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

[Walter Brisken]

Three comments here.

1. For the polarizations to be properly propagated, a vex file correctly 
identifying the polarizations is needed from the start.  Is it possible 
that the wrong polarization labels started with an incorrect vex file?

2. It is strictly true that H/V would be the more appropriate labels for 
alt-az antennas.  However, for the few HaDec mounts.  For other mount types
(e.g., X over Y, Nasmyth, RadioAstron, ...) neither prescription is 
correct.  So there is no pleasing everyone.  I think the best thing to do 
is just consider the "X/Y" that comes from the correlator as "uncalibrated 
X/Y".  Downstream calibration (when applying the parallactic angle 
correction) will then calibrate the polarization.

3. CASA also uses X/Y for all linear polarizations.  The cost of changing 
to H/V would be large, with almost no tangible gain.  I see no reason to 
pursue code changes to this effect.

 	-Walter

On Thu, 11 Apr 2019, Leonid Petrov wrote:

> Dear colleagues,
>
>> I second Chris's note that X/Y labels seem to be getting propagated
>> correctly via the FITS-IDI file to AIPS at the moment in DiFX-produced
>> files.
>
> I have VGOS experiment b17339
>
> b17339/h_1001.input :
>
> ...
> NUM REC POLS 31:    2
> REC BAND 0 POL:     X
> REC BAND 0 INDEX:   0
> REC BAND 1 POL:     X
> REC BAND 1 INDEX:   1
> ...
> REC BAND 9 POL:     Y
> REC BAND 9 INDEX:   1
> REC BAND 10 POL:    Y
> REC BAND 10 INDEX:  2
> ...
>
>  and difx2fits puts  'R' in POLTYA and 'L' in POLTYB
> I see these lines in applications/difx2fits/src/fitsAN.c (2.5.2):
>
> 	polTypeA = 'R';
> 	polTypeB = 'L';
>
>  i.e. polTypeA, polTypeB are just hardcoded! Ivan, can you look at
> your version fitsAN.c? Maybe somebody has already applied a similar patch?
>
>> But I suppose that the data may be stored in multiple places in the
>> FITS-IDI file, and it is entirely possible that one place is correct and
>> another is not!
>
>  How a program that parses FITS-IDI is supposed to learn the polarization??
> Keep in mind, this is the first step. I have mixed polarization VLBI
> experiments in my desk that difx just rejects. For this experiment incorrect
> propagation of polarization labels will be disastrous.
>
>  Ivan, thank you for your comment. I missed that some antennas really
> record X and Y. In that case if AIPS expects X and Y polarization data and
> if we will try to eat VGOS data that record H and V, it will produce totally
> wrong result! If today AIPS does not support logic for handling H and V data,
> we need ask Eric to add an error message like "Do not support H and V pol
> data yet".
>
>  Then the question, how the labels in *.input files are set an the first place?
>
> Leonid
> 2019.04.11_07:38:29
>
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-- 
-------------------------
Walter Brisken
NRAO
Deputy Assistant Director for VLBA Development
(575)-835-7133 (office)
(505)-234-5912 (cell)