[evlatests] Antenna 8 polarization

[Barry Clark]

If you want to do something drastic to evaluate absolute polarization,
just put up a receiver without the hybrid.  If the circular feed, which
is supposed to provide E_v + i*E_h and E_v - i*E_h is really providing
E_v + x*E_h and E_v-y*E_h, where x and y are complex numbers near i,
on an unpolarized source, the crossed 'hands' directly give x and y
(after calibration with the parallel 'hands').

George Moellenbrock wrote:
> Rick-
> 
> Nicely done!   That's one way to get _differential_ parallactic
> angle coverage on the EVLA a la VLBI.
> 
> A couple of points:
> 
> 1.  A rotation of exactly 90 degrees is fortuitous.  Anything
> else would require a third position angle or a careful
> consideration of the _direction_ of rotation.   (Are there
> rotation "choices"?)
> 
> 2. Was the whole dewar rotated?  I.e., the receiver stayed cold
> throughout?   Your analysis of course depends on the stability
> of the D-terms, even when the physical rotation is introduced.  In
> the VLA D-term stability analyses, departures from stability were
> matched to maintenance events that included (but not necessarily
> limited to) warming up the receiver.   Departures from stability
> induced in this manner should be ruled out.
> 
> 3. Doing this with some frequency resolution might also enable
> disentangling the receiver D-term contribution from the antenna
> (standing waves, etc.).
> 
> -George
> 
> 
> 
> On Thu, Apr 30, 2009 at 3:39 PM, Rick Perley <rperley at nrao.edu> wrote:
>>    The 6cm receiver on antenna 8 was correctly re-oriented yesterday
>> (having been found to have been mounted 90 degrees away from the
>> standard position), and a quick observation of an unpolarized source
>> made at noon today.
>>    The cross-polarization between antenna 8 and all the other wide-band
>> antennas is now around 4%, rather than the 15% that was seen on the
>> weekend data.
>>
>>    But we can do a little more with these data.  Presuming that all the
>> observed cross-polarization is due to the receiver (and that none of it
>> is due to the antenna), rotating the feed by 90 degrees flips the sign
>> of the cross-polarized contribution from that antenna.  So, if on a
>> single baseline observed an unpolarized source, the two antennas both
>> have their feeds oriented normally, the RL correlator data, after
>> calibration for parallel-hand gains, gives an output given by:
>>
>> RL = (Dr1 + Dl2*)I
>>
>> where I is the total intensity.
>> If antenna 2's receiver is rotated by 90 degrees, the relation becomes
>>
>> RL = (Dr1 - Dl2*)I
>>
>> The sum and differences give us the actual D terms amplitude and phase.
>>
>> I tried this, by hand, on a single baseline, 28 x 8.  I get:
>>
>> Dl28 = 5.8% at pa = 113.
>> Dr8 = 8.6% at pa = -52.
>>
>> Note that these are fairly high, and that their phases are nearly in
>> opposition (meaning, physically, that the R and L antenna ellipticities
>> are nearly orthogonal).  This explains why, when in the correct
>> orientation the polarization is seen to be low (the vector sum nearly
>> cancels the contributions), while when one feed is rotated by 90 degrees
>> (which rotates the D-term by 180 degrees), the observed polarization is
>> very high (the vector amplitudes very nearly add tip to tail).
>>
>> This procedure can be done, using the data in hand, to derive the
>> absolute cross polarizations for every antenna in the array.  If we want
>> to do this for every band, we'll need one antenna's feeds to be
>> similarly rotated, for a little while ...
>>
>>
>> _______________________________________________
>> evlatests mailing list
>> evlatests at listmgr.cv.nrao.edu
>> http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests
>>
>>
> 
> _______________________________________________
> evlatests mailing list
> evlatests at listmgr.cv.nrao.edu
> http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests