[evlatests] D*P contributions to total intensity

[bcotton at nrao.edu]

   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