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

[Rick Perley]

Mark et al.:

The new lin-pol AIPS code does not assume all antennas are identically oriented — but you do need to know in advance what the angles are.  That should be no problem for parallactic angles.  For misoriented dipoles of  unknown offset (such as we have on the VLA’s P-band feeds), the correction is best handled as part of the ‘d-term’ analysis.  See the below-referenced memo for details.  

I don’t know anything about how CASA deals with this.  

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).  

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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