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<p>This would need to be taken with considerable oversampling. I
have such data, taken quite a long time ago ... Not sure I can
get to it right now (but would happily offer it to any volunteer
...)<br>
</p>
<p>Rick<br>
</p>
<div class="moz-cite-prefix">On 3/30/22 14:02, Steven Myers via
evlatests wrote:<br>
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<blockquote type="cite"
cite="mid:C671F0FF-2EBB-471F-8448-27433327983F@nrao.edu">
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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.<br class="">
<div><br class="">
<blockquote type="cite" class="">
<div class="">On Mar 30, 2022, at 1:32 PM, Sanjay Bhatnagar
via evlatests <<a
href="mailto:evlatests@listmgr.nrao.edu"
class="moz-txt-link-freetext" moz-do-not-send="true">evlatests@listmgr.nrao.edu</a>>
wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<div dir="auto" class="">
<div dir="auto" class="">George:</div>
<div dir="auto" class=""><br class="">
</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" class=""><br class="">
</div>
<div dir="auto" class="">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 class="">
<div class="gmail_quote">On Mar 30, 2022 12:57 PM, George
Moellenbrock via evlatests <<a
href="mailto:evlatests@listmgr.nrao.edu"
class="moz-txt-link-freetext" moz-do-not-send="true">evlatests@listmgr.nrao.edu</a>>
wrote:<br type="attribution" class="">
<blockquote class="quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div class="">
<p class="">Sanjay-</p>
<p class="">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 class="">
</p>
<p class="">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 class="">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 class="">
</p>
<p class="">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 class="">peculiar</i> feed
rotation calibration explicitly <i class="">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
class="">
</p>
<p class="">(**is the correlator at all aware of the
pointing model? for reasons<i class=""> other
than </i>net path length, if even that?)<br
class="">
</p>
<p class="">Cheers,</p>
<p class="">George</p>
<p class=""><br class="">
</p>
<p class=""><br class="">
</p>
<p class=""><br class="">
</p>
<div class="">On 3/30/22 11:42, Sanjay Bhatnagar via
evlatests wrote:<br class="">
</div>
<blockquote class="">
<p class="">A simpler way to achieve the same
would be:</p>
<p class="">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 class="">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 class="">
</p>
<p class="">sanjay<br class="">
</p>
<div class="">On 3/30/22 10:47 AM, Steven Myers
via evlatests wrote:<br class="">
</div>
<blockquote class=""> 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
class="">
<div class=""><br class="">
</div>
<br class="">
<br class="">
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