On-line Catalogues and Image Stores

GAIA is capable of using much of the increasingly large amounts of information available at astronomy data-centres distributed around the world.

The way that GAIA accesses on-line information is two-fold for historical reasons. Using the older SkyCat facilities a preset list of useful catalogues and images are available for query, whereas using the newer Virtual Observatory query mechanisms an ever-expanding set of resources is available. Access to both these query mechanisms is found in the "Data-Servers" menu.

Local catalogues

Local catalogues are contained in files held on your local computer. These are opened using the "Open" item in the "File" menu, or by the "Local Catalogues" sub-menu in the "Data-Servers" menu (this is available in the main window and in the catalogue window). Local catalogues can be created by saving remote ones to disk (this is a good idea when you have slow or intermittent internet access), or they can be created by other programs (such as SExtractor -- SUN/225).

The native format of local catalogues is called "tab-table". This is a simple, plain-text, format that is described in the GAIA User Note (SUN/214, the command showme sun214 may display this in your web browser).

As well as this format (which is the most efficient to use), local catalogues can be stored in any format accessible by the CURSA (SUN/190) package (this includes FITS-tables), and the "ASCII_HEAD" format used by the SExtractor package.

It is also possible to "import" plain text files (that is ones that are space, comma or fixed width separated) and display them as catalogues. To do this you need to use the "Import plain text file..." toolbox that is found in the "Positions..." sub menu of the "Image-Analysis" menu. There is some specific help about the toolbox that performs the import:

Finding out more

If you'd like to know more about the "Data-Servers" menu then look in the "Help" menu on the main window and select "SkyCat...". This should start up netscape and take you to the WWW Skycat page at ESO. There's a FAQ and some other information. The description of how the WCS system works is largely correct, but GAIA uses a different back-end (the Starlink AST library) which supports more formats and the NDF WCS component. You should also note that GAIA may not be using the latest version of SkyCat, so everything described there may not be available.

A rather more pointed description of how catalogues are served over the web (and how to provide your own) is given in Starlink System Note 75 (SSN/75). This may be available on your local machine, or from Starlink (try "http://www.starlink.ac.uk/docs/ssn75.htx/ssn75.html").