[evlatests] Update on Strange R-L phase behavior

[Rick Perley]

This would need to be taken with considerable oversampling.  I have such 
data, taken quite a long time ago ...  Not sure I can get to it right 
now (but would happily offer it to any volunteer ...)

Rick

On 3/30/22 14:02, Steven Myers via evlatests wrote:
> 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
>>
>>
>>
>>
>>     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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