[evlatests] Update on Strange R-L phase behavior

[Steven Myers]

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?

> On Mar 30, 2022, at 10:39 AM, George Moellenbrock via evlatests <evlatests at listmgr.nrao.edu> wrote:
> 
> 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
> 

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