[evlatests] D*P contributions to total intensity

[George Moellenbrock]

Actually, for an unpolarized calibrator (at least, see below),
the best solution is to solve for the D-terms per _channel_,
each in its own R-L phase frame, then solve for R-L also
per channel to get the positional angle calibrated.   This
is essentially just the traditional approach invoked per channel
rather than per "IF".   We support this in CASA, and it is what
we did for the summer school tutorial using 3C84.  Note that
the R-L phase residual (post gain calibration) is not just a pure
delay---there is an interesting R-L phase bandpass at the
level of a few 10s of degrees variation on top
of any R-L delay slope (across 128 MHz), as well (look
at any bandpass phase in the parallel hands, and expect
as much for the refant's cross-hand phase bandpass).

Currently, CASA does the source polarization estimate
per spw (not per channel), so large coherence problems
(e.g., delays) need to be removed for a sensible
source polarization estimate.  To this end (and as part
of adding support for linear feeds), I've added
a rudimentary R-L delay solving option to gaincal in CASA
(checked in yesterday).  This should permit improving
spectral coherence sufficiently for a decent source pol
estimate, followed by channel-dep (or not) D-term
estimation.

-George

On Tue, Jul 27, 2010 at 7:41 AM,  <bcotton at nrao.edu> wrote:
>
>   I've been working on the wideband C band polarization test data and
> have run into a problem which has long been an issue for VLBI
> polarimetry, namely the interaction between the R-L delay and the
> instrumental polarization.
>   After the parallel hand calibration there is ideally a single delay
> and phase offset between the R and L gain systems.  This should be
> easy to determine from looking at a known polarized signal.  However,
> the instrumental polarization also contributes and for the high
> instrumental polarization for the EVLA this is a serious contribution
> even for strongly polarized sources like 3C286.
>   The R-L delay term needs to be removed before fitting D terms but
> the D terms corrupt estimation of the R-L delay.  The frequency
> structure of the D terms is one of the issues that needs to be better
> understood but it's is hard to separate from the R-L delay.
>   The optimum solution might be a joint estimation of the R and L
> gains, R-L phase and delay, bandpass and the frequency dependent D
> terms.  That's alot of data to shove into a least squares solver.
>
> -Bill
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