[daip] Position offsets in UVFIT

[Andy Biggs]

Hi Eric. I gave this a go and it works - it's now possible to fit to sources far from the phase centre. It seems that the UVFIX+UVFIT method works better, but it's much more convenient.

Cheers,

Andy

On 4 Apr 2012, at 23:12, Eric Greisen wrote:

> 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.
> 
> I have put back a corrected versin that is rather slower but uses correct maths for the phases and derivatives.  Sadly double precision sines and cosines are slow compared to linearized computations.
> Hopefully, not too slow.
> 
> Eric Greisen