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NVSS

A UK Mirror for the NRAO VLA Sky Survey

Final data update installed 2 August 2002

To facilitate faster access and downloading for UK users, MRAO Cambridge is providing a UK Mirror of the NRAO VLA Sky Survey (NVSS), produced at NRAO. For details and facilities see list below.


Principal Investigators

J. J. Condon, W. D. Cotton, E. W. Greisen, and Q. F. Yin
National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903
R. A. Perley and G. B. Taylor
National Radio Astronomy Observatory, P.O. Box 0, Socorro, NM 87801
J. J. Broderick
Physics Department, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

General Description

The NRAO VLA Sky Survey (NVSS) is a radio continuum survey covering the sky north of -40 deg declination. A detailed description appears in the 1998 May issue of The Astronomical Journal (Condon, J. J., Cotton, W. D., Greisen, E. W., Yin, Q. F., Perley, R. A., Taylor, G. B., & Broderick, J. J. 1998, AJ, 115, 1693).

Users should read the postscript preprint (about 4.7 megabytes) or the gzip compressed preprint (about 1.2 megabytes) for an overview of the NVSS.

The NVSS has been made as a service to the astronomical community, and the principal data products were released by NRAO as soon as they were produced and verified. The very few small, isolated holes in the I (total intensity) coverage have now been filled at NRAO (July 2002) and this MIRROR was updated accordingly on 2 August 2002 with the resulting new images and revised source catalogue. We understand that the few remaining holes in polarization data will not be filled.

Meanwhile UK and European users may need to check the NVSS main site in the unlikely event that the data required are not found on this MIRROR.


Current Survey Status

Observations for the 1.4 GHz NRAO VLA Sky Survey (NVSS) began in 1993 September and nearly covered sky north of -40 deg declination (10.3 sr = 82% of the celestial sphere) by the end of 1996. Reobservations to fill the most of the gaps were completed in January 1998. The 1998 data were then processed, and the resulting image and catalog updates brought even closer to completion by some reanalysis work by NRAO to fill in some of the gaps in the previous (May 1998) archive. This resulting status quo from November 1998 saw the survey extremely close to completion, with only 6 of the 217,446 NVSS snapshot fields lacking total-intensity images and a further 2094(1.0%) having total-intensity but no linear-polarization images. In July 2002 all the final few gaps in total intensity and some of those in polarization were filled at NRAO and the corresponding images and catalogue were installed on this MIRROR at 2 August 2002.


Accessing the data

The principal data products of the NVSS are:

  • A set of 2326 continuum map “cubes,” each covering 4 deg X 4 deg with three planes containing the Stokes I, Q, and U images. These images were made with a relatively large restoring beam (45 arcsec FWHM) to yield the high surface-brightness sensitivity needed for completeness and photometric accuracy. Their rms brightness fluctuations are about 0.45 mJy/beam = 0.14 K (Stokes I) and 0.29 mJy/beam = 0.09 K (Stokes Q and U). The rms uncertainties in right ascension and declination vary from < 1 arcsec for relatively strong (S > 15 mJy) point sources to 7 arcsec for the faintest (S = 2.3 mJy) detectable sources. The completeness limit is about 2.5 mJy.
  • A catalog of discrete sources on these images (over 1.8 million sources in the entire survey).
  • Processed (u,v) data sets. Every large image was constructed from more than 100 smaller “snapshot” images. All of the edited and calibrated single-source (u,v) data sets used to make the snapshot images contributing to each large image have been combined into a single multisource (u,v) file for users who want to investigate the data underlying the images.
  • The sky positions of the large NVSS images are shown in the image grid plot.

The easiest way to find and extract a particular source or region of sky from the image-set is by using our postage-stamp server. The resulting sub-image can be viewed and/or copied to your own machine.

Alternatively, you can access entire 4×4 deg downloadable images. These images are in FITS format and can be read by the major astronomical software packages (AIPS, IRAF, etc.) as well as by a stand-alone FITS viewer for Windows 3.1, Windows 95, Macintosh, and Unix/X-windows systems.


The NVSS source catalog

The authors extracted a source catalog from each image by fitting elliptical Gaussians to all significant peaks. The individual catalogs have been merged into the master catalog of sources from all images. See the catalog description for details about the cataloged source parameters. The user program NVSSlist can display selected portions of the catalog. NVSSlist corrects the raw catalog for known biases and computes errors associated with the source model parameters (position, flux density, etc.) using equations derived in Condon, J. J. 1997, PASP, 109, 166. An experimental NVSS catalog browser is available, but please exercise caution (don’t ask for a catalog of the whole sky!).

If you plan to make extensive use of the catalog it may be useful to obtain a site copy – but because of its size (approx 140 Mb) you will need to submit a scheduled anonymous ftp for a specified overnight time-slot to avoid time-outs. Ask your system manager how to do this, and remember that the catalog is in FITS BINTABLE format and the associated software is a tar file with gzip compression, so you will need to use binary mode in both cases. Check that no errors have been reported (usually via email to the initiator of the job) before using the transferred data. The ftp address is ftp.mrao.cam.ac.uk and the directory nrao/nvss/CATALOG contains both the catalog itself (CATALOG.FIT) and the software to access it (NVSSlistnnn.tar.gz , where nnn represents the version number of NVSSlist). The catalog and, more often, the software are both subject to evolution – so remember to check from time to time to make sure that your versions are up-to-date.

To avoid ambiguity, please refer to NVSS sources using the standard IAU format NVSS EHHMMSS+DDMMSS, with E = J for equinox J2000 or E = B for equinox B1950 coordinates, HHMMSS the hours, minutes, and truncated (not rounded) seconds of right ascension, the declination sign (+ or -), and DDMMSS the degrees, minutes, and seconds (truncated, not rounded) of declination. Thus the NVSS source at J2000 RA = 00 00 00.24, DEC = -20 04 49.2 (= B1950 RA = 23 57 26.34, DEC = -20 21 31.5) should be called NVSS J000000-200449 or NVSS B235726-202131.


The VLA is an instrument of the (USA) National Radio Astronomy Observatory, (NRAO), which is a facility of the (USA) National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

For more information about the NVSS, contact jcondon@nrao.edu.


Please send questions and comments about this MIRROR to Sally Hales.