a small set of objects visually identified on each plate. Comparison of classifications from multiply cataloged objects in the plate overlap areas showsthat the purity of objects classified as stars is typically 97 percent.

The GSC calibrations were obtained on a plate-by-plate basis by polynomial modeling against the photometric and astrometric reference catalogs. Photometry is available in the natural systems defined by the individual plates in the GSC collection (generally J or V), and the calibrations are done using B, V standards from the Guide Star Photometric Catalog (Lasker, Sturch, et al. 1988). In Paper II the overall quality of the photometry near the standard stars was estimated from the fits and other tests to be 0.15 mag (one sigma, averaged over all plates), while the quality far from the sequences was estimated from the all-sky plate-to-plate agreement and from comparisons with independent photometric surveys to be about 0.3 mag (one sigma), with about 10% of the errors being greater than 0.5 mag. Additionally, Ratnatunga's (1990) comparison of the GSC against totally independent J-band photographic photometry for three southern fields (20sq deg area) for 12.5 < J < 15.5 shows agreement at the 0.1-0.2 mag level. Astrometry, at equinox J2000, is available at the epochs of the individual plates used in the GSC; and the reductions to the reference catalogs (AGK3, SAOC, or CPC, depending on the declination zone) use third order expansions of the modeled plate and telescope effects. The fits to the reference catalogs lie in the range 0.5" to 0.9", and most of this is attributable to errors in the reference catalogs, to centroiding errors on the relatively large images of the reference stars, and to unmodeled astrometric effects. Paper II reported estimates of the overall external astrometric error, produced by comparisons of independently measured positions, in the range 0.2" to 0.8" (per coordinate), depending on the areas of the plate and the sky. Then from a more extensive analysis against the Carlsberg Automatic Meridian Circle data, Taff et al. (1990) found that GSC absolute positional errors from plate center to edge vary from 0.5" to 1.1" in the north and from 1.0" to 1.6" in the south, and that relative errors at half-degree separations range from 0.33" to 0.76" depending upon hemisphere and magnitude.

Paper III describes the software system used to produce the GSC. It consisted of a set of (primarily non-interactive) image-processing and calibration programs interconnected by a set of pipeline files and supported by databases organized on a plate-by-plate basis. A set of utility programs was also provided to support quality control and to correct operational problems. Object names are of the form GSC rrrrr nnnnn, where the first field specifies an internal region number and the second is an ordinal within it. For objects cataloged from more than one photographic plate, an entry was made from each image; and all entries for the same object were given the same unique name. Paper III also reviews the database for compiling statistics of objects with multiple entries and the details of the organization and structure of the GSC, including the provisions for assigning unique names, for cataloging objects lying in the plate overlap regions, for rapidly indexing positions against regions, and for recovering the original plate measurements. The separate count statistics for stellar and non-stellar objects on a plate-by-plate basis are provided in the supporting tables.

The all-sky collection of Schmidt plates that were digitized, archived to optical disc, and processed to generate the Guide Star Catalog (GSC) constitute a general image resource for astronomical research. This data set, combined with the computing environment provided by the Guide Star Astrometric Support Package (GASP), major elements of which are exported within the Space Telescope Science Data Analysis System, provides random access to a digital image in any part of the sky. The GASP environment also supports access to the GSC and to other major astronomical catalogs.

The GSC 1.1 activities performed to address a number of known problems in GSC 1.0 are summarized here and described in detail in the text file for this revision, REV_1_1.TBL;1. Two concerns related to the brighter stars arise from the heavily overexposed images on the Schmidt plates used in the GSC, namely an incompleteness and a reduced precision. Both are addressed in the domain V < 7.5 by the use of data from the INCA Data Base (Turon et al. 1992; Jahreiss et al. 1992; Grenon et al. 1992) in the Tycho Input Catalog (TIC; Egret et al. 1992). Such entries are designated by the plate identifier +056 in GSC 1.1. The limit of V < 7.5 preserves the original GSC data for objects that were used in the GSC 1.0 astrometric calibration.

Naming errors occur when objects catalogued from more than one photographic plate have positional errors sufficiently large that cross-matching of the overlapping plate areas is done incorrectly. The most significant known instances of this in GSC 1.0 were associated with overexposed (and therefore badly centroided) images of the brighter stars. A search around the positions of the INCA stars facilitated the identification of these naming errors, which were then removed in GSC 1.1.

