[Difx-users] Handling lin-pol data with difx2fits

[Mark Kettenis]

> From: Rick Perley <rperley at nrao.edu>
> Date: Thu, 11 Apr 2019 13:58:31 -0600
> 
>      All:
> 
>      AIPS now does to linear polarization quite nicely.  See the EVLA 
> Memo (#207) that Eric Greisen and I just submitted.
> 
> Many changes were needed to ensure the correct combinations, and order, 
> were made.  Eric can provide details, if needed.
> 
>      Rick

Hi Rick & others,

Of course the assumption made in 4.1 that all antennas are identically
oriented on the sky doesn't hold for a typical VLBI array.  If that
assumption is reflected in the AIPS code then calibration and
interpretation of VLBI data with linear polarisations in AIPS won't be
straightforward.  And I believe the data structures still won't be
able to represent mixed linear-circular observations.

While the MeasurementSet in principle allows proper representation of
such observations I don't think CASA can actually handle such data
properly either.

This is why I think that conversion from linear to circular after
correlation (as implemented by Ivan Marti-Vidal in his PolConvert
tool) is still the most viable strategy to handle mixed-polarisation
observations, possibly including all-linear VLBI observations.

Cheers,

Mark

> On 04/11/2019 01:44 PM, Leonid Petrov wrote:
> > Dear Walter,
> >
> >> It is true that the poltype is hard coded.  With some experimentation we
> >> might be able to change that to X/Y as aspropriate.  At one point I
> >> believe AIPS had trouble with other than R or L.  We can look into this
> >> for DiFX 2.7.
> >    If AIPS at the moment has troubles with other than R and L, we can
> > ask Eric just to patch AIPS to treat X and Y or H and V as R and L on the flight
> > temporarily until better time come, if it is necessary. It should not take more
> > than 5 minutes. From the other hand, I got an impression that some folks
> > already used AIPS for processing lin-pol data. Am I wrong? If yes, how did they
> > do it?
> >
> >    I have 6 lin-pol experiments on my table and a hundred or so to come within
> > weeks. We have to fix it now, i.e. today.
> >
> >> As noted above, the FITS format is unable to describe such data, so this
> >> is not possible.
> >    Although I heard a combinations of words "possible" and "not", I still cannot
> > grasp what do they mean together. We need amend FITS-IDI, I understand it.
> > So what? Even the US Constitution was amended, and more than once. Let us
> > discuss how to do it. And we need fix difx2fits: today it tells that it does not
> > like a combination of lin- and cir- data and stops. Yes, we already have the
> > lin-cir and cir-lin data that need to be processed, and we plan to get more data
> > like that. I already rolled my sleeves up and ready to do it. If you or anybody
> > else can provide ideas and/or guidance, warn about pitfalls, contribute code, etc,
> > I will appreciate it. But I cannot just do nothing and say: "these are the data that
> > I cannot process". Failure is not an option.
> >
> > Sincerely,
> > Leonid
> > 2019.04.11_15:38:06
> 
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