2 The present context of CDS activities

The ground- and space-based observatories, the sky surveys, and the deep field observations, produce large amounts of data, obtained at different wavelengths with different techniques. To understand the physical phenomena at work in objects, astronomers need to access this wealth of data, to understand their meaning (error bars, etc.), and to combine the use of data from different origins, especially with the development of panchromatic astronomy. At all steps of an astronomer's work, it is thus more and more necessary to re-use data obtained by others and to take into account previous results, often from other fields of astronomy.

Astronomers and funding Agencies are now well aware of the necessity of preserving and diffusing data and results. This leads to several types of developments:

• the data producing teams, which have the knowledge of instruments and observation techniques, have to preserve all or part of their data, in a form usable by all astronomers - in the majority of recent large projects, the building of the observatory archive is now considered as one of the project missions;

• in some domains, a specialized center provides access to and information about data in a given subfield of astronomy - this is the case in particular of the disciplinary NASA Centers, HEASARC for High Energy astrophysics, IRSA at IPAC for Infrared data, and MAST at the Space Telescope Science Institute, for optical and UV data;

• as a Data Center, the role of CDS is to bridge the gap between the specialized approach of the scientific teams, and the general approach of the community of researchers. In the present context, with the very rapidly growing number of on-line services of interest to astronomers, this means in practical terms the definition, development, and distribution of tools for retrieving useful information among the vast array of possible sources (Sects. 3, 5).

In addition, journals are recent, but very active actors in the on-line distribution of information in astronomy. Electronic publication has made rapid progress and many journals now have on-line versions, which often display external links. Moreover, NASA Astrophysics Data System (ADS, Kurtz et al. [2000]) has become a major reference tool for astronomers. Several aspects of the ADS are described in a set of companion papers. The bibliographic astronomy network, and the CDS role in this networking, are described in more detail in Sect. 5.2.

On the other hand, computers and networks evolve very rapidly: the high rate of increase in the volume of data and results, the irruption of the Internet and World Wide Web, the widespread usage of graphical interfaces, the lower and lower cost of information storage, completely changed the technical context of the CDS activities in the last few years.