[evlatests] Update on Strange R-L phase behavior

[Steven Myers]

Is there a R (and/or L) complex voltage pattern map sitting around somewhere to look at? Rick probably also has the equivalent from the holography runs.

> On Mar 30, 2022, at 1:32 PM, Sanjay Bhatnagar via evlatests <evlatests at listmgr.nrao.edu> wrote:
> 
> George:
> 
> 1. The effect I am thinking of is more like in the first few sentences of you second paragraph.  Source moving _systematically_ in the R and L voltage patterns.  The precise track can be written down as an expression (as also suggested by Steve).  It is not a source wander.  In general it also not a pure rotation (as you seem to imply).  Also, with significant pointing offsets antenna polarization squint matters for the kind of investigations done here (variation of R-L phase with time).
> 
> 2. CASA imaging *does* account for geometric effects (e.g. antenna offsets, squint, effects of non ideal aperture illumination as measured with holography, all of these as a function of time, etc.).  Some of these are needed, and even used for VLASS imaging.  Also these corrections, in general, can only be done during imaging (i.e., can't be done in the traditional pre imagining calibration step).  For compact sources one can approximate, I _think_, these corrections via transitional calibration (as in AIPS or CASA calibration modules) but only *after* eliminating pointing offsets 
> 
> On Mar 30, 2022 12:57 PM, George Moellenbrock via evlatests <evlatests at listmgr.nrao.edu> wrote:
> Sanjay-
> 
> I think you are describing phase variation within/across the voltage pattern, and the source wandering around in that.  Wouldn't that be quite band-dependent?   I think Rick was going to look for R/L amplitude effects which might be evidence of that sort of thing.  And we might expect that wander to be less systematic/symmetric, probably.   Still, wander around the beam, especially near zenith, is likely at least a confusing factor, indeed.
> 
> The geometric effects I've been trying to describe will operate even if the source is strictly stationary (in direction) in the voltage pattern.  But it is still rotating, or more to the point, the antenna (and thus feed) is rotating about the direction to the source in a manner that is a function of mechanical imperfections described by the pointing model (and related effects).  This rotation causes differential advance/retard of R and L phases, relative to whatever phase the vp introduces at the point the source pierces it (assuming stable pointing).    And to be clear, CASA (nor AIPS, to my knowledge) incorporates geometrical info from the pointing model to correct the differential rotation of the antennas (which gets interestingly large near zenith).  And this would be via the parallactic angle correction, which I suspect Rick hasn't been applying, else we'd probably see more interesting things, like more odd symmetry effect, if AIPS is still using geocentric latitude for the calculation (alas CASA does, too, because the overall impact is still fairly small for most observations, compared to likely posang errors from other causes). 
> 
> As for solving for the effects as Steve suggests, we may already be doing so, e.g., in the pointing model; i.e., existing terms can suffice, at least qualitatively if not to scale, and maybe some new term is needed...    My point is that we are not doing the peculiar feed rotation calibration explicitly anywhere**, and so the effects thereof must show up at some level in solved-for phases in the manner Rick has shown (possibly, or probably, confused a bit by what Sanjay describes, but not so much as to obliterate an otherwise very geometric-looking systematic effect), and may, in fact, be the actual explanation---if the required mechanical errors are significant enough to do it.  
> 
> (**is the correlator at all aware of the pointing model?  for reasons other than net path length, if even that?)
> 
> Cheers,
> 
> George
> 
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> 
> On 3/30/22 11:42, Sanjay Bhatnagar via evlatests wrote:
> A simpler way to achieve the same would be:
> 
> 1. For deriving R-L phases, use source model that includes known effects of antenna pointing offsets (from pointing measurements) and measured antenna aperture illumination patterns.  This can be done in CASA.
> 
> 2. I am less sure here, but since the celestial source is compact, I _think_ if the data is pointing offset-corrected before deriving R-L phases, it will effectively achieve almost the same as above.
> 
> sanjay
> 
> On 3/30/22 10:47 AM, Steven Myers via evlatests wrote:
> If the explanation is geometric, then can we write an equation mapping (AZ,EL) of the antenna and (HA,DEC) of the source, including the various physical offsets, to the observed R-L phase, and then solve for these offsets using the data in hand?
> 
> 
> 
> 
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