[evlatests] Update on Strange R-L phase behavior

[Barry Clark]

I think a couple of quick tests on the existing data are worthwhile, 
despite a low probability of showing anything useful.

1.  Make sure the effect shows up on a single baseline, data 
uncalibrated, from, say, baseline 1-9, just to positively exonerate all 
the calibration software.  There's probably enough SNR to do this on a 
single 2 MHz channel even.

2.  Look at the self cross correlations to make sure that the effect, 
for some mysterious reason, chose to vanish when Paul did his definitive 
measurement.

3.  Instead of calibrating each subband, transfer calibrations from one 
subband to the others.  This may tell us if the effect is a phase offset 
or a delay offset.


On 3/30/2022 10:26 AM, 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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