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
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
For ultimate flexibility, AstroGrid provides a
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
- 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
learn the basics, and example scripts of more complex queries can be
found at the bottom of that page.
WFAU, Institute for Astronomy,
Royal Observatory, Blackford Hill
Edinburgh, EH9 3HJ, UK
Tel +44 131 668 8356 (office)
or +44 131 668 8100 (switchboard)