This WWW page uses the NRAO/VLA Sky Survey (NVSS) radio source catalog to search for potential confusing sources around a position to be imaged with a radio synthesis array and then converts these sources into the AIPS commands to set up the task IMAGR to add additional imaging fields to include the confusing sources. This Web page uses the NRAO NVSS Survey which covers the sky north of -40 deg declination with a limiting flux density of about 2.5 mJy and a resolution of 45". This survey should be suitable for most low frequency radio imaging in the declination region covered.
When the form is submitted, the NVSS source catalog is searched for sources matching the request and then these sources are used to generate the input to the AIPS task IMAGR to include extra fields to cover these sources. Nearby double sources will be lumped into a single field. The file returned is wrapped in html to enable Web browsers to display it. This file can either be converted into an AIPS RUN file by stripping the html wrapper or using the window manager's cut and paste facility to enter it directly into AIPS. To remove the html wrapper, use only the lines between the PRE and /PRE markers.
If more sources are found than can be accomodated (IMAGR limit) then the strongest sources after correction for primary antenna pattern are used. The primary antenna pattern is specified in terms of the antenna diameter and the observing frequency and assumes a uniformly illuminated dish with a scale correction determined for the VLA antennas at 1.4 GHz. The antenna gain is clipped at the five percent level; that is if the computed approximation is less than 0.05 then a value of 0.05 is used. Thus, the antenna gain cannot be used to reject very strong sources at arbitrary distance from the field center.One potential source of trouble is trying to CLEAN using overlapping CLEAN fields. In order to avoid this problem, this web page requests the size of the region being CLEANed ("Size of CLEANed area" and "Cell spacing". These should be specified large enough to keep additional fields from overlapping your main field. After all, your source is likely in the NVSS catalog.
Descriptions of individual fields
This is the equinox of the coordinates to be used for celestial positions.
This specifies the Right ascention of the pointing center to be imaged. Sources will be selected in a circular region centered on this position.
This specifies the declination of the pointing center to be imaged. Sources will be selected in a circular region centered on this position.
The radius of the specified circular region about the center Right Ascension and declination is given in arcseconds. This should be at least the size of the primary antenna pattern.
Only sources with a peak brightness (mJy) higher than this value will be included. The cataloged peak brightness is adjusted by the primary antenna gain (determined from antenna size and observing frequency) before applying this selection.
In order not to specify additional fields that overlap your primary field, you must specify the region being CLEANed in terms of the dimensions of the CLEAN region in cells and the angular size of the cells. This size may be smaller that the entire region being imaged.
In order not to specify additional fields that overlap your primary field, you must specify the region being CLEANed in terms of the dimensions of the CLEAN region in cells and the angular size of the cells. The cell spacing of the images to be made are specified in arcseconds.
In order to correct for the gain of the primary antenna in determining which sources are strong enough for inclusion, the size of the antenna pattern must be given. This is specified by the antenna diameter in meters and the observing frequency in MHz. The algorithm assumes a uniformly illiminated dish of the size given and has only the rigorously checked for the VLA antennas but should be a reasonable approximation for other antennas.
The size of the additional fields are specified in the "X" (unrotated RA) and "Y" (unrotated Dec) directions by the width in cells. These values are used to set the FLDSIZE adverbs and for grouping nearby sources.
This item gives the number of the first additional field to add to the IMAGR list. Generally this is 2 but if you're using multiple fields this allows control of the field numbers. If you're using a value other than 2 you should check that the specified CLEAN size and cell spacing is sufficiently large that new fields are not added over regions already covered.
IMAGR has a limit of either 64, 512 or 4192 fields, depending on the version of AIPS you're using. Specifying this value will avoid exceeding the array bounds.
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Modified on Saturday, 20-Jul-2002 11:00:47 EDT