CDS Services in Support of MEGACAM/TERAPIX Data Access,
Identification and Validation

Francois Bonnarel¹, James G. Bartlett ¹, Jean-Marc Cochery¹, Pascal Dubois¹, Daniel Egret¹,
Pierre Fernique¹, Francois Genova¹, Soizick Lesteven¹, Mireille Louys^2,1,
Francois Ochsenhein¹, Patricio Ortiz^1,3, and Marc Wenger¹

1. CDS, 11 rue de l' Université, 67000 Strasbourg, France
2. ENSPS, Bd. Sébastien Brandt, 67400 Illkirch, France
3. European Southern Observatory, Garching, Germany

Abstract:

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.

Introduction

  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 any wavelength;
• 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 Key Services at CDS

  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

  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.

VizieR

  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.

ALADIN and access to MEGACAM pixel data

  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.

New Projects at CDS

  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). This tool 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.