[evlatests] D*P contributions to total intensity

[Barry Clark]

Yes, frequency dependent D terms are intrinsically entwined with the
R-L delay.  When you start talking about frequency dependent D terms,
I think the problem can be phrased as one of making sure the set of
orthogonal polarizations you are reducing to is the same at one end
of the band as the other.  I don't know a good way out of this.  You
can assume 3C 286 has a known linear Faraday rotation (the Faraday
synthesis enthusiasts will shake their heads), You can radiate a
linearly polarized signal from the dish vertex and rely on theoretical
estimation of the polarization effects of bouncing off the subreflector
($$).  You can install one receiver without the circular polarizer
(outrage from non-polarimetrists).

One of these days we might get organized enough to provide noise diode
delays, the equivalent of VLBA pcal delays, which I would think would
be helpful for interpolating solutions for R-L delay if you ever got
one.

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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