NVSS radio survey

NVSS version #36

README file for the NVSS FTP directory

J. J. Condon

95/02/25 version modified 95/08/31 by W. D. Cotton

Now there are 1814649 src in ver #36.

Observations for the 1.4 GHz NRAO VLA Sky Survey (NVSS) began in 1993 September and should cover the sky north of -40 deg declination (82% of the celestial sphere) before the end of 1996. The principal data products will be:

(1) A set of 2326 continuum map "cubes," each covering 4 deg X 4 deg with three planes containing Stokes I, Q, and U images. These maps 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 0.3 arcsec for strong (S > 30 mJy) point sources to 5 arcsec for the faintest (S = 2.5 mJy) detectable sources.
(2) Lists of discrete sources.
(3) Processed (u,v) data sets. Every large map was constructed from more than 100 smaller "snapshot" maps. All of the edited and calibrated single-source (u,v) data sets used to make the snapshot maps contributing to each large map have been combined into a single multisource (u,v) file for users who want to investigate the data underlying the large maps.

The NVSS is being made as a service to the astronomical community, and the principal data products are being released into a directory accessible by anonymous FTP as soon as they are produced and verified. To ensure equal access for everyone, the NVSS team members have agreed to use only these electronically released results for their own research. Users should read the postscript file "paper.ps" containing a detailed description of the NVSS. Unprocessed data are available on request. If you have any questions, comments, or special requests, please contact Jim Condon by email at Internet address "jcondon@nrao.edu" or by telephone at (804) 296-0322.

The large (4 deg x 4 deg) map cubes and total intensity only images and their associated multisource (u,v) data sets are stored as binary files in FITS format (see Grosbol et al. 1988, Astronomy & Astrophysics Supplements, v. 73, p. 359 for a description of the FITS format). Maps are stored in gzip compressed form but an uncompressed form may be requested (see below). Each map is named by the J2000 right ascension and declination of its center. Maps containing the full (IQU) polarization cube have names beginning with a "C" and those with total intensity only begin with an "I". For example, C2230P84.gz is centered on right ascension = 22h 30m, declination = +84 deg; "M" is used to denote negative declinations. The corresponding multisource (u,v) data file, in the UVDATA directory, is called J2230+84.MS. You can copy these files by anonymous FTP. The encapsulated postscript plot file bmdone.eps indicates which big maps are currently available by FTP.

The source catalog derived from the available images is kept in directory CATALOG as CATALOG.FIT. This file is a FITS dummy image file with an appended "AIPS VL" binary table containing the catalog. A description of the contents and interpretation of this catalog is given in the postscript document catalog.ps. Each survey image also has an attached binary table containing the sources derived from that image.

A terminal interface browser for this catalog, NVSSlist, is available in the CATALOG directory as NVSSlist10.tar.gz. This Fortran software requires the NASA fitsio package which is included in the CATALOG directory (fitsio406.tar.gz) as a convenience (WE DO NOT MAINTAIN THE fitsio SOFTWARE). Unix installation and use instructions are included in the README file in the CATALOG directory.

A typical FTP session might go as follows:

ftp nvss.cv.nrao.edu
 login: anonymous
 password:             (type your name or email address here)
 cd pub/nvss           (to access the NVSS public directory)
 ls                    (to list the directory contents)
 get paper.ps          (to get a postscript copy of the paper)
 get catalog.ps        (to get a description of the catalog)
 binary                (to copy binary files)
 cd MAPS               (to go to the maps directory)
 get C2230P84.gz       (to get the map cube centered on 22h 30m,
			+84 deg in compressed form)
 get I2230P84          (to get the total intensity map centered on
			22h 30m, +84 deg in uncompressed form)
 cd ../CATALOG         (to get to the catalog directory)
 get CATALOG.FIT       (to get the catalog in a FITS binary table)
 quit

Compression: The images are stored as standard FITS files compressed in GNU zip (gzip) format; the .gz at the end of the name denoting this compression. If an uncompressed version is desired, omit the ".gz" when requesting the file from ftp. Gzipped files are uncompressed using gunzip which is available for most computer systems. For more details see file pub/nvss/compress.txt.

Most professional astronomers have access to data analysis systems (e.g., AIPS) that are capable of viewing FITS-format images. For users with only PCs, Bill Cotton has recently released FITSview for MS Windows. This FITS image viewer for Windows features manipulation of the display (brightness, contrast, pseudo color), zoom and scroll, blinking images for comparison, displaying 3-D images as a "movie", and determining the celestial position and brightness of features in the image. Celestial positions are determined using world coordinate projections (WCS). All defined FITS data types are supported (8, 16, 32 bit integers and 32 and 64 bit IEEE), as are blanked pixels. Two and three dimensional simple FITS images are supported.
FITSview runs on Windows 3.1 or later and uses any multicolor (or multiple gray level) display although 256 color displays give the best results. Extensive online documentation is included.
FITSview for Windows is available at no cost via anonymous ftp as the two files:
ftp://fits.cv.nrao.edu/fits/os-support/ms-windows/fitsview/fitsv040.txt
ftp://fits.cv.nrao.edu/fits/os-support/ms-windows/fitsview/fitsv040.zip
Installation is described in file fitsv040.txt. Contact bcotton@nrao.edu if you have any questions about FITSview.
Implementations under X-windows and MacIntosh are under development.
The VLA is also making a high-resolution (5 arcsec FWHM) survey of the north Galactic cap. The FIRST (for Faint Images of the Radio Sky at Twenty-cm) survey will yield very accurate (< 1 arcsec rms) radio positions of faint sources. The FIRST survey principal investigator is R. H. Becker (UC-Davis/LLNL).