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6 VizieR interfaces
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.
From a WWW-browser, a "standard query'' in VizieR consists in a few steps:
6.1 Access from a browser
- Locate the interesting catalogues in the
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
constructed on the basis of the keywords attached to
the catalogues (Poinçot et al. ).
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
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.
- constraints based on the celestial coordinates, i.e.
location in the neighbourhood of a target specified
by its actual coordinates in the sky, or by
one of its name as known in SIMBAD (see Wenger et al.
- any other constraint on any of the columns
of the table(s), like a minimal flux value,
or the actual existence of some parameter
- which columns are to be displayed, and in which order
the matching rows are to be presented.
- 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
- Finally, there may be correlated data, like notes or remarks,
references, etc. In our example, Proxima Centauri
is related to the
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
in order to overcome the transcontinental network congestions.
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
the target sky position by a
the output format by -mime=type,
and general constraints on parameters by
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.
6.2 The ASU protocol
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. )
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
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. ).
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.
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. ).
6.4 Current developments
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