[evlatests] Update on Strange R-L phase behavior

[Rick Perley]

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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