[evlatests] Update on Strange R-L phase behavior

[Steven Myers]

Interesting.

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?

Did the data have reference pointing applied to some bands? Is there a difference there?

> On Mar 30, 2022, at 11:04 AM, Sanjay Bhatnagar via evlatests <evlatests at listmgr.nrao.edu> wrote:
> 
> 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.
> 
> "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.  
> 
> 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.
> 
> Regards,
> 
> sanjay
> 
> On 3/30/22 10:26 AM, Rick Perley via evlatests wrote:
>>  
>> 
>> 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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