To the accuracy needed when planning observations, the primary beam of
the VLA antennas can be taken to be a Gaussian with FWHM equal to
arcsec or 
arcmin. If you must
image a source, or a field of view, that is significantly larger than
the FWHM of the VLA primary beam, then you should plan to make a
mosaic from a number of pointings (see Lecture 14).
In standard VLA software, the primary beam correction
( 
, Lecture 2) is modeled by a power law expansion of the
form 
where 
and the coefficients are 
,
, 
, 
, and 
(Napier and Rots 1982). This correction is
usually not applied at small values of X for which the Napier-Rots
expansion gives 
1. It is not well determined at large
values of X for which 
0.15.