[evlatests] Update on Strange R-L phase behavior

[George Moellenbrock]

Regarding geometrical explanations, some more musings...

* 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 
/absolutely/?   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 /is driven /to track a point on 
the sky.)

* 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.)

* 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 /some/ 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)...

-George


On 3/30/22 10:26, Rick Perley via evlatests wrote:
> An update, and a suggestion...
>
> Eric cleared up some AIPS software problems, and I can now more 
> quickly and confidently make various plots.
>
> Attached is my best and clearest example of what is going on: This is 
> a plot of the R-L phase (NOT RL phase) for four carefully chosen 
> antennas -- ea01 , ea05, ea06, and ea22, at C-band, as a function of 
> elevation.
>
> The reference antenna is ea09 -- chosen because when using this one as 
> reference, the great majority of the other antennas display the 
> cleanest 'even' signature (when plotted versus time or HA). Using one 
> of these displayed antennas as reference would merely subtract what 
> you see from all the others, so that ea09 (for example) would show the 
> same effect, but with the phase declining with elevation.
>
> Key points are:
>
> 1) The four different sources (color coded) all follow the same curve 
> very closely, arguing strongly that the underlying cause is a function 
> of elevation, and not HA or parallactic angle.  (When plotting the 
> data against these, much messier plots are generated).
>
> 2) The same plots are seen at every other spectral window within this 
> band,(!!)  and in every spectral window at L and S bands. (!!!).  Not 
> only the same shape, but the same magnitude.  (!!!!) The effect (as 
> seen by these antennas, using ea09 as reference) is solely a function 
> of elevation, and is independent of observing frequency.
>
> 3) I've 'cherry-picked' the antennas to show.  About half the 
> remaining antennas show the same relation as those shown here, but not 
> as tightly as shown in these.  It's clear that the reason is that 
> there is an 'odd' factor which causes a different (R-L) phase 
> difference between the east and west sides of transit.  And for a few 
> antennas, other factors, unrelated to 'odd' or 'even' symmetries have 
> caused large phase differences.
>
> 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.
>
> So -- what to do next to isolate the cause(s)?
>
> 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.
>
> 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.
>
> 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 ...
>
> 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).
>
> Rick
>
>
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