... The National Virtual Observatory (NVO) project is an effort to federate astronomical resources, to provide seamless access to heterogeneous data at various centers throughout the world, and make them appear to the user as a homogeneous set. ... It knows how to find and communicate with a service registry, but does not have prior knowledge of which services are available. ... When the user clicks `Find Images,' the portal forms queries for each of the archive services that were selected by the user. ...