This WWW form allows the user to obtain "postage stamp" FITS or jpeg images or contour plots of selected, small fields from the NRAO/VLA Sky Survey (NVSS). This survey was made with the National Radio Astronomy Observatory's Very Large Array telescope at a wavelength of 20 cm (1.4 GHz) and produced images of the sky north of declination -40 deg with a resolution of 45". Both total intensity and linear polarization was imaged. The survey is now complete.
The images from the NVSS are produced as 4 x 4 degree, overlapping images. The desired region of a "postage stamp" to be made from these files is specified by a central Right Ascension and declination and the width of the field in Right Ascension and declination. Other aspects of the geometry in the resultant image can also be specified. The specified region of the closest NVSS field is returned as a FITS or jpeg format image or a contour plot. The resultant images will be truncated at the boundary of the closest NVSS field. 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 resulting FITS file will use this equinox for 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.
This string gives the name of the object. This is used for labeling purposes only and is not used to obtain the position. The default is "unnamed".
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.
These values specify the maximum field size in the returned image in RA and Declination in degrees. The returned image may be truncated if the requested field exceeds the boundaries of the NVSS image. There is also a limit of 262,144 pixels in the returned image; this corresponds to a 512 x 512 image.
There are a number of supported projections of the (spherical) sky onto a
NVSS images normally have a pixel spacing of 15 arcseconds in each dimension but the image can be interpolated to other spacings. Note: a larger value will lose information in the image.
The image can be rotated relative to the input image by the amount specified here in degrees. The rotation is in the sense that north rotates towards east. Note: the axis labeling on the contour plots cannot accurately represent data with a rotation on the sky. Do not use a non zero rotation for contour plots.
The returned image may be one of several types.
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. The NVSS postage stamp 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.
There are a number of programs which can read and display FITS images but most of these are inadequate for the high dynamic range of the NVSS images and lose all of the astronomical information. 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-fits 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 %sin which XFITSview is the desired external viewer.