Home | Overview | Browser | Access | Login | Cookbook | nonSurvey

WSA - Data Access

Web-based forms parse the user's input parameters and submit SQL (Structured Query Language) queries to the database.

• Archive listing - retrieve listings of the multiframes held in the WSA. Links are returned allowing the user to view the library jpegs and download the FITS files.
• GetImage - extract cut-out images around a given position
• ColourImage - specify and generate colour images around a given position
• MultiGetImage - upload a list of coordinates and extract matching cut-out images
• Region search - search the WSA object catalogues around a supplied position
• Menu query - build simple SQL queries using a series of forms
• Freeform query - submit an SQL query directly
• CrossID - upload a list of coordinates and extract matching objects

The results i.e. rows selected from the database, are displayed in an HTML table and/or written to a file (ASCII, FITS or VOTable format) that can be downloaded.

Users should note the following general points:

• HTML table output is only intended as a summary and the number of rows displayed is limited to 100. A note at the end of the table informs the user if this was exceeded.
• The number of result rows written to files is also limited and depends on how many parameters have been requested i.e. maximum rows written to file = nint(15000 / no. parameters) x 1000 So if only three parameters have been requested than the file can contain up to 5 million rows. Again users are warned if the limit was exceeded and an indication of how many extra rows were returned is provided.
• Users who exceed the file row limit should submit their query in parts e.g. by querying sections in RA or Dec or magnitude slices.
• If an email address is supplied queries are allowed to run for 30 seconds before they are placed in the background and the browser window is released. On completion an email is sent informing the user where to pick up the results.
• The tables in the database do not contain any NULL values. Where values are unavailable for a given object parameter default values have been inserted. Users should be aware of this when constructing their queries and when interpreting the results. See the schema browser for details of a given parameter's default value.
• NULL values can be returned if users JOIN tables. If this occurs the values are written out as zeroes.

For access through the Virtual Observatory, we recommend the use of AstroGrid clients. For client-side Graphical User Interfaces that provide much richer, astronomy-specific functionality than a standard web browser, use the AstroGrid Desktop Suite, which includes, for example, the powerful VOExplorer. For ultimate flexibility, AstroGrid provides a programming environment within Python which is in it's simplest form a straight-forward scripting language, but which also supplies a richly functioned, fully object-oriented programming language should you wish to write sophisticated sample selection and analysis software for your purposes. Full instructions, links and examples concerning use of AstroGrid software are available from the AstroGrid website, but for a quick-start to AstroGrid Python follow these simple steps:

• Download the VODesktop and the AstroGrid Python egg file from the downloads page;
• Install the AstroGrid Python egg file by following these instructions;
• Start up the VODesktop software (run the executable .jar file - on Windows just double click on it, on Linux run a "java -jar name-of-file.jar" command)
• Launch Python and begin scripting! Follow to the tutorial to learn the basics, and example scripts of more complex queries can be found at the bottom of that page.