[evlatests] Summary of Polarization Meeting

[Rick Perley]

    All:

    A brief summary of the Wednesday afternoon polarization meeting.  
Any errors are due to me, and those present are welcome to issue 
corrections. 

    A)  Status of measuring characteristics of EVLA polarizers.

    There is broad agreement that the wideband polarizers, although 
rather impure at L, S, and C bands, are very stable, thus enabling 
accurate 'on-axis' polarimetry.  Josh Marvel had, on the previous day, 
shown the stability characteristics derived from interferometric data.  
These are in excellent agreement with the results reported in EVLA Memos 
131, 134, 135, and 141.  It was also reported that a new design 
quadrature hybrid is known to be more stable under cryogenic conditions, 
and is thus expected to provide better purity at these low frequency 
bands.  A few demonstrator units are being installed now in the field -- 
when there is at least one of each of these bands available, Rick will 
arrange the 'rotation by 90 degrees' test to measure the absolute Ds to 
confirm the expectations. 

    There is general agreement that the long-term stability (months) is 
sufficient good to enable a table-based standard polarization correction 
to be applied as a matter of course.  There is an issue of the frequency 
grid at which these corrections should be recorded -- more discussion 
will be needed to settle this. 
    Highly accurate polarimetry will generally require 'in-situ' antenna 
polarization determinations, as has been done in the past. 

    Action Items: 

    a) Continued monitoring (Steve), and analysis (Josh) at multiple bands.
    b) 90 degrees receiver rotation trick, to determine absolute Ds at 
L, S, C (and X, when available) bands, to monitor changes and to 
determine if the new design hybrids meet expectations.

    B) Status of polarization calibration in CASA, including 
implementation of full matrix corrections. 

    This refers to the implementation of the full 4 x 4 'Mueller' matrix 
which relates the observed cross products (RR, RL, LR, LL) to the 
desired Stokes visibilites (I, Q, U, V).  The implemented software in 
both CASA and AIPS utilizes the first-order approximation, which drops 
all products between D terms and Stokes Q, U, V and other Ds.  (That is, 
it retains only the product between D and I) 
       George reports that he is nearly ready to do tests of the full 
corrections.  An importance point is that, for the VLA, it is very 
unlikely that the *absolute* Ds can be derived as a matter of course 
from ordinary observations -- by construction, all antennas view the 
sources at the same parallactic angle, making impossible, or at least 
highly unlikely, a robust method for extracting the absolute Ds.  By 
necessity, all 'Ds' determined from standard interferometry are 
referenced to a standard -- either a global mean (CASA, MIRIAD), or a 
particular antenna (AIPS).  The full matrix correction requires 
absolute, not relative Ds (which are sufficient for the linearized 
treatment).   It was agreed that a good test of the code will be to 
utilize the 'absolute' Ds determined by receiver rotation, from which we 
hope an interative process can be developed.   There is reasonable hope 
for this procedure, given the stability of the D terms.  Note that in 
general, these higher order corrections may only be needed for imaging 
in the multi-hundred thousand to one regime. 
    Other methods to determine absolute Ds were briefly discussed.   If 
time permits, I may test these. 
    George is currently busy with development of polarimetry for ALMA, 
but should soon be available for these trials.  He will report 
separately on these issues in more detail.

    C) Status of off-axis polarimetry. 

    Sanjay reports that the necessary code is available for testing 
within CASA, but that higher priority items are taking up his time. 
    Frazer has suggested that the beam polarization models (derived by 
Walter using Penticton's GRASP8 software) would be sufficient for 
testing the code.  Others opined that the beam holography observations 
made by Rick last December, and reduced by Bill Cotton, (and reported by 
Bill in an OBIT memo) would be more appropriate.  Bill offered to 
provide suitable tables for this purpose.   Bill also offered to use 
these models with his OBIT environment, to calibrate some of Frazer's 
wide-field data. 

    Time ran out before a discussion of observational methodologies 
suitable for our EVLA users (both 'internal' RSRO/ECSO, and external 
'OSRO') could be held.  We will need another meeting, perhaps in a 
couple of weeks, to review the status.