General Instructions for NVSS Image ftp service

This WWW form allows the user simplified access to images produced by the NRAO/VLA Sky Survey (NVSS). This service determines which NVSS image is desired based on a celestial position and returns either the image directly or a Web page with a link to the file. This avoids having to determine the name and/or directory of the file. This survey is being done with the National Radio Astronomy Observatory's Very Large Array telescope at a wavelength of 20 cm (1.4 GHz) and is producing images of the sky north of declination -40 deg with a resolution of 45". Both total intensity and linear polarization is being imaged. This project began in September 1993 and is now substantially complete.

The images from the NVSS are produced as 4 x 4 degree, overlapping images. The images use the SIN projection, have 15" cell spacing and are labeled in J2000 coordinates. (The NVSS Postage stamp (small FITS file) server can be used to obtain images with other geometries.) This form will allow determination of the closest of these images to a desired position. In just a few cases, isolated small regions within images may have been lost due to strong radio frequency interference and have not yet been reobserved. These regions are represented by blanked (no value defined) pixels in the returned image. In addition to the desired image size, the polarizations included in the returned image may be specified (see below).

Descriptions of individual fields on this form follow:


This is the equinox of the coordinates to be used for input coordinates. The NVSS FITS files use J2000 coordinates.


The returned FITS file can contain either Stokes' parameter I (total intensity) or Stokes' I, Q and U (total intensity and linear polarizations). In the former case, the returned image will have one plane; in the latter, three.

Central Right Ascension

The desired position on the sky is expressed in terms of its celestial position which consists of a Right Ascension (like longitude but expressed in hours) and declination (like latitude). The central Right Ascension at the specified equinox is given as hh mm ss.s (hours minutes and seconds of time). Values in the range 0 0 0.0 to 23 59 59.9 are valid.

Central declination

The central declination is given as +/-dd mm ss.s (degrees, minutes and seconds of arc). Values in the range -40 0 0 (the southern limit of the survey) to +90 0 0 are valid.


The returned image may be one of several types. "link to file" will return a page with hot links to either the image in gzip compressed or uncompressed form. "FITS image" returns the gzip compressed image in FITS format with a declared MIME type of image/x-fits allowing the use of an external viewer to display the image. "FITS file" returns the gzip compressed image in FITS format with a declared MIME type of application/octet-stream which should cause your browser to write the image to a file. Note: directly returned FITS images are gzip compressed; use "html" and ask for the uncompressed file if your system does not support gzip compression. However, uncompressed file are 2 to 12 Mbyte in size and take a long time to transfer; the compressed files are generally a factor of 7 smaller.

Installing external FITS viewers

Web browsers use external viewers to display files like FITS images which they do not understand. In order to invoke an external viewer, it is necessary to associate a particular viewer with a particular MIME type. This server can return a FITS file with the MIME type of image/x-fits; your browser needs to be told a suitable display program for FITS files. In addition, the returned image is in gzip compressed form so the external viewer should be able to handle this type of compressed file.

There are a number of programs which can read and display FITS images but most of these are inadequate for the high dynanic range of the NVSS images and lose all of the astronomical information. Most cannot process gzip compressed file so the file must be uncompressed before viewing. The limited dynamic range is a problem because the brightest pixels in a given image are typically many times brighter than the faintest; thus the general result is a few white dots on a black background. At very least, control over the contrast and brightness is needed if not some control over the range of values displayed in order to get the best display of the objects in the image. One suitable external viewer is the FITSview family of FITS image viewers is available for a variety of computer systems.

In order to use an external viewer with your Web browser you need to install the software on your computer. Then the browser needs to be told to associate MIME type image/x-image with your broswer. The details of this depend on the particular browser; consult the documentation for your browser for details. For Unix systems this can generally be done in the .mailcap file in your home directory by inserting an entry like:

image/x-fits; XFITSview %s

in which XFITSview is the desired external viewer.