SIMPLE = T / Standard FITS format BITPIX = 8 / Character Information NAXIS = 0 / No image data array present EXTEND = T / Extension exists FILETYPE= 'COMMENTS' / Indicates file type ORIGIN = 'ST ScI ' / Space Telescope Science Institute DATE = '01/06/89' / Date of issue (dd/mm/yy) COMMENT THE GUIDE STAR CATALOG COMMENT COMMENT An all-sky astrometric and photometric catalog COMMENT prepared for the operation of the Hubble Space COMMENT Telescope. COMMENT COMMENT Copyright, 1989, Association of Universities COMMENT for Research in Astronomy, Inc. COMMENT COMMENT This file contains data for one of the supporting tables for the GuideCOMMENT Star Catalog (GSC). Additional information on the GSC may be found inCOMMENT accompanying scientific publications as well as in comments and tablesCOMMENT elsewhere on this set of volumes. COMMENT COMMENT The Guide Star Catalog (GSC) was prepared by the Space Telescope ScienceCOMMENT Institute (ST ScI), 3700 San Martin Drive, Baltimore, MD 21218, USA.COMMENT ST ScI is operated by the Association of Universities for Research inCOMMENT Astronomy, Inc. (AURA), under contract with the National Aeronautics andCOMMENT Space Administration (NASA). COMMENT COMMENT COMMENT File COMMENTS COMMENT Introduction and General Comments COMMENT COMMENT This table contains text (in 80 column format) summarizing the GuideCOMMENT Star Catalog and providing references to, as well as abstracts of, theCOMMENT scientific publications describing the GSC. END XTENSION= 'TABLE ' / Table Extension BITPIX = 8 / Character Information NAXIS = 2 / Two-dimensional table NAXIS1 = 80 / Number of characters per line NAXIS2 = 206 / Number of rows PCOUNT = 0 / No Random Parameters GCOUNT = 1 / Only one group TFIELDS = 1 / One field per row EXTNAME = 'COMMENTS' / Generic Comments EXTVER = 1 / Integer Version Number TTYPE1 = 'TEXT ' / Free text TBCOL1 = 1 / Start in column 1 TFORM1 = 'A80 ' / 80 Character Field END THE GUIDE STAR CATALOG An all-sky astrometric and photometric catalog prepared for the operation of the Hubble Space Telescope. Copyright, 1989, Association of Universities for Research in Astronomy, Inc. INTRODUCTION The Guide Star Catalog (GSC) for the Hubble Space Telescope (HST) is presentedin this set of FILES, which, depending on the computer medium used, may be DECVMS Backup tapes, FITS (Flexible Image Transport System) tapes, 12-inch WORM(write once, read many times) optical disks, or CD-ROMs (compact disc, read onlymemory) in the ISO 9660 format. This issue, Version 1, corresponds to the GSC as it existed on 1 June 1989. Theassociated scientific papers, which are being issued concurrently as a ST ScIpreprint and which will be submitted to the Astronomical Journal in 1989, are asfollows: 1. The Guide Star Catalog. I. Astronomical and Algorithmic Foundations; Barry M. Lasker, Conrad R. Sturch, Brian J. McLean, Jane L. Russell, Helmut Jenkner, and Michael M. Shara. 2. The Guide Star Catalog. II. Photometric and Astrometric Calibrations; Jane L. Russell, Barry M. Lasker, Brian J. McLean, Conrad R. Sturch, and Helmut Jenkner. 3. The Guide Star Catalog. III. Production, Database Organiza- tion, and Population Statistics; Helmut Jenkner, Barry M. Lasker, Conrad R. Sturch, Brian J. McLean, Michael M. Shara, and Jane L. Russell. In addition, the following publication is in preparation at the time of issuingthis version of the GSC: 4. The Guide Star Catalog. IV. User Interfaces, Utilities, and Astronomical Applications; Brian J. McLean et al. The Guide Star Catalog (GSC) was prepared by the Space Telescope ScienceInstitute (ST ScI), 3700 San Martin Drive, Baltimore, MD 21218, USA. ST ScI isoperated by the Association of Universities for Research in Astronomy, Inc.(AURA), under contract with the National Aeronautics and Space Administration(NASA). HJ is on assignment from the European Space Agency; JLR is currently affiliatedwith the Applied Research Corporation, Landover, MD; and CRS is with theAstronomy Programs, Computer Sciences Corporation at Space Telescope ScienceInstitute. ORGANIZATION OF THE DATA FILES Generality in the correspondence of data between the files on this medium andthe tables which appear in the scientific publications is achieved by sequen-tially numbering the tables within each paper and naming the digital files. The names of the files are given by the FITS FILETYPE keyword, as follows: ---------------------------------------------------- FILETYPE Description ---------------------------------------------------- COMMENTS - Introduction and general comments. PLATES - Information on the plates used in the GSC. PROCESS - Image processing parameters. ASTR_CAL - Parameters of astrometric calibrations. PHOT_CAL - Parameters of photometric calibrations. C_UP_POP - Catalog update population statistics. ST_POP - Population statistics for stars. NS_POP - Population statistics for non-stars. REGIONS - Boundaries of GSC regions. C_RE_POP - GSC region population statistics. LG_REG_X - Index to large regions. SM_REG_X - Index to small regions. XREF_P2R - Cross-reference table, plates to regions. XREF_R2P - Cross-reference table, regions to plates. ---------------------------------------------------- These files are also available as a microfiche supplement to the GSC. Addition-ally, table PLATES appears in the printed version of Paper I; and REGIONS,LG_REG_X, and SM_REG_X, in the printed version of Paper III. SUMMARIES OF THE GSC PUBLICATIONS Astronomical and Algorithmic Foundations (Paper I) The Guide Star Catalog (GSC), which