[evlatests] Update on Strange R-L phase behavior

[George Moellenbrock]

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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