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VLSS Postage Stamp Server Documentation Help for VLSS Postage Stamp Server

General Instructions for VLSS Postage Stamp Server

This WWW form allows the user to obtain "postage stamp" FITS or jpeg images or contour plots of selected, small fields from the The VLA Low-Frequency Sky Survey (VLSS, formerly VLSS). This is a proposed survey to be done with the National Radio Astronomy Observatory's Very Large Array telescope at a wavelength of 4 m (74 MHz) and is producing images of the sky north of declination -30 deg with a resolution of 80". The survey is not yet complete.

The images from the VLSS are produced as 14 x 14 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 VLSS 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 VLSS field. Regions without successful observations are represented by blanked (no value defined) pixels in the returned image.

Descriptions of individual fields on this form follow:

Equinox

This is the equinox of the coordinates to be used for input coordinates. The resulting FITS file will use this equinox for coordinates.

Object name

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".

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 -30 0 0 (the southern limit of the survey) to +90 0 0 are valid.

Field size

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 VLSS image. There is also a limit of 262,144 pixels in the returned image; this corresponds to a 512 x 512 image.

Projection

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

  • SIN = Sine projection (default, used by VLSS)
  • 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

    VLSS images normally have a pixel spacing of 25 arcseconds in each dimension but the image can be interpolated to other spacings. Note: a larger value will lose information in the image.

    Rotation

    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.

    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.
  • "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" will return a contour plot as a postscript file of the selected field. The labeling of contour plots assumes no rotation and a rectangular image grid. 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. Contours are plotted at +/- 1 mJy/beam multiplied by 2^0, 2^(1/2), 2^1, 2^(3/2), 2^2, ... = 1, 1.414, 2, 2.828, 4, ... Using a smaller pixel size (e.g. 5 arcsec) will make a better looking plot. A cross will be plotted marking the location of the input position. The MIME type is given as "application/postscript".
  • "jpeg image" returns a jpeg image of the field with MIME type image/jpeg. The high dynamic range of most VLSS 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 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 VLSS 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 VLSS 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 %s

    in which XFITSview is the desired external viewer.