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<p>Sanjay-</p>
<p>You're saying that if the vp is registered stably to a tracked
point on the sky offset from the source, the source will travel
through the voltage pattern in a systematic way, by the
parallactic angle rate (and this is also why off-center sources
travel through beam "annuli"). Yep, and that will create some
sort of effect like this, if the pointing error is large enough,
and the vp R-L phase interesting enough in the right way on that
scale (but freq-dep...). Reference pointing is the exercise by
which we try to keep the source stationary in the vp to avoid
this. Unclear if reference pointing can be effective near the
zenith, though..... R/L Amps can say a lot, I think, if pointing
is going way bad near zenith.<br>
</p>
<p>I'm saying (in my 2nd paragraph), that on top of this, the source
itself rotates (spins in place) as it also possibly travels (as
above), and this rotation (at a stationary point in the vp, if the
pointing is good and stable), all by itself, causes the R-L phase
to evolve (think rotating helices). This is the parallactic
angle rotation itself, not its effect on a source's offset
location in the vp, and which changes R and L phase in opposite
directions. Now, because the antennas are all nominally mounted in
parallel, the gross parallactic angle phase (what we normally
think about) differences to zero in the <i>parallel</i> hands, so
doesn't matter in ordinary gain solutions (see VLBA, where parang
phase <i>will</i> show up in gain solutions if you don't
pre-apply it). However, the mechanical errors in antenna
structure and alignment mean that each antenna has a peculiar
residual in its driven rotation for the observed source and which
is modeled nowhere <i>for phase cal purposes</i>. The
differencing in the parallel hands isn't perfect, and this blows
up the R-L phase near the zenith. Can it do enough? Still
TBD...</p>
<p>These are distinct things, related through dependence on
parallactic angle, I think. Rotation must occur. Travel
through the vp <i>may</i> occur, but won't (significantly) if
pointing is good enough.<br>
</p>
<p>-George</p>
<p><br>
</p>
<p><br>
</p>
<div class="moz-cite-prefix">On 3/30/22 13:32, Sanjay Bhatnagar
wrote:<br>
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<blockquote type="cite"
cite="mid:f3fbdd24-9dbe-467d-a0be-677bef8379c6@email.android.com">
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<div dir="auto">George:</div>
<div dir="auto"><br>
</div>
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).
<div dir="auto"><br>
</div>
<div dir="auto">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 </div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Mar 30, 2022 12:57 PM, George
Moellenbrock via evlatests <a class="moz-txt-link-rfc2396E" href="mailto:evlatests@listmgr.nrao.edu"><evlatests@listmgr.nrao.edu></a>
wrote:<br type="attribution">
<blockquote class="quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div>
<p>Sanjay-</p>
<p>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.<br>
</p>
<p>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 <i>rotating</i>,
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). <br>
</p>
<p>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 <i>peculiar</i>
feed rotation calibration explicitly <i>anywhere</i>**,
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. <br>
</p>
<p>(**is the correlator at all aware of the pointing
model? for reasons<i> other than </i>net path length,
if even that?)<br>
</p>
<p>Cheers,</p>
<p>George</p>
<p><br>
</p>
<p><br>
</p>
<p><br>
</p>
<div>On 3/30/22 11:42, Sanjay Bhatnagar via evlatests
wrote:<br>
</div>
<blockquote>
<p>A simpler way to achieve the same would be:</p>
<p>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.</p>
<p>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.<br>
</p>
<p>sanjay<br>
</p>
<div>On 3/30/22 10:47 AM, Steven Myers via evlatests
wrote:<br>
</div>
<blockquote> 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?<br>
<div><br>
</div>
<br>
<br>
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