[evlatests] [Ecsv] Summary of Polarization Meeting

[Barry Clark]

I slightly disagree about the usefulness of the absolute D terms.
For purposes of estimating I (necessary for very high dynamic ranges),
the ordinary, relative ones are adequate.  The use of any sort of
D terms suffices to move the observation to two orthogonal
polarizations, and I is the sum of the powers in the two.

An interesting way of looking at D term application is that the
linear form moves the vectors along the tangent plane to the
Poincare sphere, and the higher order terms convert this motion
to a rotation preserving the radius (that is, the power).

As George pointed out, you can add arbitrary constants to the D terms,
which shoves you to a different polarization reference, but I believe
that if it is done right, it will preserve I.  (This is much easier
to see if you first think about calibrating on an unpolarized source,
and once you convince yourself that works, thinking about a polarized
one.)

The usefulness of the absolute D terms is in measuring the polarized
flux.  If our orthogonal polarizations differ from true circular by,
say, 1 dB, the naively estimated linear polarizations will be wrong
by about 1% of their values.

Rick Perley wrote:
[snip]
> 
>     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.
> 
[snip]