Primary Beam Correction

A correction is needed for falling sensitivity at the beam edges when the field of view is comparable to the single (not synthesized) beam width. Each telescope in the synthesis array has its own beam response (the so called `primary' as opposed to `synthesized' beam), the image we have produced is really the sky multiplied by the primary beam. To correct for this, so that flux densities that we measure are not biased low, we must divide the image by the primary beam. In doing this, we assume that the primary beam is the same for each telescope, and that it is circularly symmetric.

For 610 MHz data, the primary beam correction was done using the AIPS task PBCOR and for 235 MHZ data it was done using the task FLATN using the following parameters.

Coefficients to use in the PBPRM for primary beam correction of GMRT data for frequency 610 MHz & 235 MHz are tabulated here. The polynomial fit to the beam is

$$\rm {1 + (a/10^3)x^2 + (b/10^7)x^4 + (c/10^10)x^6 + (d /10^13)x^8}$$ (3..6)

where $x$ is in terms of (separation from pointing position in arc-minute $\times$ frequency in GHz). $a$, $b$, $c$, & $d$ are the coefficients which are required to be specified in PBCOR when using GMRT data. PBPARM(3) (=$a$), PBPARM(4) (=$b$), PBPARM(5) (=$c$), PBPARM(6) (=$d$). PBPARM(7)=0 for GMRT.

The coefficients used for primary beam correction for the frequencies 235 MHz and 610 MHz are shown below:

1. 235 MHz

       a= -3.366,
       b= 46.159,
       c= -29.963,
       d= 7.529,
                half power beam-width: 118.5'.
   

2. 610 MHz

       a = -3.486,
       b= 47.749,
       c= -35.203,
       d = 10.399,
                  half power beam-width: 44.4'.