General Instructions for FIRST Postage Stamp Server

This WWW form allows the user to obtain "postage stamp" FITS or jpeg images of selected, small fields from the VLA FIRST Sky Survey. 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 a large area of sky around the North Galactic Cap and a further strip around declination 0 degrees, all with a resolution of 5".4 .

The images are typically 34.5'x46.5' (0.45 sq deg) with some mutual overlap. 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 FIRST field (if it exists) is returned as a FITS format or JPEG format image or a postscript CONTOUR plot. If the requested "postage-stamp" overlaps the boundary of the closest FIRST field the area beyond the boundary is filled with blank (NaN) pixels. In addition, some fields may contain blank regions for which no observations are yet available - these regions are also represented by blank (no value defined) pixels in the returned "postage-stamp" image. For a FITS or JPEG image any blank regions should be obvious when the image is viewed. For a CONTOUR plot regions of blank pixels will not be obvious, so if the area includes blank pixels a warning message is printed along the top of the plot. If such a warning message is present the corresponding FITS or JPEG should be retrieved and viewed to check which part of the CONTOUR plot is affected. If you have hit the edge of a FIRST field, the missing data may be available from an adjacent field by submitting an offset target position. Note that single isolated blank pixels occur occasionally but are generally not a problem as the CONTOUR will interpolate across them.

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.


FIRST only provides images of Stokes' parameter I (total intensity)

Object name

This string gives the name of the object, no embedded blanks are allowed (use '_' instead). This is used for labeling purposes only and is not used to obtain the position. The default is "No_Name".

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.sss (hours minutes and seconds of time). Values should be expressed in the range 0 0 0.000 to 23 59 59.999 .

Central declination

The central declination is given as +/-dd mm (degrees, minutes and seconds of arc).

Image size

These values specify the field size for the returned image in RA and Declination in degrees. The returned image may be padded with blanks if the requested area overlaps the boundaries of the original FIRST field nearest to the target position.


There are a number of supported projections of the (spherical) sky onto a flat image:

  • SIN = Sine projection (default, used by FIRST)
  • TAN = Tangent projection
  • ARC = Arc projection
  • NCP = North Celestial Pole projection
  • GLS = Global sinusoidial projection
  • MER = Mercator projection
  • AIT = Aitoff projection
  • STG = Stereographic projection

    Pixel spacing

    FIRST images normally have a pixel spacing of 1.8 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.

    Image type

    The returned image may be one of several types.

  • FITS image returns the image in FITS format with a declared MIME type of image/x-fits allowing the use of an external viewer to display the image. Don't select this option unless you have your browser configured with an external FITS viewer. (See Installing external FITS viewers section below if you need to configure your browser - Note that there may already be a suitable system-wide configuration set up at your site).
  • FITS file returns the 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.
  • CONTOUR map will return a contour plot of the selected field as a postscript file - declared MIME type application/postscript. Contours are plotted at +/-0.5 mJy/beam multiplied by
    2^0, 2^(1/2), 2^1, 2^(3/2), 2^2, ... = ie: 0.5x(1, 1.414, 2, 2.828, 4, ...) Using a smaller pixel size (e.g. 0.6 arcsec) may make a better looking plot. A cross will be plotted marking the location of the input position. The labeling of the axes assumes zero rotation and a rectangular coordinate grid. Since the sky is curved this approximation breaks down towards the celestial pole, but it should be a good approximation over a (small) postage-stamp area, given that the upper dec limit of the FIRST survey is about 65 degrees. Occasionally the plot may include a region of blank pixels. Please therefore take a moment to read the following two caveats :
    1. Please note that the CONTOUR plot axis labelling, being performed by PGPLOT, is rectangular and shows intervals calculated for the central RA and Dec (your target position). This simplification might possibly become noticeable if you were actually using the axis grid to infer astrometric positions of outlying sources in a postage-stamp contour of a high-dec target, particulary if you had extracted a very large area. However, the LABELLING and REGISTRATION of the TARGET POSTION with the MARKED CROSS are ALWAYS EXACT, irrespective of declination.
    2. If your contour plot runs into the edge of the COADD image or a blank area within the image, there will be a warning about blank pixels printed along the top of the plot frame - if you get this message you should retrieve a FITS or jpeg image of the same area and display it to check the location of the blank region. If required it may then be possible to get alternative coverage from an adjoining COADD image by offsetting the target position in the appropriate direction. Occasionally an isolated blank pixel will occur within an image and is spanned by the interpolation process, not compromising the contour plot - this is why we have NOT adopted the situation of making blank regions stand out on the contour plot by substituting an extreme value!
  • JPEG image returns a jpeg image of the field with MIME type image/jpeg. You may need to use "Reload" on your browser if you have fetched more than one. The high dynamic range of most FIRST fields generally results in a few white dots on a black background if a simple conversion of the pixel values is used. To avoid this problem, only a portion of the range of pixel values (-2 to 30 mJy) is given in the jpeg file and a nonlinear (square root) mapping of intensities to colors emphasizes the lower portion of the brightness range. This may not be completely adequate in all cases and use of the "FITS image" option together with 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.

    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. The FIRST 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 dynanic range of the FIRST 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. For flavours of Unix saoimage, saotng and ds9 are also generally available.

    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 %s

    in which XFITSview is the desired external viewer.