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Science with the Virtual Observatory |
This is a short exercise for finding resources in the VO using a VO Registry. A registry functions as a sort of yellow pages, or high-level directory, of astronomical catalogs, data archives, data-providing organizations, and computational services . You can search the registry to find data of interest, review the data source descriptions, and in many cases make direct position-based data requests.
A resource is a general term referring to a VO (Virtual Observatory) element that can be described in terms of who curates or maintains it and which can be given a name and a unique identifier. Just about anything can be a resource: it can be an abstract idea, such as sky coverage or an instrumental setup, or it can be fairly concrete, like an organization or a data collection. This definition is consistent with its use in the general Web community as “anything that has an identity” (Berners-Lee 1998, IETF RFC2396). We expand on this definition by saying that it is also describable. There are several types of resources published in the VO registries: standard VO data services (SIAP, Cone, SSAP, and SkyNodes, data collections, organizations, other registries are even considered resources.
An IVOA proposed recommendation, Resource Metadata for the Virtual Observatory, referred to as the "RM document", defines standard metadata concepts that describe resources, independent of any particular encoding format. It also describes the general resource model upon which the concepts are based and can be located on the IVOA Twiki site here.
There are several registries available throughout the IVOA community. We'll be working through this exercise using one of the Fully Searchable NVO registries. You may also start with the main NVO Home page.
The Registry portal can be accessed directly from the NVO-Home page or you can go to the URL http://nvo.stsci.edu/VORegistry. We will start from the NVO-Home page at the Start Using NVO section.
Click on the Full Registry Interface to go to the Home page of the Registry portal. Note the link on the front to http://voservices.net/registry/ . This is the mirror site at JHU.
The Registry is a searchable index of data sources and other resources accessible through the NVO. This particular registry is based on a relational database and uses text indexing. Other VO Registries have portals which interface to XML databases and support additional paradigms for querying. Begin to explore which resources are available for a specific topic by entering in the Search text box the words “abell cluster” and Click on the Search button.
Your search results appear on the page. In the horizontal row below the text box you will see a listing of the different Resource Types available. Cone resources are data archives that can be searched by position. The number next to the word Cone represents the number of these services available. Click on the word Cone to see only this subgroup of resources. You may see other resource types appear in this row listing depending on the available types of resources found from the keyword Search. For a list of all available resource types currently implemented in the registry you can click on the Contents tab.
The results table contains the title and a brief description of the resource. To see additional specific metadata descriptions about an individual resource you can select View at the left side of a resource. Click on View next to the Abell CofG and you will see the Vizier Abell catalog resource metadata. These are standard terms that are used in all IVOA (International Virtual Observatory) registries. A more complete description/definition of each term can be found in the Resource Metadata document located in the left panel Help page.
Click Back on your Browser page to go back to the listing of Cone resources.
To look at data from this service Click on the Try-It button on the left side of the Abell Clusters (ABELL). You will now see a data access test page. This is a very simplified interface (work in progress) that requires some a priori knowledge of coverage. . Enter the values ra 195, dec 28 and 5 deg for a search radius to retrieve data from the Coma Cluster then Click the Submit. You will see the html table representation for the Abell catalog. You can also go back and Click on VOPlot to launch the plotting tool. You can experiment with other Cone services to see the various outputs formats (Hint: Repeat step 2 and Try “GSC2 Cone” ). The XML output option shows the VOTable that is the standard XML representation of the VO cone search query result.
Click on the QUERY tab at the top. You now see the same interface as the Home page except there are options for AND/OR on the text searching. Click on the Advanced search option. This query form allows you to enter SQL-like predicates or select from a pull down list. A table of available metadata fields which predicates can be constructed on can be seen in the VOMetadata link. Select from the pull down list the entry ResourceType like ‘%CONE%’ and subject like ‘%AGN%’ and then Click Execute Query. Once again the search results appear below. The percent symbols in the predicate are a method for specifying a partial text match.
If you have a VO data service to publish, an archive collection, or perhaps an organization, you can enter this locally via the PUBLISH form and your resource will automatically become available in the VO registry network. For detailed information on how to publish to the NVO you can select the Help tab on the left panel and read Overview of Publishing to the VO. The NCSA registry has a more complete publishing user interface which will be demonstrated in the following talk Publishing to the VO.
To look at the publish interface, Click on the PUBLISH tab at the top. You will see a few fields already populated. We will not go through all these steps. It is often helpful to have examples for getting started. If you Click on the CONTENT tab at the top of the page you will see again the available resources (and total count at top). You can use a sample template by selecting one of the resources, for example SIAP, then from the search results page Click on Copy next to a sample resource, enter your own identifier and correct the metadata fields. Submit your new resource from this page. *Note you also are required to assign a Password that will allow you to manage and curate(make changes) to your resource.
The registry also has programmatic interfaces to support remote Applications that require dynamic access to VO resources/services. If you Click on the DEVELOPER tab at the top it will take you to a new page with web services links and also some client resources. Go ahead to this page.
Applications which use web service clients interface via the W3C Soap protocols for searching the Registry. Click on the Search Services link. You will now see a page with several web services listed. Click on the Service Description link and it will load the Registry endpoint http://nvo.stsci.edu/VORegistry/registry.asmx?WSDL. This is the Web Services Description Language and not designed to be a friendly human readable format. For application developers using SOAP web services this is the required description “ endpoint” for generating client interfaces to the Registry. This is language independent.
Click back on your browser and go to the Keyword Search. Since these are .NET web services there is automatically a convenient simple test interface provided, so you can again enter search text. Try this service by entering “coma cluster” in the keywords box and type “true” for the andKeys which says ‘AND’ the text words rather than OR’ing them. Click Invoke. You will see an XML page with metadata for each resource retrieved. This is again only a test form page and the actual use of these services is for programmatic interface. This test interface is valuable for performing quick checking of services.
There are several Registries available in the IVOA community. All registries store and exchange resources in a standard format. The other NVO searchable registry(Carnivore) is developed at Caltech and is based on XML database technology. You can explore this registry and others by going to the side panel Projects Link. Click on Projects and you will see the two major categories, Searchable and Publishing. Links to these other registries are provided here and you are encouraged to explore them.
The NVO Summer School is made possible through the support of the National Science Foundation and the National Aeronautics and Space Administration.
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