\subsection{\xlabel{online_catalogue_capabilities}On-line catalogue capabilities}

Using the facilities provided by
\htmladdnormallink{SkyCat}{http://archive.eso.org/skycat}, GAIA is
capable of downloading images from several surveys and catalogues from a
wide range of sources, so for instance if your image has a world
coordinate system associated with it, you can query the ESO catalogue of
HST guide stars about any located within the bounds of your image. These
will then be listed and plotted over your image.  Similarly you can
query the
\htmladdnormallinkfoot{NED}{http://nedwww.ipac.caltech.edu}
(NASA/IPAC Extragalactic Database) and
\htmladdnormallinkfoot{SIMBAD}{http://simbad.u-strasbg.fr/simbad}
databases and see lists of all the objects that they know about on your
image. You can also display an image returned directly from the
\htmladdnormallinkfoot{Digital Sky Survey}
{http://arch-http.hq.eso.org/dss/dss}(and then perform queries in other
catalogues about this field). Named objects from certain catalogues can
have their full information displayed in a web browser and may even have
bibliography links to the
\htmladdnormallinkfoot{ADS}{http://adswww.harvard.edu} abstract
service.  Finally the
\htmladdnormallinkfoot{HST}{http://cadcwww.dao.nrc.ca/hst} archives
are also available. Using these you can see what observations have
been made of any objects on your image (and preview any generally
available).

More recently GAIA has been extended to offer similar features,
catalogue and image access, using the developing Virtual
Observatory. These like the features above are found in the
\mytt{Data-Servers} menu of the main window.

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{\bf Note:} you should take time to find what your obligations, in
terms of the correct acknowledgement of copyright, use of service
{\em etc.} are, whenever you use facilities such as those mentioned
above.

