Up: Home Page
CDS Services in Support of MEGACAM/TERAPIX Data Access,
James G. Bartlett 1, Jean-Marc Cochery1,
Pascal Dubois1, Daniel Egret1,
1. CDS, 11 rue de l' Université, 67000 Strasbourg, France
Pierre Fernique1, Francois Genova1,
Soizick Lesteven1, Mireille Louys2,1,
Francois Ochsenhein1, Patricio Ortiz1,3, and
2. ENSPS, Bd. Sébastien Brandt, 67400 Illkirch, France
3. European Southern Observatory, Garching, Germany
CDS already provides the astronomical community with services
like SIMBAD, VizieR and ALADIN.
New projects are under way for mining survey data, and managing very
large catalogues, like the TERAPIX/MEGACAM point source catalogue.
Within the scope of the TERAPIX pipeline, CDS is responsible for providing
public access to the survey and pointed mode catalog data, as well
as to``summary" images for the pixel data.
The Strasbourg astronomical data center (CDS) has developed
a series of tools for:
- managing and making accessible published data extracted from sky
surveys, deep fields, or more generally, very large catalogues at
- helping cross-identification and validation of data
from new surveys.
The main CDS services are briefly reviewed in section 2, while section 3
describes the new projects currently developed for the management of very
large catalogues and cross-referencing.
The different CDS services are completely inter-connected (Genova
et al. 1998), and one can navigate easily between them (for
example from ALADIN to VizieR and then from VizieR to SIMBAD).
SIMBAD is the reference database for the identification and bibliography
of astronomical objects (Egret et al. 1991).
It contains about 2,000,000 objects, 5,000,000 object names, and more
than 100,000 bibliographical references containing 2,600,000 citations
of objects (as of July 1998), making it an invaluable tool for
cross-identification of objects referenced in the literature.
The VizieR and Astronomer's Bazaar services (Ochsenbein 1998) provide
access to a wide library of data tables, including
key catalogues and surveys (such as: Hipparcos, Tycho, IRAS,
VLA FIRST, etc.), and allow respectively sampling and
full retrieval of the catalogues over the network (1815 catalogues
available in July 1998).
They are well suited to the managment of MEGACAM ``pointed mode" catalogs.
In the frame of the VizieR service, dedicated management techniques
have been developed for fast coordinate access to very large catalogues
(of size > 20 Million objects) such as GSC, USNO A1.0, or later TERAPIX survey mode catalog. This approach gives very good response times, for example,
in USNO A1.0 (488,006,860 sources):
- 0.09 seconds for a search radius of 2.5 arcmin (14,000 records);
- 0.67 seconds for a search radius of 30 arcmin (200,000 records).
Although no indexing is made for magnitudes and colors, a query based on
color criteria can be done on the complete catalogue in about 35 minutes,
thanks to heavy data compression.
The ALADIN (Bonnarel et al. 1998) interactive sky atlas provides
access to reference images from the major Schmidt surveys (DSS I and,
later, DSS II, and high resolution MAMA scans) as well as a set of tools
to aid source diagnostic and identification.
The user can also visualize his/her own digitized images, and
overlay catalogue (or user) data on the images, making it a very
useful tool for multi-wavelength cross-identifications.
An X-Window interface to ALADIN is available and a JAVA interface is
currently under developement.
Experience with ALADIN proves that access to some kind of survey pixel
data (for example, MEGACAM in the future) can
be very useful in some cases to check that nothing important has
been missed in the original source extraction. Within the scope
of the project, CDS is studying various compression techniques
and working on adaptative solutions, depending on the relevant information
content of the pixel data.
Two new projects are currently under way at CDS:
- a very large catalogue management system, fully object-oriented,
storing several very large catalogues together, with
fast multi-criterion access. Current studies at CDS make use of a
commercial DBMS: Objectivity.
As a part of this project, new methods for sky partitioning and multi-dimensional indexing are also being studied. For the latter, the idea is
to organize data in such a way that objects which are close in parameter space are
grouped in the
same containers so that requests constrained on these parameters
can be restricted to a minimal list of containers;
- an automatic cross-referencing engine for catalogues (collaboration
between ESO and CDS in the framework of the VLT science environment).
will allow the user to perform cross-correlation between ``any'' kind
of data (ESO and VizieR catalogues, user table, SIMBAD...).
These projects will be interfaced in the future and will allow CDS to
provide access and data mining tools for large survey catalogues
such as MegaPrime/Terapix, DENIS, 2MASS, etc.
Up: Home Page