Since the sky is curved this approximation breaks down near the celestial pole. The plot labeling should be accurate at the center of the filed but may become less accurate away from the field center close to the pole. A cross will be plotted marking the location of the input position. The high dynamic range of most NVSS fields generally results in a few white dots on a black background if a simple conversion of the pixel values is used. This may not be completely adequate in all cases and use of an external viewer with some control over brightness and contrast is recommended.
FITS files contain the full range of values in the correct physical units.
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. 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. 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.