VSA - Data Access
Users can access the data held in the VSA through web-based forms.
These 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 VSA. 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
- MultiGetImage - upload a list of coordinates and extract matching cut-out images
- Freeform query - submit an SQL query directly
Query 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. 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(300,000 / 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
WFAU, Institute for Astronomy,
Royal Observatory, Blackford Hill
Edinburgh, EH9 3HJ, UK