NVO Frequently Asked Questions
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What is the Virtual Observatory?
The Virtual Observatory is a framework that enables new astronomical research by greatly enhancing access to worldwide data and computing resources.
What is the NVO?
The National Virtual Observatory is the US Virtual Observatory project. NVO is one of 17 national and international initiatives that have joined together in the International Virtual Observatory Alliance (IVOA) to agree upon standards and protocols such that astronomical data worldwide can be located, retrieved, and analyzed within a common framework.
Who is behind the NVO?
The NVO development project is funded by NSF’s Information Technology Research Program. Dr. Alex Szalay of The Johns Hopkins University and Dr. Roy Williams of the California Institute of Technology are the co-principal investigators. Seventeen astronomy and computer science organizations in the US and Canada have been involved in the development of the NVO.
How do I get started using the NVO?
The NVO home page has links to a number of easy-to-use applications. These let you locate sources of astronomical data and catalogs, compile information about particular objects or positions on the sky, cross-correlate survey catalogs, generate your own object catalogs from images you have, search for and analyze spectra from several large spectral surveys, determine the overlapping sky coverage of various surveys and catalogs in regions of interest, repair inaccurate coordinate information in image headers, make mosaics from several sky survey image collections, and collect information about transient events.
How do I find out what other applications are available?
Follow the links labeled “Tools” and “Data” at the top of the NVO home page.
What is a VOTable?
A VOTable (VO table) is an XML-based encoding scheme for astronomical tables and catalogs. The VOTable format was established by the International VO Alliance in order to provide an unambiguous way to transmit tables between computer programs.
What is XML?
XML is the eXtensible Markup Language. It is a generalization of HTML, the language used for making web pages. Whereas HTML defines certain tags for formatting text and controlling page layouts, XML provides mechanisms for programmers to define their own tags to describe potentially complex content and relationships. XML is a World Wide Web Consortium standard and has become the lingua franca for exchanging information between computer programs running on the Internet.
Why use VOTables?
VOTables carry extra information about column names and contents to make it easier for computer programs to compare data from various sources. Because VOTables are encoded in XML, many widely used computer programs and web browsers can display them.
Can I use ASCII tables and FITS tables?
Many VO applications support input and output of tables in a variety of formats, including VOTable, HTML, ASCII, and FITS. The TOPCAT application will convert tables from one format to another. Note that if a VOTable is converted to ASCII or FITS some of the extra information that makes it easier to compare information from different telescopes or observatories could be lost.
What is the Registry?
The Registry is basically the “yellow pages” for the VO. The Registry is used to collect basic information about archives, data collections, databases, tables from the literature—basically any resource that is 1) available electronically and 2) accessible through VO standard protocols. Users can search the Registry for information by topic, by data provider, by bandpass, or by a number of other criteria. Many VO applications use the Registry to check for newly available data collections and to determine how to automatically request data. There are now over 12,000 resources in the VO Registry.
What is a Cone Search?
A Cone Search is the simplest VO request for information. For a given position in the sky (right ascension, declination) and a radius about that position, what information is available? VO data providers have implemented hundreds of Cone Search services, allowing users to examine subsets of catalogs and see where various telescopes have been pointed. The results of a Cone Search request are delivered in a VOTable.
What is SIAP?
SIAP is the Simple Image Access Protocol. SIAP takes the Cone Search request to the next level, providing a way to determine what images are available in a given data collection within a specified radius of a given position on the sky. Users can request images of a certain size or review the contents of an archive. The results of an SIAP request are delivered in a VOTable. Each row in the table is a link to an image that has some sky coverage within the area specified in the request. The images themselves can be in FITS format, or may be in a standard graphics format such as GIF or JPEG if they are preview images or thumbnails.
What is SSAP?
SSAP is the Simple Spectrum Access Protocol. SSAP is the analog to SIAP, but for spectral data. Like the Cone Search and SIAP, SSAP searches for spectra taken of object within some radius of a given position on the sky. The results of an SSAP request are delivered in a VOTable. The VOTable may contain links to spectral data files stored elsewhere, e.g., in FITS files, or may contain the actual data (wavelengths, fluxes, uncertainties).
What is a SkyNode?
A number of astronomy survey projects produce databases containing source catalogs. Some of these databases contain a billion or more entries, and for each entry, tens or even hundreds of columns may be stored. A VO SkyNode is a standard interface to such databases. SkyNodes are needed because these databases can be in different systems, such as SQL Server, Oracle, Sybase, MySQL, etc. To avoid requiring users to learn different database systems, or to get database access accounts for many servers, these databases are provided to the VO through the SkyNode interface. Also, databases that are published as SkyNodes can be cross-correlated based on the positions of objects.
What is ADQL?
ADQL is the Astronomy Data Query Language. It is the query language understood by SkyNodes. It looks very much like SQL, the standard Structured Query Language used by many database systems. ADQL does not support all features of SQL, but it adds several important capabilities specialized for astronomy. These include a cross-match function, XMATCH, that allows users to find objects in different databases that are spatially coincident to within some tolerance, and a REGION function that allows users to constrain their queries to a particular region on the sky.