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Astronomical Data Analysis Software and Systems VII
ASP Conference Series, Vol. 145, 1998
R. Albrecht, R. N. Hook and H. A. Bushouse, e
Ö Copyright 1998 Astronomical Society of the Pacific. All rights reserved.
ds.
The VizieR System for Accessing Astronomical Data
Fran›cois Ochsenbein
Centre de Donn’ees Astronomiques, Strasbourg Observatory, France
Abstract. The recently reshaped VizieR 1 system, a unified query inter­
face to an increasing number of catalogs (presently # 1500), is presented.
1. Historical Background
VizieR was first presented at the AAS meeting in early 1996 (Ochsenbein et
al. 1995), as the result of a joint e#ort between CDS 2 and ESA­ESRIN 3 (the
European Space Agency's Information Systems division) in order to provide the
astronomical community with a dedicated tool for retrieving astronomical data
listed in published catalogs and tables --- a follow­up of the ESIS (European
Space Information System) project.
Shortly after this first version, which has been e#ectively accessible since
February 1996, new needs for performance and for standardisation led to ba­
sic changes in the system: the ASU 4 (Astronomical Standardized URL), re­
sulting from discussions between several institutes, was adopted as the way to
specify constraints in the new version of VizieR, which was introduced on 29
May 1997 --- just in time for the distribution of the results of Hipparcos cata­
logs. The basic concept of ASU is a standardized way of specifying catalogs (as
­source=catalog designation), target positions (as ­c=name or position,rm=ra­
dius in arcmin), output format (as ­mime=type), and general constraints on pa­
rameters (as column name=constraint).
Besides the adoption of this new protocol, the most visible changes in this
new version of VizieR are an easy access to notes, and possibilities of navigation
between the tables of a catalog.
The quantitative daily usage of VizieR is presently (September 1997) about
1,000 external requests from 75 di#erent nodes; 1,000 di#erent nodes e#ectively
submitted queries to VizieR during the first 3 months of the new installation
(June to August 1997); among all queries, about 40% of the hits concern the
recent results of the Hipparcos and Tycho missions.
1 http://vizier.u­strasbg.fr
2 http://cdsweb.u­strasbg.fr
3 http://www.esrin.esa.it/esrin/esrin.html
4 http://vizier.u­strasbg.fr/doc/asu.html
387

388 Ochsenbein
2. How to Query in VizieR
The ``standard query'' in VizieR consists in a few steps:
1. Locate the interesting catalogs in the VizieR Service 5 . This page presents
various ways of finding out the interesting catalog among this large set:
(a) from one of their usual acronyms, like GSC, HD, HIC, etc. . .
(b) from a set of words (author's names and/or words from the title of
the catalog), and/or keywords attached to each catalog;
(c) or by clicking in a Kohonen Self­Organizing Map 6 , a map created by
neural network techniques which tends to group in nearby locations
those catalogs having similar sets of keywords. This technique is
the same as the one used by Lesteven et al. (1996) to index the
bibliography.
2. Once a catalog table -- or a small set of catalog tables --- is located (for
instance the Hipparcos Catalog 7 resulting from the Hipparcos mission),
constraints about the input and/or output can be specified, as:
. constraints based on the celestial coordinates (location in the neigh­
bourhood of a target specified by its actual coordinates in the sky, or
its name, using SIMBAD 8 as a name resolver);
. any other qualification on any of the columns of the table(s); the
standard comparison and logical operators are available, detailed in
the VizieR help pages 9 ;
. which columns are to be displayed, and in which order the matching
rows are to be presented.
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 10 .
3. Obtaining full details about one row is achieved by simply clicking 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 11 .
5 http://vizier.u­strasbg.fr/cgi­bin/VizieR
6 http://vizier.u­strasbg.fr/cgi­bin/VizieR#Qkmap
7 http://vizier.u­strasbg.fr/cgi­bin/VizieR?­source=I/239/hip main
8 http://simbad.u­strasbg.fr/Simbad
9 http://vizier.u­strasbg.fr/cgi­bin/Help?VizieR/intro
10 http://vizier.u­strasbg.fr/cgi­bin/VizieR?­source=I/239/hip main&­sort=­
Plx&Plx=%3e=200
11 http://vizier.u­strasbg.fr/cgi­bin/VizieR­5?­source=I/239/hip main&HIP=70890

