[evlatests] D*P contributions to total intensity

[bcotton at nrao.edu]

rperley at nrao.edu writes:
 > 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.

   This might help but there is also likely separate single-band and
multi-band R-L delays which would confuse things.  It would be best to
actually measure the D terms over wide bands rather than assuming
properties.  I think this is critical to devising an optimal strategy
for calibrating it out.

 > 
 > 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...

   That would be great, multiple calibrators is good.

-Bill

 > 
 > 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
 > >
 >