[evlatests] D*P contributions to total intensity

[rperley at nrao.edu]

Bill:

Wouldn't a better way be to use the 'multi-band' phase slope (in RL and
LR) for a strongly polarized source like 3C286?  The argument would be
that the phase structure in the 'D' terms would be at least somewhat
randomized over so broad a frequency span.

I have now 2 GHz-wide data on Herc A (90 minutes worth) at C-band,
including 3C286 and four other calibrators.  I can FITTP you an
appropriate version of this (averaged to 10 seconds) for
experimentation...

Rick

>
>    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
>