The VizieR System for Accessing Astronomical Data 389
4. Finally, there may be correlated data, like notes or remarks, references,
etc. . . . In our example, Proxima Centauri is related to the # Cen system,
which components can be viewed from the CCDM link appearing in the
detailed page.
It should be noted that the usage of the ASU protocol allows to write
anywhere in a text (like it is done in this short article) a hyperlink to the
result of a query: for instance, all parameters from the Hipparcos catalog for
the star HIP 12345 can be pointed to by a call to VizieR with parameters:
­source=I/239/hip main&HIP=12345 12 ; or a pointer to all Tycho stars closer
than 0.5 # to Sirius, ordered by their increasing distance to the brightest star,
can be written by a call to VizieR with parameters:
­source=I/239/tyc main&­c=Sirius,rm=30&­sort= r 13
3. VizieR Structure
VizieR is based on the usage of a relational DBMS: the data tables are stored
as relational tables, and a set of META tables --- a structure which was called
Reference Directory in the previous version of VizieR --- contains the necessary
descriptions of all items:
. METAcat is a table providing the description of catalogs (a catalog is a
set of related tables, like a table of observations, a table of mean values,
a table of references, . . . ); METAcat details the authors, reference, title,
etc. . . of each stored catalog. There are presently # 1500 rows in this table.
. METAtab is a table providing the description of each table stored in VizieR:
title, number of rows, how to access the actual data, the equinox and epoch
of the coordinates, etc. . . There are presently # 3500 rows in this table,
i.e. an average of 2 1
3
tables per catalog.
. METAcol is a table providing the description of each column stored in
VizieR: labels, units, datatypes, etc. . . There are presently # 45000 rows in
this table, i.e. an average of 13 columns per table.
. about 10 more tables exist in the system to detail other parameters, like
the definitions of keywords, the acronyms associated with the catalogs, the
notes, etc...
4. The VizieR Feeding Pipeline
It is of course not possible to enter all details describing the # 45000 columns
by hand: the VizieR feeding pipe­line is fully automatic using as input the stan­
dardized description 14 of the catalogs shared by the Astronomical Data Centers,
12 http://vizier.u­strasbg.fr/cgi­bin/VizieR?­source=I/239/hip main&HIP=12345
13 http://vizier.u­strasbg.fr/cgi­bin/VizieR?­source=I/239/tyc main&­c=Sirius,rm=30&­
sort= r
14 http://vizier.u­strasbg.fr/doc/catstd.htx

390 Ochsenbein
and used for the description of the electronic tables published by a large fraction
of the major astronomical journals (A&A, ApJ, AJ, PASP).
The addition of a new catalog into VizieR consists in two steps: (1) adding
into the META tables all items describing the new catalog: catalog title, authors,
table captions, etc. . . and details about every column of each table; and (2)
converting the data of each original (ASCII) file making up the catalog into a
relational table. In this step, the existence of data (the so­called NULL values)
is carefully tested, and some important parameters like astronomical positions,
magnitudes and color indexes are converted to uniform units. We however take
care of storing, as much as possible, the catalogs in their original form, and for
instance the coordinates are stored in their original equinox and epoch.
5. Access to the Very Large Catalogs
Very Large Catalogs are defined here as catalogs made of more than 10 7 objects
--- a size which can hardly be managed by the existing DBMs. Dave Monet's
(1977) USNO­A1.0 catalog 15 gathering 488,006,860 sources is a typical example:
it consists originally in a set of 10 CDROMs (about 6Gbytes) with 12­bytes
binary records (positions, magnitudes, and a couple of flags).
This catalog was losslessly compressed by grouping small regions of the sky,
allowing to store only position o#sets instead of full­range values positions: the
resulting catalog occupies only 3.4Gbytes, allowing therefore faster access since
the queries are heavily i/o­limited: on a Sparc­20 (72MHz), the average search
time (radius of 2.5') is less than 0.1s, and the whole 488 â 10 6 objects are tested
in about 40 minutes (i.e., 5µs per object).
6. VizieR developments
The current new developments include: (a) more connectivity between catalogs,
with Simbad, and more remote connectivity with external databases, Observa­
tory Archives, and other search engines; the GLU (Fernique et al. 1998) will
most likely be extensively used for this purpose; (b) the creation of shared in­
dexes on fundamental parameters like celestial positions in order to allow queries
directed to a large number of tables; (c) a possibility to submit large lists for
queries; and (d) a facility to present the results in graphical form.
References
Fernique, P., Ochsenbein, F., & Wenger M. 1998, this volume
Lesteven, S., Poin›cot, P., Murtagh, F. 1996, Vistas in Astronomy 40, 395
Monet, D. 1997, ``The USNO A1.0 catalog'', http://psyche.usno.navy.mil/pmm
Ochsenbein, F., Genova, F., Egret, D., Bourekeb, I., Sadat, R., Ansari, S.G., &
Simonsen, E. 1996, Bull. American Astron. Soc. 187 #9103
15 http://vizier.u­strasbg.fr/cgi­bin/VizieR?­source=USNO­A1.0