[daip] Position offsets in UVFIT

[Eric Greisen]

Andy Biggs wrote:
> Hi there. I'm trying to fit to the u,v visibilities of a source *far* from the phase centre with UVFIT. If I shift the data to the phase centre with UVFIX then the fitting goes fine with GPOS=0. However, if I try fitting to the un-UVFIX'd data, then I can't get sensible answers. I use IMDIST to tell me the x and y offsets of the source relative to the phase centre and use these in UVFIT - should that work?
> 
> I want to subtract the source from the data, taking into account its variation with HA due to the primary beam. I do this by model fitting as a function of time and don't want to UVFIX the data twice.
> 
> Cheers,
> 
> Andy
> 
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I have examined the maths used in UVFIT and they are better than pure 
linear but not up to the full field problem.  I will give it a whirl to 
see if I can make it better (and get it to work).  Note that until 
recently I did not think that UVFIT worked.  I also did not realize that 
OMFIT worked either but it does - although it is hard to drive.

Eric Greisen