[daip] FARAD question

[Eric Greisen]

Hanno Spreeuw wrote:
> Hello, I have  a question about FARAD for WSRT data.
> After changing the Stokes value from -5 to -1 in the header using PUTHEAD,
> RL=Q+iU is now XY=-U+iV.
> Assume V=0 for calibrators and target. XY=YX=real if the XY phase offset 
> has been corrected for properly. Ionospheric Faraday rotation will then 
> not affect the XY phase (~0), just the amplitude, if I am not mistaken.
> 
> Also XX and YY are affected because Q is affected by ionospheric Faraday 
> rotation. For the VLA, RR and LL are not affected by ionospheric faraday 
> rotation if I understand correctly.
> 
> Is it correct to infer that FARAD will not work for WSRT data because 
> the XY phase does not change by ionospheric faraday rotation?
> Is there a workaround? I mean, the RMs calculated by FARAD could still 
> be used but would have to applied differently.

I think it is safe to say that AIPS only works minimally with linear 
polarization telescopes.  It can do some useful stuff with polarized 
calibrators on WSRT data since they are equatorial.  But polarization 
calibration itself would be beyond our capabilities and of course FARAD 
cannot do something correct for linear polarizations when the header has 
been buggered to lie about the polarization...

If you know someone who is expert in linear polarization who would like 
to work in AIPS, we might be able to find some funding....

Eric Greisen