[evlatests] Double-D term at C-band?

[George Moellenbrock]

Rick-

A few points:

1. The D*D contribution to the parallel hands is constant in time
(per baseline) for stable D-terms because the parallactic angle
factor on that term contains the parallactic angle _difference_
between antennas in the baseline. These differences are
very nearly zero for the VLA, so unless the D-terms themselves
have a diurnal periodicity (recall the apparent R-L phase instability
I've reported for C-band, which could be a result of D(t)), the D*D
term cannot explain your 1% residual.  Also, this term cannot
be a function of source, nor is is purely antenna-based.

2. The  D*P contribution to the parallel hands contain non-unity
parallactic angle factors, and so can contribute time- and source-
dependent effects.  A ~10% linearly polarized source (e.g. 3C286,
3C138) and ~10% D-terms would induce a ~1% diurnal residual,
but it will not factor perfectly as an antenna-based _multiplicative_
gain.

3. It is worth noting that while the switched power system
is subject to the degree of polarization isolation in the
signal path it samples, the characterization of it as a
multiplicative gain correction does not admit a proper
polarization isolation correction (obviously it cannot
correctly model effects proportional to source polarization).

4. How well are the antenna- and poln-relative Tcals known?
As good as 1%?  Seems to me an uncorrected 1% residual
is easier to explain by a relative scaling error in the switched
power gain estimate (as a function of poln and antenna).  And
if a single mean Tcal is used for both polarizations on each
antenna, this would explain the polarization asymmetrization
you note.  (Pre-applying a commonly estimated opacity
correction---your "elevation-dependent correction"---
can also asymmetrize the residuals on the polarization
axis.)

-George





On Fri, Dec 31, 2010 at 8:05 PM,  <rperley at nrao.edu> wrote:
> I reported yesterday on a strong temporal variation in electronic gains at
> C-band, most likely due to temperature sensitivity, which is mostly -- but
> not completely -- removed by application of switched power.  The basic
> effect gives a ~ 7% variation in gain.  After correction, about 1%
> variation remains.  Nearly all antennas show this effect, to the levels
> quoted.
>
> It proves to be difficult to calibrate more accurately than about 1% in
> the face of this variation.  In particular, separating temporal from
> elevation effects will require software more sophisticated than what we
> have now.  I was able to approximately effect this for the C-band data by
> utilizing the night-time hours (during which the gains were fairly stable)
> to solve for the calibrator fluxes and elevation effects, after which the
> residual temporal changes are easier to identify.
>
> Doing this to the C-band data revealed a most curious effect in the gain
> residuals:  These are not randomly distributed, but often show an opposite
> pattern between two sources -- e.g. source 'A' being 1% above the mean,
> source 'B' being 1% below, for a period of a few hours.  If the sources
> are observed altenately, the result is a zig-zag pattern.  The really
> interesting thing is the opposite polarizations have opposite residuals:
> so in the example noted above, the RCP residuals are of the sense noted,
> while the LCP residuals are of equal and opposite sign!  This is seen in
> all antennas, throughout the entire 30 hour run!!
>
> The magnitude of the effect is about right for the 'double-D'
> cross-polarization -- which leaks RCP into LCP and vice versa.  With the
> cross-polarization currently at 10%, the 'double-D' is expected to show up
> at the 1% level.
>
> Note that this effect is not seen in the X-band data -- for which the 'D'
> terms are at the 1 -- 3% level.
>
> The opposing sense of this for RCP and LCP (which I would not have
> expected offhand) means that plots of Stokes 'I' vs (say) UV distance look
> very nice, but individuals plots in RCP or LCP are quite a bit worse.  The
> difference of course shows up well in plots of Stokes 'V'.
>
> Further efforts to reduce the 'calibrator run' will have to await my
> return from holiday and the AAS, in mid January.
>
>
>
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