[evlatests] [Ecsv] Summary of Polarization Meeting

[Barry Clark]

So, another bad way of getting "absolute" D terms.  Look at your
calibrator at half power points as well as beam center, and adjust
your D terms to get a circular pattern.

I don't think this is a matter of practical importance.  The error
introduced is the correction from relative to "absolute" D terms times
the "dish polarization", which will amount to a few tenths of a percent
at most (less than half a percent), probably less than the error in the
estimates of the "dish polarization."  (That is, comparable to the
effects arising from the feed legs.)

Walter Brisken wrote:
> Hi all,
> 
> Perhaps I didn't get the whole story as I did not attend this meeting, but 
> I suspect Barry's response here is limited in applicability to the portion 
> of the primary beam that can be well approximated as having no 
> instrumental polarization.  Absolute polarization calibration does start 
> to matter when observing near the edges of the beam, perhaps even at the 
> half power point.  This is because the natural basis polarization states 
> become chosen for you (and are no longer arbitrary), and thus you can no 
> longer rely on a polarization basis that fits your needs.  Since the 
> natural polarization basis states will change over the primary beam, only 
> properly calibrated data should produce wide-field images without 
> artifacts.
> 
> I propose a simple observation to test this proposition.  Measure and 
> calibrate a database on a bright source observed on bore sight.  Then 
> attempt to apply the resultant polarization calibration to observations of 
> the same source observed at a few -6dB points around the center of the 
> beam while incorporating the full-beam polarization corrections in the 
> form of the models Sanjay has in his imaging code.  I suspect that 
> artifacts will appear.
> 
> This concept forms the ground layer of an idea I've had (but have yet to 
> test) for polarization calibration for the VLBA.  One can use the 
> instrumental polarization of the off-axis antenna beams to produce an 
> effective source of known polarization angle from an unpolarized source. 
> When I get enough time I plan to try this.
> 
> -Walter
> 
> On Fri, 16 Jul 2010, Barry Clark wrote:
> 
>> 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]
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