Possible problem with plotting polarizations in uvspec?

[dominics]

Hi,

I'm using uvspec to plot Stokes XY and YX, and I noticed that Stokes U is very different from what I would expect based on XY and YX, i.e. U = 1/2 (XY + YX). I've included a screenshot of the uvspec window here:

0201-440_Stokes.tiff (33.86 KiB) Viewed 34792 times

I'm plotting only the real part of the visibilities, but this cannot be the explanation since
Real(U) = Real( 1/2(XY + YX) ) = 1/2[ Real(XY) + Real(YX) ]
: the real part of Stokes U should still be the average of the real parts of XY and YX, which is not the case.
Could this be a plotting problem in uvspec? I'm using version 1.0 21-Apr-10. The command line code I use is
uvspec vis=0201-440.1384 device=/xs interval=100000 axis=freq,real nxy=1,1 options=nobase,avall stokes=xy,yx,u,v

On Jamie's advice I used uvaver to convert the visibilities from the original data set to Stokes I,Q,U,V, but the spectrum of Stokes U and V of the new data set shows the same issues.

Let me know if I can help with solving this problem.

best,
Dominic

[Mark.Wieringa]

Hi Dominic,

I think the problem is that the real relation between XY,YX and U,V is given by



If you look at uvplt of time,ampl of XY and YX you can see a clear parallactic angle effect, whereas U and V tend to be flat.

Cheers,

Mark

[dominics]

Hi Mark,

Thanks for your answer. I know about the parallactic angle effects in uvplt, I was now looking into how uvspec might be used to identify bad data (in particular bandpass problems, so I was looking at XX/YY/XY/YX instead of the Stokes parameters, where such problems are more difficult to trace back)

thanks again,
Dominic