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WSA - Data Access

Users can access the data held in the WSA through (a) web-based forms, or (b) through Virtual Observatory clients. The former provide simple, easy-to-use interfaces that allow sample selections up to the limitations imposed by the various forms and the use of Structured Query Language, while (b) can be particularly useful if you need to do larger scale and/or repetitive operations, especially those more easily achieved in a programming environment.

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(150000 / no. parameters) x 1000
    So if only three parameters have been requested than the file can contain up to 50 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 Topcat for working with catalogue data and Aladin for working with images. Topcat provides the ability to perform cone searches and ADQL queries against our data. Simply use the "VO" menu, select the chosen access method, and search for UKIDSS to find our services. Topcat can also be scripted to perform data retrieval and analysis in an automated fashion, and to do this refer to the documentation on Topcat's website. Aladin provides the ability to view and manipulate image files. You can search by going to "File -> Open" and selecting UKIDSS from the left menu, or select "All VO" at the top and search by position. Results can be immediately downloaded and used.

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Links | Credits

WFAU, Institute for Astronomy,
Royal Observatory, Blackford Hill
Edinburgh, EH9 3HJ, UK