[evlatests] Antenna 16, Q-band, bad polarizer?

[Ken Sowinski]

On Thu, 24 Feb 2011, Rick Perley wrote:

>    More interesting -- or at least curious -- information from the
> short test today.
>
>    In plotting the visibility amplitudes at Q-band, I found large
> 'closure errors', of the order of a few percent.  A quick check showed
> that *all* of these come from antenna 16.  I suspected bad delay or
> bandpass (since I created a 100 MHz-wide continuum by integrating across
> the spectrum) -- but no, that wasn't the problem, since the closure
> errors remained after proper delay and bandpass calibration.
>    The problem is a bad polarizer.  In plotting the cross-polarization
> amplitudes, I find that this antenna shows a 10% cross-polarization!  I
> guessing that this is cross-coupling the correlations, either through
> the D*P  mechanism, or the D*D mechanism.  Neither is corrected with the
> current way we handle polarization calibration ...

I would not get too excited about this yet.  ea16 has never had
collimations properly set since it came from the barn because its
X band receiver was removed as soon as it left the master pad.
The collimation run a night ago showed large errors at K, Ka and
Q bands.  The corrections were only introduced after your observations
today.  Might this affect the quality of polarization data?