NED is accessible on the World Wide Web at http://ned.ipac.caltech.edu. The Web interface is by far the most common way that most users access NED; usage has grown from about 105 requests per month in 1997 to an average of over 106 requests per month in 2001. The rate of growth of access to the NED Web server is also rising quickly. There is also a VT-100 (ASCII) and X-Window graphical user interface available via telnet login session (`telnet ned.ipac.caltech.edu'). These legacy services provide access to the same database updates as available through the Web interface, but they are no longer maintained or enhanced with new services. Limited resources to build new tools are going into the much more heavily used Web and batch access modes.
3.1. Web Query Services
Figure 1 shows the NED Web interface main menu (home page), where the main functionality is organized in columns as follows.
Figure 1. The NED Web interface main menu available at http://ned.ipac.caltech.edu. The annotations refer to example query results which are displayed in Figures 2-6. Click here for a high resolution Postscript version of the image.
In the OBJECTS column NED can be searched for extragalactic objects using menus designed for searches `By Name', `Near Name' (input a name and search radius), `Near Position' (input celestial coordinates and a search radius), `IAU Format' (object names), or `By Reference Code' (a 19 digit code developed jointly by NED with the ADS (6) and CDS to uniquely identify publications in astronomy). The `Skyplot' service generates finder charts indicating NED objects and star positions in a specified field of view. Figure 2 illustrates an example of the Essential Data that is available from NED after a query of the galaxy NGC 4151.
Figure 2. Essential Data returned by NED from a query of NGC 4151: includes coordinates and redshift (with uncertainties), multi-wavelength survey cross-identifications and object types, size, magnitude, classifications, Galactic extinction along the line of site, as well as links to query references, notes, photometry, positions, velocities, and images. This information is followed by links to External Archives (Figure 3). Click here for a high resolution Postscript version of the image.
3.1.2. Global Archive Connectivity
Figure 3 shows hyperlinks to `External Archives and Services', easily found when the user scrolls down below the Essential Data that comes directly from the NED database. Links here allow the user to retrieve images and query original catalog data or observation log entries. The first set of hyperlinks are to data related directly to an object name; the second set of links are to services that can be queried at the object's coordinates. A summary of the available resources includes: original catalog record entries in VizieR at CDS/France; the NVSS, and FIRST catalog and image servers and the Observation Log of the VLA telescope from the National Radio Astronomy Observatories (NRAO); infrared mission archives at IRSA/IPAC (2MASS, IRAS, etc.); visual and UV mission archives at MAST/STScI (HST, IUE, etc.); high energy mission archives at HEASARC/GSFC (ASCA, CGRO, Einstein, etc.); and the IMPRESS focal plane plotting service for NASA astrophysics missions at the NASA Goddard Space Flight Center/Astrophysics Data Facility (GSFC/ADF) 7. This tool makes it very easy for researchers to determine whether an object has been observed by one or more of the major surveys or observatories. New archive services are being added as they become available. With this service, users now have at their fingertips distributed data dynamically cross-linked (federated, fused) using source names and positions indexed and maintained by NED. This innovative service is a major step toward federating distributed archives as discussed in reference to a National Virtual Observatory portal.
Figure 3. Queries to globally distributed archive data are conveniently available with a single mouse click. Such links are generated dynamically during object query report generation. The first set of hyperlinks are to data related directly to an object name; the second set of hyperlinks are to archive services that can be queried at the coordinates of the NED object. This example is a continuation and extension of the report from a query of NGC 4151 (Figure 2). Click here for a high resolution Postscript version of the image.
3.1.3. Sample Building with `By Parameter' Queries
The `By Parameters' menu allow the user to query NED using joint constraints on redshift range, sky area, object types, or survey/catalog name prefixes. Casual users of NED may not realize the benefits of querying NED using a catalog name prefix. With this feature one can dynamically regenerate a classic catalog sample that contains not simply the original catalog measurements (available elsewhere, such as VizieR), but rather the most precise and currently available source positions and redshifts, with links to up-to-date references, multi-wavelength photometry, images, etc. For example, today anyone can use NED to generate an up-to-date compilation analogous to the `Catalog of Markarian Galaxies'  or generate a current data set for the entire Third Cambridge (3C) radio galaxy sample, with the click of a mouse. Since entries in NED are continuously updated through a synthesis of the literature and large surveys, errors in original catalogs are often found and documented. Therefore, for many studies it is more efficient and effective to cross-correlate new observations against the data synthesis in NED rather than against catalogs in their original published forms. A visualization of one example of this powerful feature is shown in Figure 4. Other example queries that can be performed with the `By Parameters' tool are (including the number of objects returned at the time of writing):
Planned upgrades to this functionality are reviewed below (Section 4.2).
