Virtual Observatory Glossary
This page briefly describes some of the VO terminology and technology that underlies the NAVO efforts and which may be referenced in the NAVO summary pages and other NAVO publications. This glossary is intended to provide a sense of what the terms mean. NAVO has defined the specific VO standards that will be used in NAVO interfaces in our Agreed VO Standards pages. Our goal here is not to rigorously define these standards but to show how they are used in sufficient detail so that the reader can understand where and why they are used and the relationships among standards are clear.
The VO is the collaboration of astronomers and astronomy data providers using standard interfaces. These interfaces allow the discovery and retrieval of information for users who wish to get data, and allow for the publication and dissemination of data by data providers. These standards have been developed by the International Virtual Observatory Alliance (IVOA), a consortium of national virtual observatory institutions. More information is available at the IVOA.
We can break down the VO standards into three basic areas:
- Registry standards discuss how we populate and query the VO registry. The VO registry is the mechanism by which users can discover VO services and capabilities.
- Data protocols (or data access protocols as they are referred to in IVOA documentation) are the protocols that allow users to query and retrieve information from data providers. These tend to be the protocols one most directly associates with the VO.
- Content protocols generally define formats or elements of the files retrieved through the content services. They may also help define the syntax of the parameters that are used in querying services. Some content protocols define content that data providers should provide.
There are two kinds of registries - searchable registries, which applications will query, and publishing registries, where resource providers place their information. If you want to know of all the registries that exist, the IVOA maintains a "Registry of Registries".
A registry is a list of all the services made available by different data providers. Each entry records some information about the type of service, who provided it, and what kind of data it contains.
There are two methods of publishing your service into a registry. The most common method is to enter service information into a Fully Searchable Registry. The method, typically used by archive data centers with many services, is to create a Publishing Registry.
A publishing registry is the vehicle for making a resource available to the virtual observatory (VO). In particular, it can create new descriptions of resources and share them with the rest of the VO through the harvesting process. A data center, which may curate a number of data collections and offer a variety of services to access them, may operate their own publishing registry. A searchable registry can also support the publishing function, which the VAO Registry at STScI does.
Registry Searching - Services put all sorts of useful information in their registry entries, and some tools make use of this. So for example, you can look specifically for image services, for X-ray data, for data curated by HEASARC, for services that are new this month, and so on.
There are five major data protocols implemented at NAVO centers, Simple Cone Search, Table Access Protocol, Simple Image Access Protocol, Simple Spectral Access Protocol, and DataLink.
Simple Cone Search - This offers the simplest access to astronomical catalogs. The input is sky position and radius. A subset of the catalog within that radius is returned using the VOTable format defined below. This protocol is very easy to implement and query but scales poorly since a separate query is required for every position.
Table Access Protocol (TAP) - This service offers more flexible access to data tables, along the lines of those used to make queries to other databases. The input is a query in Astronomical Data Query Language (ADQL), which is basically a standardized version of SQL. A data table is returned.
Simple Image Access Protocol (SIAP) This offers access to pixel data. The input is a position and a size. If it's a "cutout" service, the result will be an image, centered at the requested position, with the requested size. If it is an "atlas" service, which holds a collection of standard sized data frames, then the size is used to look for frames centered within that distance of the requested position, and the whole data frame(s) returned.
Simple Spectral Access Protocol (SSAP) - This service provides access to spectra. The input is a position and size. Like the "atlas" version of image access, the result will be any spectra whose target positions are within the stated distance of the requested position.
DataLink - This is an intermediate data access service that connects discovered data-sets to the downloadable data files, services that can act on the data files, and links to related resources.
The content standards used in the virtual observatory are VOTable, Data Models, ObsTAP, and UCDs.
VOTable - VOTable is designed as a flexible storage and exchange format for tabular data, with particular emphasis on astronomical tables. All VO tools interact with VOTable. VOTable has more flexible descriptive metadata than CSV or FITS tables.
Data Models - The IVOA is developing a suite of standard data models. This way data returned in a search service follows some standard structure and uses standard ways of specifying what kind of quantity is in a column, so that a lot more can be done automatically.
ObsTAP - ObsTAP is a Table Access Protocol (TAP) interface to observation data model. ObsTAP and the observation data model describe observations in a broad sense (exactly what comprises an "observation" is not well defined within astronomy and is left up to the data provider to define for their data). ObsTAP also describes archive data products (e.g., actual archive files). In general an "observation" may be composed of multiple individual data products.
UCDs - The Unified Content Descriptor (UCD) is a formal vocabulary for astronomical data that is controlled by the International Virtual Observatory Alliance (IVOA). The vocabulary is restricted in order to avoid proliferation of terms and synonyms, and controlled in order to avoid ambiguities as far as possible. It is intended to be flexible, so that it is understandable to both humans and computers. UCDs describe astronomical quantities, and they are built by combining words from the controlled vocabulary.