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

[Rick Perley]

    Could do so, if the pointing error is bad enough -- putting the 
source beyond the half-power or so ...

Ken Sowinski wrote:
> 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?