<html><head><meta http-equiv="Content-Type" content="text/html; charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">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="">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="">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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