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.

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.

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.

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.

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.