As an alternative to using a Web browser, the principal NVSS data products can be obtained from this MIRROR archive by freestanding anonymous FTP:
ftp ftp.mrao.cam.ac.uk (login as anonymous, with your e-mail address as password) cd nrao/nvss (this directory includes the relevant documentation files and subdirectories) ls (to list these) binary (if transferring FITS or gzipped data)
The files you can see in this directory (nrao/nvss/) are:
paper.ps and paper.ps.gz (uncompressed and compressed postscript versions of the main survey paper) catalog.ps, catalog.ps.gz (uncompressed and compressed postscript descriptions of the catalog) bmdone.eps (postscript plot of layout of 4x4 deg images, showing which are available)
The subdirectories visible at this level are:
holes.plots/ containing postscript plot files which summarize where there are still gaps within available images. MAPS/ containing all the images (see notes below). UVDATA/ containing UV data (see notes below). CATALOG/ containing the raw source catalog CATALOG.FIT and the NVSSlist software needed to access and interpret it (apply corrections etc). Note that CATALOG.FIT is about 140Mb, so you will need to schedule ftp for a specified overnight time slot to avoid interruption of your transfer by timeouts. Ask you system manager how to set this up and remember to invoke binary mode! Check your messages afterwards to verify transfer was successful. The catalog and, more frequently, the software are both subject to evolution, so check the currency of the copies in use at your site from time to time. The files gpaper.ps, gpaper.ps.gz are uncompressed and compressed postscript versions of the paper 'Errors in Gaussian Fits' (J.J.Condon, 1997 PASP, 109, 166), which gives the derivation of the errors and corrections used in the NVSSlist program.
The large (4 deg X 4 deg) maps and their associated multisource (u,v) data sets are stored as binary files in FITS format. Maps are stored in gzip compressed form. Each map is named by the J2000 right ascension and declination of its center, and the first letter specifies the polarization plane(s). For example, C2230P84.gz is the map cube with Stokes I, Q, and U planes centered on right ascension = 22h 30m, declination = +84 deg. I0224M32.gz contains only the total-intensity plane centered on right ascension 02h 24m, declination -32 deg. The corresponding multisource (u,v) data file is called J0224-32.MS. The compressed map cubes are about 1.7 megabytes in size, and the total-intensity maps are smaller (about 0.7 megabytes) for faster access by users not interested in linear polarization. A postscript plot shows the maps currently available. The postscript file paper.ps (about 4.7 megabytes) contains a detailed description of the NVSS.
A sample anonymous FTP session is shown below:
ftp ftp.mrao.cam.ac.uk login: anonymous password: (type your full email address here) cd nrao/nvss (to access the NVSS public directory) ls (to list the directory contents) get paper.ps (to get a postscript copy of the paper) binary (to copy binary files) cd MAPS (to access the MAPS directory) get C2230P84.gz (to get the compressed map cube centered on 22h 30m, +84 deg) get I0224M32.gz (to get the total-intensity map centered on 02h 24m, -32 deg in compressed form) cd ../CATALOG (to go to the CATALOG directory) If you wish to transfer CATALOG.FIT, the FITS binary table containing the source catalog (140Mb), you will need to schedule an overnight ftp for a specified (quiet) time slot to avoid timeouts and check your messages to verify that a successful transfer was achieved. (Ask your system manager how to set up the job). quit
Compression: The images are stored as standard FITS files compressed in GNU zip (gzip) format, the .gz extension denoting this compression. Gzipped files may be uncompressed with the command gunzip which is available for most computer systems. See the text file nrao/nvss/compress.txt for a more detailed description of gzip.
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