General Instructions for NVSS Catalog Search

This WWW form allows the user to list selected portions of the NRAO/VLA Sky Survey (NVSS) radio source catalog. 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 the sky north of declination -40 deg with a resolution of 45". The catalog contains sources with a peak brightness in excess of 2.5 mJy/beam. Both total intensity and linear polarization is being imaged. This project began in September 1993 and is now substantially 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 (45") 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 contains a very large number of entries; the final version is expected to contain approximately 1,800,000. 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 either 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 as strong radio frequency interference has caused some regions to be rejected and reobservations of these regions are needed. 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. If a central search position is given, 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 next entries in the output describe the linear polarization of the component. The first of these ("P_Flux") is the integrated linearly polarized flux density followed by the position angle of the "E" vectors ("P_ang) on the sky if the source was detected (1 sigma) in linear polarization. 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 or region is to be searched fill in the selection boxes in the sectiuons marked "Single position or regioun 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 (mJy) higher than this value will be listed.

Minimum percentage polarization

Only sources with a percentage polarization 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 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 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 NVSS catalog not yet including that area. A reasonable value for this second parameter is 720" = 12'. Now that the catalog is nearly complete this is no longer very important. In order to be absolutely certain that a region has not yet been covered by the NVSS survey it is necessary to look at the image. A postage stamp server which returns FITS files of limited regions from NVSS 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 NVSS 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.