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

[Leonid Petrov]

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