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

[Mark Kettenis]

> From: Rick Perley <rperley at nrao.edu>
> Date: Fri, 12 Apr 2019 07:42:15 -0600
> 
> Mark et al.:
> 
> I don’t know anything about how CASA deals with this.

It should handle linears since that's what ALMA has ;).  But I'm
fairly confident most of the calibration code would fall flat on its
face in the case of mixed XY/RL.

> I’ve also thought about the post-correlation basis conversion
> method, (and have analyzed this in one of my many unseen memos) —
> but I would like to think a bit more about the consequences in
> calibration.  (It’s obviously completely valid for pure linearly
> polarized feeds, but not so clear for ‘real,dirty’dipoles).

As far as I can tell Ivan does consider leakage in his paper:

  https://arxiv.org/abs/1601.04266

But I don't think this method has been tested on dipoles...

Cheers,

Mark

> I’m cc’ing Eric on this.
> 
> Rik
> 
> Sent from my iPad
> 
> On Apr 12, 2019, at 6:42 AM, Mark Kettenis <kettenis at jive.eu> wrote:
> 
> >> 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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> 
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