Figure 4. Objects that meet the search criteria [Redshift > 4.0 AND Glat < -60°] returned after submitting a query using the `By Parameters' menu (see Figure 1). Click here for a high resolution Postscript version of the image.
The DATA column in the main NED menu allows the user to enter an object name (e.g., `NGC 4151', `APMUKS(BJ) B003425.77-334949.0', `SN 1993G', `2MASXi J1132350+582422', `SDSS J1044-0125', `Antennae') and query NED for `Photometry & SEDs', `Catalogs', `Positions', `Redshifts', `Notes', or `Images. In the report resulting from a query of any NED object, hyperlinks are included that provide access to this same information; the DATA search menus simply provide more direct access. Figure 5 illustrates an example of multi-wavelength photometry data and Spectral Energy Distribution (SED) plots available from NED.
Figure 5. Multi-wavelength photometry and spectral energy distributions (SEDs) covering gamma-ray through radio wavelengths. The data are available in original units as published, and also in common units (Hz, W m-2 Hz-1) used to construct SED plots. The data include uncertainties and references. The SED plots are dynamic, with configurable axis units (e.g., wavelength or frequency for the abscissa and f, f or f for the ordinate) and optional error bars. Click here for a high resolution Postscript version of the image.
Figure 6 shows an example of the rich variety of multi-wavelength FITS images available for immediate download or visualization using the Aladin Java applet. With Aladin the user can: superimpose entries from NED, the USNO catalog and various other astronomical catalogs; interactively link to information for all known objects in the field; measure interactively positions and distances in celestial coordinates. Aladin's interoperability with NED and other distributed data services provides a visual summary of the multi-wavelength sky. Aladin was developed at the CDS (Strasbourg, France) and is configured with the NED interface through a cooperative agreement. The NED team has made significant contributions to testing and debugging of Aladin using the richness of FITS image types in the NED archive. This is a successful example of software tool reuse to meet common goals and interests through an international collaboration.
Figure 6. Multi-wavelength galaxy images, including previews (GIF) and science-grade data (FITS). Image overlays and graphical inter-activity between sky coordinates (from information in the FITS image header) and object markers (from NED and other databases) are available using the Aladin Java applet (CDS). Click here for a high resolution Postscript version of the image.
In the LITERATURE column of the main NED menu, the user can: (1) enter an object name and access the `References' related to that object; (2) enter an `Author Name' and retrieve a list of references; (3) search journal article `Abstracts' by year, volume or page numbers; (4) search `Thesis Abstracts' by year range; (5) use the `Text Search' tool to perform a keyword search on the indexed contents of the journal and thesis abstracts in NED or the full text content of LEVEL5; and (6) access the LEVEL5 `Knowledgebase'.
LEVEL5 can also be accessed directly at /level5. It contains hyperlinked review articles and documents of current and lasting interest to cosmologists and extragalactic astronomers. Contents include a glossary of terms, essays, recent research articles, detailed monographs and extensive reviews (where copyrights allow). Within each article, cited extragalactic objects are cross-linked to NED Basic Data frames, and all available citations are hyperlinked to NASA's Abstract Data Service (ADS), to on-line NED abstracts, or to preprints on astro-ph. Tabular data, images and graphs are being progressively linked to and from relevant essays and review articles.
The TOOLS column of the main NED menu contains a `Coordinate & Extinction Calculator' that performs coordinate conversions and precession, and displays line-of-sight Galactic extinction estimates using two modern techniques [3, 4]. The `Velocity Calculator' computes conversions between redshifts for extragalactic objects in different reference frames: heliocentric, Local Group, Galactic Standard of Rest, and 3K Microwave Background. There is also a link to NED's public FTP site, primarily used by users to pick up their output from the NED batch mode.
The INFO column of the main menu contains links to a Frequently Asked Questions (`FAQ') document, an `Introduction', summary of `Features', `News', information about the NED development `Team', forms and documentation for submitting NED `Batch jobs', and finally a page of useful `Web Links' relevant to extragalactic astronomy.
