[evlatests] Update on Strange R-L phase behavior

Rick Perley rperley at nrao.edu
Wed Mar 30 16:30:11 EDT 2022 

     For those thinking of invoking the small spatial phase gradient 
withing the R and L beams, combined with pointing offsets (which 
certainly do exist near the zenith), I remind that the data (at least, 
the data I'm looking at) show all three bands to have identical R-L 
phase differences for L, S, and C bands.  Although it is true that any 
pointing offsets are common to the three bands, I would not expect the R 
and L beam phase gradients for each band (and for every antenna) to be 
the same!  Naively, I would expect an R-L phase difference to show a 
gradient along the squint axis (which is dramatically different between 
these three bands).

     No impossible, but unlikely ...

     Nevertheless, it would be useful to see the spatial (differential) 
phases over the main beams.  It's been a very long time since I've tried 
to do this.  I recall gradients of a few degrees are common.  About the 
right magnitude ...

On 3/30/22 14:14, pjaganna via evlatests wrote:
> I do have some complex voltage maps on disk. For S band on the ready 
> thanks to VLASS.
> I can point you to it. There is also repeated scans on 3C147 in that 
> which can be used
> to derive the same numbers that Rick has independent of Rick and AIPS 
> software.
>
>
> ---
> Preshanth Jagannathan
>
> Associate Scientist,
>
> Algorithm Research & Development Group.
> NRAO Socorro, New Mexico, 87801, USA.
>
> Phone: +1-575-835-7497
> Email: pjaganna at nrao.edu
>
> On 2022-03-30 16:02, 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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