ADC/CDS Standard Document for Catalog:/catalogs/1/1220/
ADC/CDS Standard Document for Catalog:
Superseded The HST Guide Star Catalog (Lasker+ 1992)
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The Guide Star Catalog Version 1.1 - An all-sky astrometric and photometric
catalog to support the operation of the Hubble Space Telescope
<The Association of Universities for Research in Astronomy, Inc.
([c] 1989, 1992)>
The Guide Star Catalog Version 1.0 is described in a series of papers:
I. Astronomical foundations and image processing
Lasker B.M., Sturch C.R., McLean B.J., Russell J.L., Jenkner H., Shara M.M.
<Astron. J., 99, 2019 (1990)>
=1990AJ.....99.2019L
II. Photometric and astrometric models and solutions
Russell J.L., Lasker B.M., McLean B.J., Sturch C.R., Jenkner H.
<Astron. J., 99, 2059-2081 (1990)>
=1990AJ.....99.2059R
III. Production, database organization, and population statistics
Jenkner H., Lasker B.M., Sturch C.R., McLean B.J., Shara M.M., Russell J.L.
<Astron. J., 99, 2082 (1990)>
=1990AJ.....99.2082J
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ADC_Keywords: Surveys ; Positional data
Abstract:
The Guide Star Catalog (GSC), which has been constructed to support the
operational need of the Hubble Space Telescope contains nearly 19
million objects brighter than sixteenth magnitude, of which more than 15
million are classified as stars. This catalog provides positions and
magnitudes for these stars.
Introduction:
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.
Astronomical and Algorithmic Foundation:
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 degree s 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 catalogued objects in the plate overlap areas shows that
the purity of objects classified as stars is typically 97 percent.
Photometric and Astrometric Calibrations:
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 (20 sq 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.
Production, Database, Organization, and Population Statistics:
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 catalogued 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
cataloguing 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.
User Interfaces, Utilities, and Astronomical Applications:
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 GASP is part of the STScI SDAS package which,
together with IRAF, can be obtained through the STScI World Wide Web
pages.
See also:
Superseded by catalog 1254:
The HST Guide Star Catalogue, Version 1.2 (Lasker+ 1996)
at URL ftp://adc.astro.umd.edu/pub/adc/archives/catalogs/1/1254
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