The Centre de Données astronomiques de Strasbourg (CDS) develops and maintains a set of widely used, interlinked services, in particular SIMBAD, the reference database for astronomical objects, the VizieR catalogue browser, and the ALADIN interactive sky atlas. The CDS services are described in Genova et al. (2000) and the set of companion papers (Wenger et al. 2000; Ochsenbein et al. 2000a; Bonnarel et al. 2000). The services are available from the CDS Web site, together with the Dictionary of Nomenclature, mirror copies of journals and of the ADS, Yellow Page services, documentation, and other services.
The CDS services are interconnected, and each service is itself a hub connected to remote reference services.
On one hand, they are part of the astronomy bibliographic network, together with the ADS (Kurtz et al. 2000) and the electronic journals. One can easily navigate from the list of references in a published paper to the ADS, and to the list of objects from this paper in SIMBAD. This link is directly implemented in the journal in some cases. CDS is still more organically linked to electronic publication for tables: it participates to the Astronomy & Astrophysics publication process by preparing and publishing on-line tables in the VizieR catalogue service; it also distributes tables from the AAS and ASP journals.
Links are also being implemented with major data archives. Observation logs can easily be implemented in VizieR, like any tabular data. Active links to the archives, located at the observatory site or in a data center, are installed, and an update mechanism is implemented for evolving logs. ALADIN gives access to these lists of observations through VizieR, and also to archive images with the full ALADIN functionalities when feasible (Bonnarel et al. 2001). Links with archives are implemented in SIMBAD on a case by case basis, with an effort to implement links that actually respond, with HEASARC for objects having a ‘high energy' name, with IUE, and soon with ISO, after cross-identification and inclusion of the logs in SIMBAD.
In addition, user data can be used as input in the services: query by lists of objects for SIMBAD and VizieR, integration of user catalogues and images in ALADIN, when using the Standalone version.
Interoperability between heterogeneous, distributed astronomy information services is based on a few de facto standards, such as the 19-digit bibcode (e.g., 1999A&A...349..236E; Schmitz et al. 1995) to describe bibliographic references, first defined by NED and SIMBAD, then extended and widely used by ADS, and accepted by the journals. To be able to integrate data from different origins, one must be able to understand their contents. For that purpose, FITS is of course a major standard of astronomy. The completion of World Coordinate System (WCS) coding in FITS (with which ALADIN is fully compatible) is a major step towards standardisation of image astrometry. Another example is the standard description of tables (http://vizier.u-strasbg.fr/doc/catstd.htx), defined by CDS and shared by data centers and journals.
XML is certainly a key tool for metadata management in the future. CDS has lead the Astrores consortium, which defined an XML standard for tabular data in 1999 (Ochsenbein et al. 2000b). ALADIN is fully Astrores/XML compatible, and VizieR results can be retrieved in XML format (among many others) to be interpreted and used by any other service.
Another problem for interoperability is the maintenance of links. The Générateur de Liens Uniformes (GLU; Fernique et al. 1998), first developed by CDS to maintain links between its own services, is widely used for that purpose. A shared dictionary describing links is implemented and updated, and the GLU mechanism allows service providers to use symbolic links in their html pages. These symbolic links are translated to physical links using the GLU dictionary. The GLU dictionary can easily be made compatible with XML syntax, and it can also manage metadata.
The world-wide astronomy web already gives access to many on-line resources, from observational data to published results. One main objective of the Virtual Observatory is to include survey data, and tools to manage them, in the astronomy web. Survey catalogues can be included in VizieR, where at present time they can very efficiently be searched by position (Derriere et al. 2000). R&D studies are under way to implement the capability to search very large catalogues by other criteria. Remote on-line survey images will soon be accessible through ALADIN, provided that they can be retrieved by an HTTP query, with a FITS WCS description. For instance, ALADIN is already provided by NED as a tool to access their image collection (Bonnarel et al. 2001).
Another aspect of the Virtual Observatory is the need to be able to mine through large amounts of heterogeneous data. This was addressed by the ESO-CDS Data Mining project (Ortiz & Ochsenbein 2001). The project defined a set of Uniform Content Descriptors (UCD), to describe hierarchically the contents of tables (i.e., catalogues, tables, surveys, observation logs); and implemented a prototype of fast cross-correlator, able to identify relevant tables in VizieR by their UCDs and to correlate user tables with these tables.
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