<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="">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><br class=""><blockquote type="cite" class=""><div class="">On Mar 30, 2022, at 10:39 AM, George Moellenbrock via evlatests <<a href="mailto:evlatests@listmgr.nrao.edu" class="">evlatests@listmgr.nrao.edu</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">
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<div class=""><p class="">Regarding geometrical explanations, some more musings...</p><p class="">* An important objection to my suggestion that antenna-dependent
collimation offsets might be pushing the effective pointing of
each antenna's primary away from the target source (such that the
drives effectively rotate them for a direction different from that
of the source, and differentially among antennas) is that we might
have expected this to show a more interesting band-dependence.
But what if the whole feed cabin, as a unit, were not quite
centered properly at the vertex (probably by rotation, not
translation, about the primary's focus point), such that all of
the feeds nominally point to a convergence point not on the
symmetry axis of the primary (i.e., the boresight line). In this
case, I think all of the feeds (per antenna) would suffer the same
collimation rotation (w.r.t. the primary's axis) relative to
nominal, and thus force the same net boresight pointing offset.
Would it be interesting to compare the scale of inter-antenna
collimation offsets with the scale of (per-antenna) inter-band
collimation offsets? If the latter are smaller than the former,
then a common (per antenna) collimation offset could be relevant.
Do we have any idea what the (band-independent) per antenna
collimation offsets are <i class="">absolutely</i>? I.e., how the feed
cabin is oriented w.r.t. the primary? What is the spec on
placement of the whole EVLA feed cabin (I seem to recall it was a
tight fit....)? I don't know how this is easily pinned down now
if degrees of freedom in pointing will compensate for it.
(Remember, what we are seeing are net differential effects between
antennas, in their peculiar departures from the simple spherical
geometry that governs how Az/El motion <i class="">is driven </i>to track
a point on the sky.)<br class="">
</p><p class="">* I think maybe displacement of the subrefector mount (w.r.t. the
primary's symmetry axis), so as to similarly asymmetrize the
primary optics (and possibly done deliberately to accommodate the
feed cabin positioning?), could also do something like this, but
again one must think carefully about what band-dependence should
be expected. (I haven't managed to conclude anything on that.)<br class="">
</p><p class="">* And what about perpendicularity of the elevation axes w.r.t.
the Az axis? For a reasonably correct Az axis orientation
(vertical w.r.t. array center location coords), I think the Az
rotation compensation required near the zenith to keep up with the
source would yield the even symmetry effect. I.e., at transit, (I
think) a tilt in the elevation axis in a vertical E-W plane would
look like a longitude offset in the antenna position (which causes
even). Is the effect correlated with the elevation axes'
perpendicularity? This strikes me as a promising possibility if
the even symmetry is the dominant one. Residuals to this are then
due to Az tilt (which must have <i class="">some</i> effect) and other
similar things at lower levels, including the hysteresis evident
in most of Rick's vs-Elevation plots of antennas with large even
symmetry effects (i.e., falling el actually doesn't quite match
rising el)...</p><p class="">-George</p></div></div></blockquote></div><br class=""></body></html>