[daip] FLATN and primary beam correction

[Eric Greisen]

Andy Biggs wrote:
> Hi. I'm using FLATN to combine 4 separate GMRT 610-MHz pointings together. It is generally acting sensibly, but there is a strange pattern around the main area of interest that I don't think should be there. Please see attached figure. Note that each pointing is very large and should be greatly truncated by FLATN i.e. the pixels with the holes in the centre probably correspond to real data.
> 
> I am using the recommended parameters for the GMRT beam and I thought at first that the problem might be due to the GMRT polynomial increasing back above my cutoff of 0.1 at large distances. However, I just tried it again with PBPARM(6)=0 and that shows the same problem.
> 
> Are my inputs okay?

I agree that this looks wierd - it is as if outside some ellipse it 
allows values from the outer facets even though one would hope that they 
would be blanked.

PBPARM(1)=0 and PBPARM(1) = 0.1 should do the same thing according to 
the help file.  Try PBPARM(1) = 0.02 and 0.2 instead and maybe I can get 
some idea what is going on - it would help to see an IMHEADER of an 
outer facet of say the top left pointing plus the IMHEAD of the center 
facet.

Curious - BTW a pixel is a single point like your bright "dots" in the 
good part.  Of course the worry is that the good part includes outer 
facet messed up from the more distant pointings...

I will start looking although I have been told to write a very long and 
tricky document so software is taking a back seat.

Eric Greisen