has been constructed to support theoperational need of the Hubble Space Telescope for off-axis guide stars,contains 18,819,291 objects in the seventh to sixteenth magnitude range, ofwhich more than 15 million are classified as stars. The GSC is primarily based on an all-sky, single epoch, single passbandcollection of Schmidt plates. For centers at +6 degrees and north, a 1982 epoch"Quick V" survey was obtained by the Palomar Observatory, while for southernfields, materials from the UK SERC J survey (epoch approximately 1975) and itsequatorial extension (epoch approximately 1982) were used. The plates were digitized into 14000-square rasters at 25 micron sample inter-vals using modified PDS microdensitometers. The sky-background was approximated with a bi-dimensional cubic spline approxi-mation to the modal level. Then an object finder, based on locating connectedpixels at a certain threshold above the background, was used to obtain, for eachplate, a list of positions, sizes, intensities, and related descriptive para-meters. Images with multiple peaks were deblended by an algorithm based oncorrelations against a library of stellar images. The identified objects were classified as stars or non-stars by an interativemultivariate Bayesian classifier that used image features from the object-detection steps and was started from a small set of objects visually identifiedon each plate. Comparison of classifications from multiply cataloged objects inthe plate overlap areas shows that the purity of objects classified as stars istypically 97 percent. Photometric and Astrometric Calibrations (Paper II) The GSC calibrations are obtained, on a plate-by-plate basis, by polynomialmodeling against the photometric and astrometric reference catalogs. Photometry is available in the natural systems defined by the individual platesin the GSC collection (generally J or V), and the calibrations are done usingB, V standards from the Guide Star Photometric Catalog. The overall quality ofthe photometry near the standard stars is estimated from the fits and othertests to be 0.15 mag (one sigma, averaged over all plates), while the qualityfar from the sequences is estimated from the all-sky plate-to-plate agreementand from comparisons with independent photometric surveys to be about 0.30 mag(one sigma), with about 10% of the errors being greater than 0.50 mag. Astrometry, at equinox J2000, is available at the epochs of the individualplates used in the GSC; and the reductions to the reference catalogs (AGK3,SAOC, or CPC, depending on the declination zone) use third order expansions ofthe modeled plate and telescope effects. The fits to the reference catalogs liein the range 0.5 arc-sec to 0.9 arc-sec, and most of this is attributable toerrors in the reference catalogs and to centroiding errors on the relativelylarge images of the reference stars. Estimates of the overall external astrometric error, produced by comparisons ofindependently measured positions without regard to location on the GSC plates,are in the range 0.4 arc-sec to 0.6 arc-sec; similar studies, restricted to theplate edges, give values of about 0.7 arc-sec. Additionally, the analysis ofpair-separations, based on experiments in the Praesepe astrometric test region,indicates that for GSC pairs from the same plate the relative errors overhalf-degree fields are about 0.3 arc-sec. Production, Database Organization, and Population Statistics (Paper III) The software system used to produce the GSC consists of a set of (primarily non-interactive) image-processing and calibration programs interconnected by a setof pipeline files and supported by databases organized on a plate-by-platebasis. A set of utility programs is also provided to support quality control andto correct operational problems. Object names are of the form GSC rrrrr nnnnn, where the first field specifies aninternal region number and the second is an ordinal within it. For objectscataloged from more than one photographic plate, an entry is made from eachimage; and all entries for the same object are given the same unique name. The database for compiling statistics of objects with multiple entries and thedetails of the organization and structure of the GSC, including the provisionsfor assigning of unique names, for cataloging objects lying in the plate overlapregions, for rapidly indexing positions against regions, and for recovering theoriginal plate measurements are described. As a tool to support user evaluation of the completeness of the GSC, separatecount statistics for stellar and non-stellar objects are provided on a plate-by-plate basis. User Interfaces, Utilities, and Astronomical Applications (Paper IV) The all-sky collection of Schmidt plates that were digitized, archived tooptical disc, and processed to generate the Guide Star Catalog (GSC) constitutea general image resource for astronomical research. This data set, combined with the computing environment provided by the GuideStar Astrometric Support Package (GASP), major elements of which are exportedwithin the Space Telescope Science Data Analysis System, provides random accessto a digital image in any part of the sky. The GASP environment also supportsaccess to the GSC and to other major astronomical catalogs. The applications of this system to Hubble Space Telescope operations and toother observing programs, eg., preparing accurate positions and finding charts,as well as to several long-term programs in variable stars, galactic structure,and extragalactic astronomy, are described.