GSC 1.0 contains many pairs of objects (from single plates) with separations significantly smaller than the expected resolution of the catalog, which Garnavich (1991), based on a study of four northern plates, estimates at ~ 10" for 8.0 < V < 14.0. Visual inspection shows that these are generally blends that have been properly resolved, but then affected by a centroider defect that made the separations artificially small. Such components of blends with incorrect separations are given a classification of 2 (blend; cf. the text file for a full listing of the codes).

For stars with V < 8, image-processing artifacts near the diffraction spikes exist in GSC 1.0. In GSC 1.1, potential artifacts were identified by use of a purely geometrical criterion (proximity of the object to the spike), and were assigned a classification of 5.

Small areas around southern stars brighter than V ~ 3 are not processed from the Schmidt plates and were left blank in GSC 1.0. For these, GSC 1.1 contains entries from supplemental astrograph plates taken with the GPO (Gran Prisma Objectif) telescope on La Silla, and the astrograph at the Black Birch Observatory (BBO) in Blenheim, New Zealand. Because of their smaller fields, the photometric and astrometric calibrations of data from most GPO and a few BBO plates were performed against nearby GSC entries based on the Schmidt plates.

A number of specific errors in GSC 1.0 have been identified by the user-community and by analyses of HST operational problems. These generally involve naming errors, plate flaws, misclassifications, and multiple stars; most are individually corrected in GSC 1.1. Also, the photometric error parameter in GSC 1.1 is now correctly described by equation (3) in Paper II; i.e., the erratum of footnote 5 therein is no longer pertinent.

Plans for GSC maintenance beyond version 1.1 include an astrometric recalibration (cf. Taff, Lattanzi, and Bucciarelli 1990), and investigation of erroneous double entries that may exist for V > 7.5.

The Guide Star Catalog is subdivided into regions that are bounded by small circles of right ascension and great circles of declination, and that are numbered consecutively from 0001 to 9537. Data for each region are stored as separate files; these files are contained in directories, each of which subtends a 7.5 degree zone of declination.

The Guide Star Catalog is distributed as a two CD-ROM set, divided at a declination of -7.5 degrees. An introductory file (README.TXT;1) and the supporting tables are duplicated on both discs. All data files (i.e., with the exception of the file README.TXT;1 and the directory files) are in FITS (Flexible Image Transport System; Greisen et al. 1981; Wells et al. 1981; Grosbol et al. 1988; and Jahreiss et al. 1992) table format. The root directory contains the following files::

Directory GSC contains directories for the 7.5 degree zones in declination; these directories in turn contain the GSC region files in FITS format for the respective zone, with file identifiers of the form nnnn.GSC;1, where nnnn is the 4-digit decimal region number, with leading zeroes used as required to fill the field. The directories are named as follows:

Directory TABLES contains the following supporting files for the GSC, written in FITS table format:

The primary authors would like to thank the large number of persons who have participated in the Guide Star Selection System and the Guide Star Catalog developments over the past years. In addition to the many people previously acknowledged, we would like to thank the following who made contributions in GSC 1.1:

o by collaboration in merging INCA data into GSC 1.1: Daniel Egret and the HIPPARCOS INCA and Tycho groups;

o for providing astrograph plates for southern bright stars: Geoffrey Douglass, Robert Harrington, G. Monderen, and Otto Richter;

o by contributing to the GSC 1.0 error discussion or by reporting specific errors in GSC 1.0: Beatrice Bucciarelli, Daniel Egret, Holland Ford, Peter Garnavich, Roberto Gilmozzi, Roberta Humphreys, Mario Lattanzi, William Owen, Barry Rappaport, Kavan Ratnatunga, Larry Taff, Patrick Wallace, Fred Walter, and Archibald Warnock;

o and by making essential contributions to the project's infrastructure: Marian Iannuzzi, Greg McLeskey, Dave Paradise, Don Stevens-Rayburn, and Patty Trovinger.

Finally, special thanks are due to Riccardo Giacconi, Director of the Space Telescope Science Institute, for his vision and continuing support of this project.

