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

[Ivan Marti-Vidal]

Dear Leonid,

The only issue I see with the H/V labelling is that it assumes that the
polarizers are horizontal and vertical. However, there can be cases
(e.g., ALMA) where the feed is rotated in the antenna mount, so that X
and Y are not truly H and V. IMHO, keeping X and Y would avoid potential
misunderstandings.

   Best Wishes,

          Iván

On 4/11/19 5:40 AM, Leonid Petrov wrote:
> Dear colleagues,
>
>   DiFX has a bug and does not propagate polarization labels to the 
> output FITS-IDI. The fix is rather trivial. 
>
>   However when I discussed this issue with Rick Perley, he pointed out 
> that another fix is needed as well. According to famous AIPS 114 memo, 
> page 8, horizontal and vertical polarizations shall be labeled X and Y:
>
> X Horizontal linear
> Y Vertical linear
>
> i.e. fixed in the topo-centric, site-dependent coordinate system.
>
>   This is contrary to the IAU 1976 resolution that defines X polarization
> along direction to the Northern Pole and Y polarization along the 
> right ascension axis (East), i.e. rotating in the topo-centric, 
> site-dependent coordinate system.
>
>   I suggest to rename the polarizations recorded at the VLBI stations as 
>
> H Horizontal linear
> V Vertical linear
>
>   and reserve X and Y labels for space telescopes.
>
>   In addition to just complying with the IAU nomenclature, it reduces chances 
> of misunderstanding in communication with station personnel: who knows which 
> polarization a station guy considers as "X", but unless he is operating an 
> orbiting VLBI telescope, horizontal and vertical polarizations are unambiguous.
>
>   Any objections? Other thoughts?
>
>   Since we only started using FITS-IDI data with lin-pol, this change will
> be relatively painless.
>
> Leonid
> 2019.04.10_23:34:51
>
> ------------------------------------ Patch ---------------------------------
>
> --- applications/difx2fits/src/fitsAN.c~	2019-04-06 21:14:14.733263707 -0400
> +++ applications/difx2fits/src/fitsAN.c	2019-04-06 21:44:13.932896491 -0400
> @@ -106,8 +106,22 @@
>  	stop  = D->mjdStop  - (int)D->mjdStart; 
>  
>  	arrayId1 = 1;
> -	polTypeA = 'R';
> +	polTypeA = 'R'; /* Default */
>  	polTypeB = 'L';
> +	if ( D->nPolar == 1 )
> +	{
> +             polTypeA = D->polPair[0];
> +	      polTypeB = '?'; /* Mark missing polarization */
> +        }        
> +        {
> +             polTypeA = D->polPair[0];
> +	      polTypeB = D->polPair[1];
> +        }
> +        if ( polTypeA == 'X' ) { polTypeA = 'H'; } /* Rename linear polarizations */
> +        if ( polTypeA == 'Y' ) { polTypeA = 'V'; }
> +        if ( polTypeB == 'X' ) { polTypeB = 'H'; }
> +        if ( polTypeB == 'Y' ) { polTypeB = 'V'; }
> +
>  	time = 0.5 * (stop + start);
>  	timeInt = stop - start;
>  	for(bandId = 0; bandId < nBand; ++bandId)
>
> _______________________________________________
> Difx-users mailing list
> Difx-users at listmgr.nrao.edu
> https://listmgr.nrao.edu/mailman/listinfo/difx-users
-- 

-----------------------------------------------------------------
Dr. Ivan Marti-Vidal

Observatorio de Yebes - IGN   | E-mail: ivan.marti-vidal at oan.es
Cerro de la Palera S/N        | Phone:  +34 949 237 076
19141 Yebes (Guadalajara)     | Fax:    +34 949 290 063
Spain                         | Skype:  marti-vidal
-----------------------------------------------------------------
Webpage:       http://mural.uv.es/imarvi
-----------------------------------------------------------------
PGP Key ID:    0xC5B38704
ORCID:         0000-0003-3708-9611
Researcher ID: A-8799-2017
-----------------------------------------------------------------

-