<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="">Interesting.<div class=""><br class=""></div><div class="">Would there be a chromatic effect over the band(s) as the pointing offset would place these in different places of the beam? Of is the dominated by the angle structure?</div><div class=""><br class=""></div><div class="">Did the data have reference pointing applied to some bands? Is there a difference there?<br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Mar 30, 2022, at 11:04 AM, 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="">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" class="">
<div class=""><p class="">I have been thinking about this issue from geometric/optics point
of view, and was waiting for the investigations for the source
being in the electronics/temp. variations etc. to settle down.</p><p class="">"over the top" tests would be quite useful, I agree. From what I
can make out, none of the on-sky tests so far included correction
for antenna pointing offsets. If so, I suggest that in the next
set of tests we include pointing correction. <br class="">
</p><p class="">With antenna pointing offsets, which can be significant,
celestial sources trace a curve about the nominal optical axis.
Such a trace (by hand -- so not accurate but good enough) on
antenna R- and L-voltage patterns from holographic measurements
show a R-L phase difference of 2-3 deg. which is in the ball-park
of Rick's measurement. Examination of the Rick's plot with and
without correcting for pointing offsets will help determine if
intrinsic R-L phase in the antenna aperture illumination pattern
could be contributing (partly or entirely) to Rick's R-L phase
plots.</p><p class="">Regards,</p><p class="">sanjay<br class="">
</p>
<div class="moz-cite-prefix">On 3/30/22 10:26 AM, Rick Perley via
evlatests wrote:<br class="">
</div>
<blockquote type="cite" cite="mid:cf6cb9a3-58a8-b589-c92a-688c2fb67199@nrao.edu" class=""> <br class="">
<br class="">
Barry has opined for an antenna-based problem (something within
the electronics which is strongly elevation-sensitive). But, in
an experiment run by Paul two days ago, no elevation-dependency on
the 'auto-cross' phase was seen. (This monitors the phase
difference in the injected noise-diode signal -- and so is not an
astronomical observation). Arguments based on a temperature
effect in my data are hard to sustain, as the outside temperatures
on the night of my observations were exceptionally uniform
throughout the period -- and it was quite breeezy as well. These
results argue for an origin preceding the injection of the noise
diode signal.
<br class="">
<br class="">
So -- what to do next to isolate the cause(s)?
<br class="">
<br class="">
I'd like to try the 'over-the-top' observation. If the effect is
truly due to an elevation-dependent effect within the antenna,
then it should continue to increase as the antenna is tilted 'over
backwards' -- the antenna elevation is then greater than 90
degrees. This should cleanly separate effects due to elevation
from those due to HA or parallactic angle. Observing OTT also
reverses the orientation of the R and L 'squint' beams, so should
be definitive in eliminating that origin.
<br class="">
<br class="">
I suggest we do this with sources which transit both to the north,
and to the south of the zenith. All my current examples are from
sources which transit on the south side.
<br class="">
<br class="">
Doing this with the current 'A' configuration might also
illuminate any dependencies on antenna placement -- despite all
our antennas nominally having parallel azimuth axes, sources will
transit at slightly different times. I don't think this is an
issue -- but who knows? We might be surprised ...
<br class="">
<br class="">
This is a fair investment of time -- a few hours. But I think we
need to do something like this to make any progress in isolating
the origin(s).
<br class="">
<br class="">
Rick
<br class="">
<br class="">
<br class="">
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