The Guide Star Catalog is partially based on data obtained at Palomar Observatory, operated by the California Institute of Technology; at the UK Schmidt Telescope, operated by the UK Science and Engineering Research Council and by the Anglo-Australian Observatory; at the Cerro Tololo Inter- American Observatory and the Sacramento Peak Observatory, mboth operated by the Association of Universities for Research in Astronomy, Inc., under contract to the National Science Foundation; and at the Mount Lemmon Observatory, operated by the University of Arizona.

Egret, D., Didelon, P., McLean, B. J., Russell, J. L., and Turon, C.:Tycho Input Catalog - Cross- matching the Guide Star Catalog with the Hipparcos INCA Data Base; Astron. Astrophys., 258, 217-212 (1992).

Garnavich, P.: The Stellar Angular Correlation: Clues to Wide Binary Star Properties; dissertation, University of Washington (1991).

Greisen, E. W., Harten, R. H.: An Extension of FITS for Groups of Small Arrays of Data; Astron. Astrophys. Suppl. Ser., 44, 371-374 (1981).

Grenon, M., Mermilliod, M., Mermilliod, J. C.: The Hipparcos Input Catalogue. III. Photometry; Astron. Astrophys., 258, 88-93 (1992).

Grosbol, P., Harten, R. H., Greisen, E. W., Wells, D. C.: Generalized Extensions and Blocking Factors for FITS; Astron. Astrophys. Suppl Ser., 73, 359-364 (1988).

Harten, R. H., Grosbol, P., Greisen, E. W., Wells, D. C.: The FITS Tables Extension; Astron. Astrophys. Suppl. Ser., 73, 365-372 (1988).

Jahreiss, H., Requieme, Y., Argue, A. N., Dommanget, J., Rousseau, M., Lederle, T., Le Poole, R. S., Mazurier, J. M., Morrison, L. V., Nys, O., Penston, M. J., Perie, J. P., Prevot, L., Tucholke, H. J., de Vegt, C.: The Hipparcos Input Catalogue. II. Astrometric Data; Astron. Astrophys., 258, 82-87 (1992).

Jenkner, H., Lasker, B. M., Sturch, C. R., McLean, B. J., Shara, M. M., Russell, J. L.: The Guide Star Catalog. III. Production, Database Organization, and Population Statistics; Astron. J., 99, 2081-2154 (1990).

Lasker, B. M., Sturch, C. R., Lopez, C., Mallama, A. D., McLaughlin, S. F., Russell, J. L., Wisniewski, W. Z., Gillespie, B. A., Jenkner, H., Siciliano, E. D., Kenny, D., Baumert, J. H., Goldberg, A. M., Henry, G. W., Kemper, E., Siegel, M. J.: The Guide Star Photometric Catalog. I.; Astrophys. J. Suppl., 68, 1-90 (1988).

Lasker, B. M., Sturch, C. R., McLean, B. J., Russell, J. L., Jenkner, H., Shara, M. M.: The Guide Star Catalog. I. Astronomical and Algorithmic Foundations; Astron. J., 99, 2019-2058 (1990).

Ratnatunga, K. U.: Comparison of GSC Photometry in Three Southern Fields; Astron. J., 100, 280- 290 (1990).

Russell, J. L., Lasker, B. M., McLean, B. J., Sturch, C. R., Jenkner,H.: The Guide Star Catalog. II. Photometric and Astrometric Calibrations; Astron. J., 99, 2059-2081 (1990).

Taff, L. G., Lattanzi, M. G., Bucciarelli, B.: Two Successful Techniques for Schmidt Plate Astrometry; Astrophys. J., 358, 359-369 (1990).

Taff, L. G., Lattanzi, M. G., Bucciarelli, B., Gilmozzi, R., McLean, B. J., Jenkner, H., Laidler, V. G., Lasker, B. M., Shara, M. M., Sturch, C. R.: Some Comments on the Astrometric Properties of the Guide Star Catalog; Astrophys. J., 353, L45-L48 (1990).

Turon, C., Gomez, A., Crifo, F., Creze, M., Perryman, M. A. C., Morin, D., Arenou, F., Nicolet, B., Chareton, M., Egret, D.: The Hipparcos Input Catalogue. I. Star Selection; Astron. Astrophys., 258, 74- 81(1992).

Wells, D. C., Greisen, E. W., Harten, R. H.: FITS - A Flexible Image Transport System; Astron. Astrophys. Suppl. Ser., 44, 363-370 (1981).