[evlatests] Antenna 8 polarization

[George Moellenbrock]

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 ...
>
>
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