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Read, M. A. & Hambly, N. C. 2001, in ASP Conf. Ser., Vol. 238, Astronomical Data Analysis Software and Systems X, eds. F. R. Harnden, Jr., F. A. Primini, & H. E. Payne (San Francisco: ASP), 182
The SuperCOSMOS Sky Surveys
Mike Read and Nigel Hambly
WFAU, Institute for Astronomy, University of Edinburgh, Blackford Hill,
Edinburgh, EH9 3HJ, UK, E-mail: mar@roe.ac.uk
Abstract:
The SuperCOSMOS Sky Surveys (SSS) programme is digitising multi-colour (BRI),
multi-epoch Schmidt survey plates with the ultimate aim of covering
the entire sky. The R-band is covered at two epochs.
The object catalogue and
compressed pixel data are stored
online and are accessible via the WWW. Coordinates, magnitudes,
morphological data and proper motions are available for all objects
down to B
![$\sim22$](img2.gif)
, R
![$\sim20$](img3.gif)
and I
![$\sim19$](img4.gif)
.
The survey is available on-line at:
http://www-wfau.roe.ac.uk/sss.
SuperCOSMOS is an ideal machine for large scale survey work. Important
characteristics of the machine include:
- fast scanning time: SuperCOSMOS scans a
mm area of the
Schmidt plate in
hours.
- positional accuracy: the design of the machine and environmental
control enable relative positional measurements at a precision of
m
- good dynamic range and 16-bit digitisation in
10
m pixels: these are
well matched to the image quality of the photographic atlases.
The SSS data consist of object catalogues to the plate limits from BRI plates
including astrometric and photometric calibration. Colours, proper motions,
image classification and morphological parameters are available.
H-compressed pixel data are
available in small FITS file subsets with included object catalogues.
In relation to the SSS data, the Wide-Field Astronomy Unit (WFAU)
currently offers online queries for:
- small area (up to 15arcmin) pixel images with attached object catalogue,
- medium-scale (up to 10
) object
catalogues with user-specified magnitude range and output format (FITS,
ASCII or tab-separated),
- multiple object small pixel images in batch mode.
Whole-sky catalogue access and analysis online is currently being planned.
Absolute positions are calibrated with respect to the Tycho-2 catalogue while
proper motions are zero-pointed on the extragalactic frame using the images
of galaxies. The initial photometric calibration is based on the Guide Star
Photometric Catalogue I; in due course we plan to recalibrate using GSPC-II.
Figure 1
shows
histograms of the mean RMS residual per star per plate in either
coordinate for Tycho-2 reference catalogue stars.
Figure 1:
Astrometric residuals from Tycho-2 standards. Five panels
show: (a) SERC J/EJ, (b) SERC ER/AAO-R, (c) SERC-I, (d) ESO-R and
(e) POSS-I E. Modal values are in the range 0.1 to 0.2arcsec.
![\begin{figure}
\plotone{P3-17a.eps}
\end{figure}](img10.gif) |
For quasar images from Veron-Cetty & Veron (1998),
Figure 2 shows distributions of residual X- and
Y-displacement (between first and second epoch plates) and indicates
that the zero point of proper motions is truly extragalactic.
Figure 2:
Distributions of residual X- and Y-displacement as a function
of B
magnitude in QSO images for the entire South Galactic Cap survey.
![\begin{figure}
\epsscale{0.8}
\plotone{P3-17b.eps}
\end{figure}](img12.gif) |
Most people will interrogate the South Galactic Cap (SGC) database via the World Wide Web. An
interface has been set up to enable subsets of the large database to be
extracted. The primary data format is FITS for both images and binary
tables of object catalogues, which enables porting of the data into
established software packages. Object catalogues are additionally
available in plain text format for readability. Basically, the user
specifies the region of interest (up to 15arcmin for images and up to
several 100square degrees for object catalogues) and a primary colour;
FITS files are then efficiently extracted and sent back to the user's
home institute for browsing and manipulation.
As is usual with measurements from wide field Schmidt plates, the
external astrometric and photometric accuracies of the data are limited
by position- and magnitude-dependent systematic errors. However, the
internal accuracy (i.e., when comparing image data in restricted
position and magnitude ranges) is unaffected by this, and for some
astronomical applications it is the internal accuracy that is
important. Typical numbers for the astrometric and photometric
precision for well-exposed stellar images are as follows:
-
arcsec;
mag;
milliarcsec/yr (internal)
-
arcsec;
mag;
milliarcsec/yr (external)
Completeness and reliability of image classification of the SGC data have
been assessed by comparison against external datasets. We find that
the completeness is essentially 100% to within 1.5mag of the nominal plate
limits (B
=21.5, R=20.5, I=17.5). For the B
data we find that the image classification is
% reliable to
B
. The limiting factor for
reliable image classification is the resolution of the photographs coupled
with the typical angular size of galaxies as a function of magnitude: at
B
=20, the typical scale size of a galaxy is comparable to
the resolution.
In the example shown in Figure 3,
a 10arcmin region
has been extracted in the B band
from a south galactic pole field containing a rich cluster of galaxies.
Figure 3:
Extraction example using the GAIA/SkyCAT software.
![\begin{figure}
\plotone{P3-17c.eps}
\end{figure}](img21.gif) |
The data have been displayed using the GAIA/SkyCAT software. In this way, it is possible
to browse the data by clicking on objects in the image display or the
catalogue list to highlight them in both. All the catalogue parameters for
a given object can then be examined (e.g., position, shape, brightness and
classification).
A 5000-square degree area centred on the SGC is available online now. By
early 2001 we will have the entire southern sky available online in both
J and second epoch R. The four-colour southern sky survey will be complete
by mid-2002. It is hoped that we will then expand the survey into the
northern hemisphere to cover the entire sky using the Palomar first and
second epoch surveys.
References
Veron-Cetty, M.-P. & Veron, P. 1998, ``A Catalogue of
Quasars and Active Nuclei'', Garching, ESO Scientific Report Series
Vol. 18
© Copyright 2001 Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, California 94112, USA
Up: Sky Surveys
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