In modern astronomy, cross-matching data acquired at different wavelengths is often the key to the understanding of astronomical phenomena, which means that astronomers have to use data and information produced in fields in which they are not specialists. The development of tools for cross-identification of objects is of particular importance in this context of multi-wavelength astronomy.
A detailed description of the CDS on-line services can be found, e.g.,
in Egret et al. ([1995])
and in Genova et al. ([1996], [1998], [2000]),
or at the CDS web site![[*]](/icons/foot_motif.gif){kind=icon}
.
Several sites currently provide on-line access to digitized sky surveys at different wavelengths: this is, for instance, the case of Digitized Sky Survey (DSS) at STScI (Morrison [1995]), and of similar implementations at other sites, providing quick access to cutouts of the compressed DSS images. SKYVIEW at HEASARC (McGlynn et al. [1997]) can generate images of any portion of the sky at wavelengths in all regimes from radio to gamma-ray. Some of these services provide simultaneous access to images and to catalogue data. The SKYCAT tool, recently developed at ESO (Albrecht et al. [1997]), addressed this concern in the context of the European Southern Observatory scientific environment (in view of supporting future users of the Very Large Telescope); SKYCAT uses a standardized syntax to access heterogeneous astronomical data sources on the network.
ALADIN has been developed independently by the CDS since 1993 as a dedicated tool for identification of astronomical sources - a tool that can fully benefit from the whole environment of CDS databases and services, and that is designed in view of being released as a multi-purpose service to the general astronomical community.
ALADIN is an interactive sky atlas, allowing the user to visualize a part of the sky, extracted from a database of images from digitized surveys and observational archives, and to overlay objects from the CDS catalogues and tables, and from reference databases ( SIMBAD and NED), upon the digitized image of the sky.
It is intended to become a major cross-identification tool, since it allows recognition of astronomical sources on the images at optical wavelength, and at other wavelengths through the catalogue data. Expected usage scenarios include multi-spectral approaches such as searching for counterparts of sources detected at various wavelengths, and applications related to careful identification of astronomical objects. ALADIN is also heavily used for the CDS needs of catalogue and database quality control.
In the case of extensive undertakings (such as checking the astrometric quality for a whole catalogue), it is expected that ALADIN will be useful for understanding the characteristics of the catalogue or survey, and for setting up the parameters to be adjusted while fine tuning the cross-matching or classification algorithms, by studying a sample section of objects or fields.
A discussion of the usage of such a tool for cross-identification can be found in Bartlett & Egret ([1997]), where it is shown how training sets are used to build likelihood ratio tests.
The ALADIN interactive atlas is available in three modes: a simple previewer, a Java interface, and an X-Window interface. We describe here mostly the Java interface which is publicly accessible on the World-Wide Web.
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