[evlatests] Strange R-L phase symmetries

Mon Mar 28 10:57:00 EDT 2022

Here is a question on the thermal origin: I don't understand why a temperature effect should show any symmetry (even or odd) with hour angle for an "arbitrary (in relation to solar)" target in the sky? 

On 3/28/22, 10:25 AM, "evlatests on behalf of Barry Clark via evlatests" <evlatests-bounces at listmgr.nrao.edu on behalf of evlatests at listmgr.nrao.edu> wrote:

    I haven't had so much fun in years.  We should all thank Rick for 
    providing such grand entertainment.  My candidate for first prize is 
    Paul's suggestion of observing again recording the self cross 
    correlations.  My candidate for second prize is Rob's suggestion that 
    the effect may be thermal rather than flextural - differing thermal lag 
    times can generate patterns that look like derivatives of other patterns.

    On 3/27/2022 9:25 PM, Rick Perley via evlatests wrote:
    > Well — I certainly didn’t think I’d get so many suggestions!  A healthy sign.
    >
    > Regarding AC/BD:  Sadly, the data taken used only the AC side.
    >
    > The thinking seems to point to the antenna, rather than some geometrical origin.  To separate these effects, perhaps tracking 3C286 through transit in two different ways may help — (a) in the normal mode, and (b) using ‘over the top’.  If the effect is due to geometry (related to parallactic angle), these two should give the same results.  If due to the antenna, the different elevations (86 and 94 degrees at transit) should clearly show up as giving different magnitudes.
    >
    > I agree that software is unlikely — but to be sure, I can generate these plots with no calibration at all (since these are differential plots, the atmosphere and most electronics effects should cancel out).
    >
    > I’ll plot these phase differences against elevation — if a true elevation effect, all traces should lie on the same curve.  (I should have done that on Friday!).
    >
    > Regarding the choice of reference antenna — ea10 looks ‘reasonable’.  I will use a different antenna as reference (clearly, one of the ‘odd’ ones) — but the results are easy to anticipate — the current plots will have the new reference antennas’s curve added.  So I can hope that all (or most) of the ‘odd’ profiles will head to ‘zero’ (no elevation/HA effect), while the ‘even’ profiles will change in a way that I hesitate to predict … (depends on the magnitude of the ‘odd’ profile being added to the large ‘even’ profile).
    >
    > I probably won’t be able to do these checks until Monday afternoon.
    >
    > Rick
    >
    >
    >
    > Sent from my iPad
    >
    >> On Mar 25, 2022, at 10:23 PM, Craig Walker <cwalker at nrao.edu> wrote:
    >>
    >> 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
    >>     1305 Vista Dr.             Array Operations Center
    >>     Socorro NM  87801  USA     National Radio Astronomy Observatory
    >>     cwalker at nrao.edu           P.O. Box O
    >>     Phone  575 835 3972        Socorro, NM 87801  USA
    >>                                575 835 7247
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