[evlatests] Update on Strange R-L phase behavior
pjaganna
pjaganna at nrao.edu
Thu Mar 31 06:46:54 EDT 2022
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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