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6 VizieR interfaces

\end{figure} Figure 3: Excerpt of the VizieR first search page

Several interfaces are currently available for an access to the data stored in VizieR: directly from a Web browser, via a construction of the query using the ASU conventions, or the developing XML interfaces.

6.1 Access from a browser

From a WWW-browser, a "standard query'' in VizieR consists in a few steps:
Locate the interesting catalogues in the VizieR Service[*]. This can be done in various ways illustrated in Fig. 3: from well-known catalogue acronyms like HIP or GSC, from a choice in the set of predefined keywords, from authors' names, or from a self-organizing (or Kohonen) map constructed on the basis of the keywords attached to the catalogues (Poinçot et al. [1998]). New possibilities for locating catalogues of interest for the user are currently under development;

Once a catalog table - or a small set of catalog tables -- is located (for instance the Hipparcos Catalog[*] resulting from the Hipparcos mission), constraints about what to search and how to present the results can be specified, as: By pushing the appropriate buttons, it is for instance easy to get the list of Hipparcos stars closer than 5 parsecs to the Sun, ordered by their increasing distance[*].

Obtaining full details about one row is achieved by a mouse click in the first column of the result: for instance, the first row of the search for nearby stars described above leads to the VizieR Detailed Page with Hipparcos parameters and their explanations concerning Proxima Centauri[*].

Finally, there may be correlated data, like notes or remarks, references, etc. In our example, Proxima Centauri is related to the $\alpha$ Cen multiple star system, which components can be viewed from the link to the double and multiple stars (CCDM)[*] that appears in the detailed page.

The quantitative monthly usage of VizieR is presently (October 1999) about 40 000 external requests from 2700 different nodes; mirror copies were installed recently in the US[*] and in Japan[*] in order to overcome the transcontinental network congestions.

6.2 The ASU protocol

The uniform access to all catalogues is based on the so-called ASU[*] (Astronomical Standardized URL) protocol resulting from discussions between several institutes (CDS, ESO, CADC, Vilspa, OAT). The basic concept of ASU is a standardized way of specifying queries to remote catalogues in terms of HTTP requests: the target catalogue is specified by a      -source=catalog_designation parameter, the target sky position by a      -c=name_or_position,rm=radius_in_arcmin parameter, the output format by -mime=type, and general constraints on parameters by     column_name=constraint. It should be noticed that the representation of a target by the name of an astronomical object (typically a star or galaxy name, e.g.3C 273) implies the usage of a name server converting a target name into a position in the sky, which is typically achieved by a call to SIMBAD.

6.3 The XML interface

The output of a query to VizieR as presented in Sect. 6.1 can hardly be used by an independent application for further data processing, such as the ALADIN[*] visualisation tool (Bonnarel et al. [2000]) which allows to superimpose the catalogued sources on top of actual image of the sky: the application requires an accurate interpretation of the catalogued output in terms of celestial positions in order to find out the exact location of each source. This means that ALADIN has to figure out not only which are the columns representing the celestial coordinates, but also accurate definitions of the system used to express the coordinates, their accuracy, etc. -- in other words the metadata about the celestial coordinates.

XML (eXtensible Markup Language) is an emerging standard which allows to embed markup "tags'' within a document; the key advantages of this language are that the same document can either be parsed by simple-minded programs (XML uses hierarchical structuring), or can be displayed in the new generation of browsers (via an XSL style sheet which maps the markup "tags'' into typographical specifications). This language presents other potential interests, especially regarding interoperability issues facilitated by the emergence of generic tools able to process XML documents.

The XML layout of astronomical tables was discussed extensively with interested collaborators, and the agreed definitions were presented at a recent ADASS meeting (Ochsenbein et al. [1999]). The output of VizieR is readily available in this format[*], currently used by the Aladin image applet; it is hoped that it will facilitate the usage of the astronomical data in new contexts.

6.4 Current developments

With the large set of homogenized catalogues, VizieR plays a central role in a data-mining project currently in development as a collaboration of ESO and CDS, in two main directions: (i) make use of the VizieR large set of described columns (over 70 000 currently) to build up new methods for locating the catalogues which are the best suited to a particular research topic; and (ii) develop automatized cross-correlation tools which can take into account the largest possible set of meaningful parameters (Ortiz et al. [1999]).

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