General Instructions for VLSS Catalog Search

This WWW form allows the user to list selected portions of the VLA Low frequency Sky Survey (VLSS, formerly 4MASS) radio source catalog. This radio survey will use the NRAO Very Large Array telescope and will cover the sky north of a declination of -30 degrees at a frequency of 74 MHz, a resolution of 80" and a limiting peak source brightness of about 500(?) mJy/beam. The VLSS survey is not yet complete.

The source catalog consists of Gaussian model components fitted to the survey images and measures of the linearly polarized emission. If the source is significantly larger than the resolution of the survey (80") then it may be represented by several of these components. If requested (the default) the fitted parameters will be deconvolved from the instrumental resolution producing source sizes (or upper limits with 99% confidence) and the integrated flux density of the model. Error estimates for each of the source parameters are also given.

The source catalog will (eventually) contain a large number of entries. To limit the size of the output, the region of the sky of interest should be limited. This form allows specifying the central Right Ascension and declination at one of several equinoxes and a circular ("Search radius"). All entries in this region meeting the other criteria are listed.

Until the completion of the survey the source catalog will NOT cover the entire region of the survey and care must be exercised in interpreting the results of a catalog search. The presence of other sources in the general area usually indicates that the current catalog includes the region of interest, although this is not an absolutely reliable indicator. The second parameter in the "Search radius" box or one of the optional items in a position list can be used to specify a region in which to look for other sources. If no match is found in the specified region then the distance to the closest source is given.

The details of the output listing depend on whether the raw, fitted, or deconvolved model parameters are requested. In the first two cases, the sizes are those in the image which include the instrumental response and the strength of the component is the peak surface brightness. In the case of deconvolved parameters, the size is that of, or an upper limit (99% confidence) to the source size and the strength is the integrated flux density of the component. Raw parameters are not corrected for known biases and no error estimates are given. The output listing gives the Right Ascension ("RA") and declination ("Dec") at the requested equinox. The distance from the specified central Right Ascension and declination ("Dist") is given in arcseconds. The position angle (from north through east) of the model component from the central position ("Ori") is given in degrees on the line under the separation. Next are the strength of the source ("Peak" or "Flux"), its major ("Major") and minor ("Minor") axis sizes and the orientation ("PA") of the major axis (in degrees from north through east) on the sky. Following this is a possible code ("Res", P*, R*, or S*) indicating structure more complex than can be fitted by the Gaussian model. If this field is nonblank then the model is inadequate to completely describe the source and the original image needs to be examined; in this case, the entry on the error line under the Res flag will give the offending value (P=peak, R=RMS, S=integrated) in 100s of microJy. The final entries ("Field", "X_pix", "Y_pix") give the name of the original survey image field from which the component was derived and the "X" and "Y" (RA, dec.) pixel numbers of the center of the component. Below each parameter (except upper limits) is given the estimate of its standard deviation.

If only a single position is to be searched fill in the selection boxes in the sections marked "Single position to search". If searches on a number of positions is desired, then fill in the position list in the text box in the section labeled "List of positions to search".

Descriptions of individual fields on this form follow:


This is the equinox of the coordinates to be used for all input and output.


"Deconvolved" gives deconvolved component size, integrated flux density. "Fitted" gives fitted component size, peak flux density. The deconvolved parameters more accurately describe the actual source values. "Raw" is like "Fitted" except that no amplitude bias corrections are made and no error estimates are given.

Minimum peak flux density

Only sources with a peak brightness (Jy) higher than this value will be listed.


This string gives a label for this field in the output.

Central Right Ascension

The listing can be limited to a specified region in RA-Dec The center right Ascension at the specified equinox is given as hh mm ss.s. A search radius or box size must also be specified.

Central declination

The listing can be limited to a specified region in RA-Dec The center declination is given as +/-dd mm ss.s. A search radius or box size must also be specified.

Search radius

The radius of the specified circular region about the center Right Ascension and declination is given in arcseconds. Too large a value will increase the probability of a false association of a radio source and the target source; values larger than an arcminute (60") should be avoided if an accurate identification is desired. The default (blank or 0s) is 15".

Verification box

An optional second value gives the halfwidth of a box in arcseconds to examine for sources. Sources not in the search radius but inside this box will not be listed but the distance to the closest entry will be given if there are no sources meeting the specified criteria inside the search window. This allows distinguishing between the cases of: 1) no source at that position and 2) the VLSS catalog not yet including that area. A reasonable value for this second parameter is 720" = 12'. In order to be absolutely certain that a region has not yet been covered by the VLSS survey it is necessary to look at the image. A postage stamp server which returns FITS files of limited regions from VLSS images is suitable for this purpose.

List of positions

If many fields are to be searched, you may list their positions in a text box ("List of positions..."). This list contains one line per field, and each line has up to 5 logical columns containing, in order:
1. Field center right ascension as hh mm
2. Field center declination as +/-dd mm ss.s
3. (Optional) Search radius in arcsecond (defaults to 15")
4. (Optional) Verification box in arcsecond (default 0).
5. (Optional) Field label (defaults to blank)
An example of a position list entry is:
12 34 56.78 -00 12 34.5 15 720 My Star
This will search for VLSS sources within 15" of RA=12 34 56.78, Dec=-00 12 34.5. If no source meeting the selection criteria is found within the 15" but there is a source within 720" (12 arcmin.), then a note to this effect is made.

Note: it may be most convienient to create this list in a text file and using the cut and paste facility of your system to enter the list into this box.