3.2. Batch Mode
NED can process requests for large amounts of data through its `Batch Job' option. Using this mode simply involves submitting to NED via email a "batch form" containing a list of objects or positions, or other constraint parameters (e.g., redshift, object type, or survey/catalog name prefix). (8) After the request has been processed, NED sends the user a notice by return email indicating where the resulting data files may be copied via FTP. There are two types of batch job forms available. One form is to search any of the main data categories in NED - Objects, Basic Data, References, Photometry, Positions, and Redshifts; the second form is to constrain searches By Parameters - positions, names, object types, and redshifts. The input forms are flexible enough to support several different searches with a single form. Though the batch processor will currently support only 3,000 input requests per job, it will return up to 10,000 objects per request. See Section 4.2 for a summary of planned upgrades to the batch mode.
3.3. Client/Server Mode Connectivity
For many years NED has provided a `server mode' with custom client (C) software 9 that has been used by computer programmers all over the world to build applications that issue queries and retrieve data from the NED database in a format that can be integrated into their services. Astrophysics archive centers and observatories use NED's server mode extensively to resolve extragalactic object names into celestial coordinates, and to retrieve lists of objects by specifying an input position and search radius. A number of sky visualization tools also make use of NED's client/server mode, including IPAC's IRSKY and SIRTF Planning Observations Tool (SPOT) 10, and the Scientist's Expert Assistant (SEA) developed at GSFC and STScI for HST and NGST proposal planning 11. The NED client/server service is being updated with a new approach as discussed below (Section 3.4).
3.4. Current and Future Technologies
NED is currently operated using UNIX (Sun/Solaris) servers, a combination of Sun D-1000 disk arrays and miscellaneous SCSI disk "shoe boxes" (totaling over 650 GB, including a backup system), two Informix relational database servers, Apache Web servers, and custom Web interface software (CGI) written mostly in C. Perl is also used for some applications, as well as Java applets. New technologies are evaluated as time allows, and applied only when there is a clear benefit in capabilities or performance over current approaches. As for most operational systems with a large base of users, limited resources require a gradual transition to new technologies, because legacy services have to be maintained during any transition. A relatively new technology planned for future NED interface enhancements is a Web application server (`middle-ware') using Java Servlets and Enterprise Java Beans (EJBs); this will enable a more sophisticated interface that includes customized options stored across user sessions, load balancing across multiple servers, and better performance and scalability compared to current CGI-based Web services. The NED team, in coordination with the CDS, IRSA, and the ADC/ADF group at GSFC, is also designing new formats for data output from the NED Web (HTTP) servers based on eXtensible Markup Language (XML). XML is a markup language designed for producing logical, well structured documents and data services that separate data content and metadata issues from those of display and presentation 12. This development will simplify the ability of client computer programs to be written that can parse and utilize NED data in more efficient ways than is possible using the classic NED `server mode' (described in Section 3.3), or by parsing the HTML output which is expected to undergo frequent changes to provide improved presentation for NED's interactive users. Providing XML output from NED will enable, among other things, the construction of software "agents" that perform automated data mining by streaming and combining data from NED and other services, improved interoperability with other services that are planned by developers of new Virtual Observatory applications, and support for the next generation of Web browsers and data analysis software that will increasingly utilize XML rather than HTML. Providing XML output from NED will establish the next generation `server mode`, superseding the need to enhance the custom NED client/server package (Section 3.3), because builders of future observatory and archive center tools will find it more convenient and powerful to connect their software with NED using industry-standard XML development tools.
6 http://adswww.harvard.edu Back.
7 http://cdsweb.u-strasbg.fr, http://www.nrao.edu, http://irsa.ipac.caltech.edu, http://archive.stsci.edu, http://heasarc.gsfc.nasa.gov, http://adf.gsfc.nasa.gov/adf/adf.html Back.
8 The forms are available from the `Batch Jobs' link in the INFO column on the main NED menu. The forms can be emailed to email@example.com for processing. Back.
9 The NED client C code is available at ftp://ned.ipac.caltech.edu/pub/ned/client.2/. Back
10 For information about Skyview and SPOT see the IPAC Web site at http://www.ipac.caltech.edu . Back
11 http://aaaprod.gsfc.nasa.gov/sea Back
12 For information about XML see http://www.xml.org, http://www.w3.org/XML, and the astronomical XML resources at http://xml.gsfc.nasa.gov. There are dozens of books published on the subject. Back