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We present a comparison of the Sloan Digital Sky
Survey Early Data Release, covering 460 square degrees of sky in five
bands, and the Guide Star Catalogue 2.2, covering the entire sky in two
bands, both released in June 2001. |
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The GSC2.2, while somewhat shallower and with
fewer bands than the SDSS, covers the entire sky with data having at least
comparable astrometric accuracy to SDSS, and with photometric and
classification accuracies suitable for many scientific projects. |
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GSC2.2 and SDSS objects were retrieved over the
internet. GSC2 objects were matched to SDSS based on position with a 2” or
5” search radius. |
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Astrometric and photometric comparisons were
performed for objects classified as stellar by both catalogs, to avoid
blurring results due to classification differences. |
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GSC2.2 plate-based effects were investigated by
examining the 0° zone of the SDSS, where GSC2.2 plate boundaries can be
easily discriminated by RA. |
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Both SDSS and GSC2 positions are reported in the
ICRF reference frame. |
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The GSC2.2 contains a single position for each
object that was selected from all available observations. Selection was
based on distance from plate center, to obtain the most reliable position. |
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SDSS-GSC2 position residuals for stars showed a
significant difference in RA, inspiring a further comparison with FIRST. |
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FIRST positions, derived from radio data,
provide an independent set of measurements in the ICRF reference frame. |
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SDSS data, being linear, is easier to calibrate
photometrically. However, GSC2 approaches SDSS accuracy for stars by means
of a Chebyshev fit using the GSPC2, which contains photometric sequences of
~100 stars per plate to ~18th magnitude. |
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Galaxy photometry has been deferred to GSC2.3
due to resource limitations. Thus, GSC2.2 magnitudes for galaxies can be
too bright by several magnitudes, since they were calibrated as if they
were point sources. |
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The GSC2.2 contains a single magnitude per
bandpass for each object that was selected from all available observations.
Selection was based on distance from plate center, to avoid vignetted plate
areas. |
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Both SDSS and GSC2.2 discriminate between
“stars” and “nonstars”, which may include blends. |
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Resolution (data) and crowdedness (sky) affect
results: a pair of stars may appear blended on GSC2 material, and distinct
on SDSS data. Thus not all disagreements are “errors”. |
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Unlike astrometry and photometry, classification
results are based on all available plate data through a multi-plate voting
scheme. |
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“High confidence” GSC2 classifications are
identified by a nonzero value of the 7th digit of the source
status flag, indicating unanimous agreement on all GSC2 plates. |
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Completeness and contamination are complementary
measures of classification performance. |
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Completeness measures select a sample of known
stars(/nonstars), and examine the percentage that are classified
stellar(/nonstellar). |
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Contamination measures select a sample of
objects that are classified stellar(/nonstellar), and examine the
percentage that are known to be stars(/nonstars). |
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For this analysis, we take SDSS classifications
as known, and perform both completeness measurements (on SDSS selected
samples) and contamination measurements (on GSC2 selected samples). |
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The Guide Star Catalogue is a joint project of
the Space Telescope Science Institute and the Osservatorio Astronomico di
Torino. Space Telescope Science Institute is operated by the Association of
Universities for Research in Astronomy, for the National Aeronautics and
Space Administration under contract NAS5-26555. The participation of the
Osservatorio Astronomico di Torino is supported by the Italian Council for
Research in Astronomy. Additional support is provided by European Southern
Observatory, Space Telescope European Coordinating Facility, the
International GEMINI project and the European Space Agency Astrophysics
Division. |
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Funding for the creation and distribution
of the SDSS Archive has been provided by the Alfred P. Sloan Foundation,
the Participating Institutions, the National Aeronautics and Space
Administration, the National Science Foundation, the U.S. Department of
Energy, the Japanese Monbukagakusho, and the Max Planck Society. The SDSS
Web site is http://www.sdss.org/. The Participating Institutions are The
University of Chicago, Fermilab, the Institute for Advanced Study, the
Japan Participation Group, The Johns Hopkins University, the
Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for
Astrophysics (MPA), New Mexico State University, Princeton University, the
United States Naval Observatory, and the University of Washington. |
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Results presented in this poster will be
available after the meeting at
http://www-gsss.stsci.edu. |
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Notes