[evlatests] Strange R-L phase symmetries

[Craig Walker]

This is an interesting puzzle.  Here are a few thoughts on the problem:

The higher dec sources have a very high rate of change of Azimuth and PA 
at transit.  The sharp peak in the R-L phase effect makes me think it is 
related.  The effect at the antennas with the single peak is much larger 
than the effect with the two peaks (one negative).  If all antennas, 
including the reference, have a peak at transit but of random sign and 
with slight and maybe random offsets from actual transit, you might get 
what is seen.  When an antenna's peak is of opposite sign from that of 
the reference antenna, the effects add and you get a single large peak. 
  When they are of the same sign, so they try to cancel in the 
difference, the slight offsets from actual transit give the two peak 
character.

The fact that the effect is scattered randomly over the array (really 
true?) suggests that it is some hardware effect not related to observing 
geometry.  Also it may be important to remember that the pads are tilted 
so that Az, El, and PA are the same at all antennas despite the Earth 
curvature over the array.

My first thought was that this all points to the azimuth cable wrap. 
But the fact that the values far from transit are the same on both sides 
doesn't match this too well.

With the VLBA, we get an amplitude effect that looks a bit like this at 
the point when the source is off the end of a baseline and the fringe 
rate goes through zero.  Then any clipper offsets, pulse cal tones or 
other signals that are the same at the sites correlate.  Could there be 
something in the VLA system of the sort that acts at transit?  That is 
definitely grasping at straws.

Definitely a puzzle.

Cheers,

Craig



On 3/25/22 11:53 AM, Rick Perley via evlatests wrote:
>      This is a long circular -- apologies to all, but the subject is a 
> bit complex ...
> 
>      Many will remember a meeting called by Frank a few years ago where 
> the subject was the very peculiar phase differences seen between the RCP 
> and LCP phases when observing a source passing by the zenith.  The 
> general conclusion was that 'we have no idea of what is going on'.
> 
>      In preparation for an upcoming trip, I am reviewing my extensive 
> observations, taken over the past decade or more, from projects with the 
> goal of measuring, and implementing the 'absolute' D-terms.  (In other 
> words, dispensing with the usual method of measuring the antenna 
> polarizations with respect to an assumed standard (usually zero)).
> 
>      One observation, taken in January 2019, is especially well suited 
> to this task.  I observed four sources, through transit, for five hours, 
> at three bands -- L, S, and C.
> 
>      The four sources were:
> 
>      3C286   dec = 30.5
> 
>      OQ208  dec = 28.5
> 
>      3C287    dec = 25.2
> 
>      3C273    dec = 2.0
> 
>      Note that OQ208 is completely unpolarized, while the others have 
> varying degrees of polarization.  All sources transit south of the zenith.
> 
>      The data are of exceptionally good quality.  The array was in the C 
> configuration.
> 
>      The attached plots show the R-L phases, using ea10 as the reference 
> antenna.  Note that these are *not* the RL or LR correlation phases -- 
> they are the differences between the antenna phase solutions using the 
> RR and LL data, using ea10 as the reference.  This means the R-L 
> dependence of ea10 is impressed on all the other antennas.  We are 
> looking at differentials.
> 
>      The plots show two antennas -- ea01 and ea12, which represent the 
> two different symmetries seen in the data.  The x-axis is HA -- plots 
> against time and parallactic angle jumble the results -- the 
> dependencies seen are purely a function of HA.
> 
>      Colors:  3C286 is red, Light green is OQ208, blue is 3C287, dark 
> green is 3C273.
> 
>      ea01 is of the even symmetry type.  Antennas 1 3 5 6 8 15 and 22 
> have this symmetry.
> 
>      ea12 is of the odd symmetry type.  All other antennas show this, 
> with the same sign -- positive difference before transit, negative 
> difference after, with the possible exception of ea18. (For this 
> antenna, the amplitude of the effect is very small, so the signature is 
> hard to discern).  Three antennas were out of the array at the time:  7, 
> 24 and 28.
> 
>      Key points:
> 
>      1) The phase signatures are *identical* for each band.  Same width, 
> same height, same values, same symmetry.
> 
>      2) The magnitude of the effect is sharply dependent on how close 
> the zenith the source transits.  For 3C273, the effect is almost 
> completely absent.
> 
>      3) The effect is independent of source polarization.  OQ 208 has 
> less than 0.1% polarization, and shows the same symmetry signature as 
> the strongly polarized sources 3C286 and 3C287.
> 
>      4) The location of the antennas is not related to the signature -- 
> the 'even' antennas were located all over the array: W6, W18, E14, N6, 
> N1, E12, and W12.
> 
>      One conclusion is clear:  The effect has nothing to do with the 
> beam squint.  And it is very hard to see how differences in the antenna 
> pole direction can do this -- the required tilt magnitudes are just 
> unreasonable.  And in any event, the parallactic angle is not a function 
> of polarization -- it's an antenna quantity.
> 
>      I have shown these data to two of our serious pundits (Barry and 
> Steve), hoping for some insight.  None was forthcoming.  We are 
> completely stumped.  It seems clear that the signatures are geometric in 
> origin -- but how does this translate into such a clear signature in the 
> phase *difference* between polarizations?
> 
>      Any and all suggestions will be taken seriously!
> 
>      Rick
> 
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     R. Craig Walker            Scientist Emeritus
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     Socorro NM  87801  USA     National Radio Astronomy Observatory
     cwalker at nrao.edu           P.O. Box O
     Phone  575 835 3972        Socorro, NM 87801  USA
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