[evlatests] Strange R-L phase symmetries
Paul Demorest
pdemores at nrao.edu
Fri Mar 25 16:14:04 EDT 2022
Yes, I was also going to suggest maybe it's a cable being
stretched/flexed, for reasons similar to what Barry mentioned. In
addition to the AC/BD comparison, a few more ideas:
1. Look at the noise diode R-L phase vs elevation to see if the effect
can be seen there. If so I think that pretty definitively rules out
optics-based causes. Also, this is a single-antenna measurement so
removes reference antenna confusion. And it does not require
astronomical sources so can be done any time.
2. Repeat the test at X and Ku bands. L/S/C all share a very similar
signal path so the results at some higher bands might be a informative
comparison.
-Paul
On 2022-03-25 13:43, Barry Clark via evlatests wrote:
> In previous looks at this phenomenon we have been looking for clever
> and sophisticated problems. These data look more like stupid
> problems. The fact that all bands look the same suggests that the
> problem does not arise in the optics or front ends. The main stupid
> thing in the rest of the system is the software. I am inclined to
> discount it on the grounds that in the relevant phase calculations the
> software does not know what polarization it is dealing with. But of
> course one never knows about software. The other remaining stupid
> items are pieces of wire. In particular, it might be the pieces of
> wire connecting the L101 or L102 synthesizers to their respective
> mixers. These ought to be well behaved at this level (couple of ps),
> but pieces of wire can be almost as treacherous as software.
> Something to look at is whether IFs BD behave the same as AC -
> different L102s.
>
> On 3/25/22 11:53, 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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