[evlatests] Update on Strange R-L phase behavior

Rick Perley rperley at nrao.edu
Thu Mar 31 10:22:46 EDT 2022 

The scale of the variation is about right.  We're going to need the 2-D 
phase surface images, in both R and L.    Also the differences ...

I have both L and C band data, taken the same way ... should anybody be 
bold enough to want to take this on ...

(I'm a bit tied up with other issues at the moment ...)

Rick

On 3/31/22 04:46, pjaganna via evlatests wrote:
> Hello,
>
> As promised here are the file locations for the voltage beams.
>
> ea01 - /lustre/pjaganna/evla/holography/beams_spw17/Ant0.im
> ea06 - /lustre/pjaganna/evla/holography/beams_spw17/Ant5.im
> ea22 - /lustre/pjaganna/evla/holography/beams_spw17/Ant20.im
>
>
> These images contain the jones cube for that particular IF.
> Which in this case is at the central frequency 3948MHz
> starting frequency.
>
> I went ahead and picked the central channel and plotted the
> phase across the main-lobe of the voltage pattern in R.
> The phase across the main-lobe varies within about 10degrees
> for the two antennas which are known to have better
> pointing performance namely ea01, ea22. For antenna ea06
> it has significant pointing offsets that are not removed
> to derive this voltage plot and consequently has phase
> changes nearly twice the magnitude of the other two antennas
> Unfortunately the other antenna in ricks plot is a reference
> antenna for the holography observation and does not have
> a beam as a result.
>
> These data are
> S-Band holography data in Ricks EVLA memo 195. That I have
> analyzed in CASA. I am trying to extract the RL Phase from
> 3C147 which is the phase calibrator for the holography run
> and consequently has been visited across a wide range of
> elevations. More on that when I get through the data tomorrow.
>
> Cheers,
>
> ---
> 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 16:14, pjaganna 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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