[evlatests] D*P contributions to total intensity

[Bill Cotton]

George,

   Certainly solving for the D terms per channel allows for an
arbitrary function of frequency but can suffer from low SNR.
If the instrumental polarization is a relatively slow function of
frequency as you suggest, then it can be parameterized.  This can
result in better SNR while including the frequency dependence.
I wonder from your description if a R-L phase bandpass for the
reference antenna would leave a more nearly constant instrumental
polarization.  If what you describe is, in fact, merely the R-L phase
spectrum of the reference antenna that would greatly simplify
calibration. 

-Bill

"GM" == George Moellenbrock <gmoellen at aoc.nrao.edu> writes:

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

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

GM> -George

GM> 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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