National Aeronautics and Space Administration

Most NAVO services are accessed simply by formatting a simple URL and parsing the results. The results are normally a VOTable which is an XML file format, which is most conveniently read with an appropriate parser but is usually human readable. The URLs comprise a base URL which varies from center to center, and a query payload which varies according to the protocol. The base URLs are given in the table below. In some cases the base URL is characterized by the table or mission that is to be queried.

For cone searches the query payload comprises the three keywords RA, DEC and SR giving the ICRS right ascension, ICRS declination and search radius, all in decimal degrees. E.g., to query the xmmmaster HEASARC table using a cone search near 3c273 we substitute the table name for XXXXX in the base query below and use the RA and Dec of 3c273 and then pick a search radius appropriate for our needs. For XMM a radius of 0.5 degrees might be sensible. So we end with the URL:
https://heasarc.gsfc.nasa.gov/xamin/vo/cone?table=xmmmaster&RA=187.25&DEC=2.05&SR=0.5 .

Another example - Retrieve basic data for Arp 220:
http://ned.ipac.caltech.edu/cgi-bin/objsearch?objname=arp+220&of=xml_main&RA=0&DEC=0&SR=0.5 .

Simple image queries have a payload of POS and SIZE where POS is a string with the RA and Dec separated by a comma and SIZE is the size of the region in which images are requested in decimal degrees. The result of this query is table describing images in the region. You can look at and parse the table to find the images you want. One of the columns will be a URL where the image described in that row can be downloaded.
http://irsa.ipac.caltech.edu/ibe/sia/twomass/allsky/allsky?POS=20,40&SIZE=0.5 .

To do a general SQL-like query in a table access service you can just add /sync?REQUEST=doQuery&LANG=ADQL&QUERY=xxx where xxx is the URI-encoded query string where most non-alphanumeric characters are replaced by hex escapes. You can get a dump of the entire database schema (tables and columns available) by appending /tables to the base.
https://heasarc.gsfc.nasa.gov/xamin/vo/tap/sync?REQUEST=doQuery&LANG=ADQL&QUERY=select+*+from+xmmmaster .

Simple spectral queries have a payload of POS and SIZE where POS is a string with the RA and Dec separated by a comma and SIZE is the size of the region in which spectra are requested in decimal degrees.
http://archive.stsci.edu/ssap/search2.php?id=FUSE&pos=151.0,69.0&size=2.0 .

These interfaces support additional constraints and query modes which are discussed in the detailed protocol specifications available at the IVOA website but this can get you started.

Limits:

1. No limits on size of query
2. 100,000 row limit on results
3. Result limit can be overridden
4. Services not indexed in registry
5. 0.75 degree radius limit on query
6. 50,000 row limit on results
7. Echoes input position in results
8. 200 row limit on results
9. 25 currently in registry

Limits:

1. Variable limit on query size but typically of order 10 degrees.
2. By default results returned depend on ratio of query size and intrinsic pixel resolution of survey datasets.
3. Default limit of 2,000 rows returned.
4. Default can be overridden.