[evlatests] Strange R-L phase symmetries
Dave Parker
dparker at nrao.edu
Tue Mar 29 16:33:21 EDT 2022
The power supplies in the racks have external temp/rack temp sensors.
The P301's are in the LO rack, the P302 in the Utility rack.
Dave
On 3/29/2022 2:15 PM, Rick Perley via evlatests wrote:
> I'll note here that the data showing the strange high-elevation phase
> effects were taken between 3 and 8AM MST, on 22 January 2019. The
> outside air temperature was a steady -3.6 C throughout, and it was
> rather breezy all night (about 15mph). The vertex room is far from
> airtight, so we might reasonably conclude that the vertex room air
> temperature was rather constant throughout.
>
> Sadly, so far as I know, there are no vertex room air temperature
> monitors. At high frequencies, the receiver gains are in fact quite
> good proxies for the cabin air temperature (!) -- but, sadly, these
> observations were taken with the low frequency receivers. The C-band
> system is fairly temperature sensitive (but not nearly as much as Ku, K,
> Ka and Q). I checked the switched power for the C-band -- as expected,
> there are no diurnal gain changes visible (to 1%).
>
> I'll also note that if the effect were thermal in origin, it would have
> affected all four sources in my data fairly equally, as I was switching
> quite rapidly amongst them (timescale 20 minutes for a complete cycle).
>
> Rick
>
>
> On 3/29/22 13:57, Barry Clark via evlatests wrote:
>> If the effect is thermal, the time constant is probably pretty long
>> (or you wouldn't get the derivative type effect). Doing the test ten
>> times slower would be more convincing.
>>
>> On 3/29/2022 1:36 PM, Paul Demorest via evlatests wrote:
>>> hi folks,
>>>
>>> Yesterday we ran a test to look for changes of the noise diode
>>> (switched power) R-L phase as a function of pointing direction.
>>> tldr/summary is that no obvious systematic trends are seen, and any
>>> such variation is at the ~0.2 degree level at most (10x smaller than
>>> Rick's variations). This implies Rick's effect is likely happening
>>> "upstream" of the noise cal injection point.
>>>
>>> For the test we pointed the array at dec = 31deg, and hour angle
>>> ranging from -2h to +2h in 20 equal steps. At each location we
>>> recorded 1min of data at S-band with widar binning mode at 10Hz to
>>> separate noise diode on and off states. The maximum elevation was
>>> about 87 degrees.
>>>
>>> A couple example plots for two antennas are attached. The top two
>>> panels show amplitude and phase vs frequency for the "on minus off"
>>> cross-hand autocorrelation data. These are just to show that the
>>> expected signal is indeed detected, and looks good, aside from the
>>> usual RFI at S-band.
>>>
>>> The bottom panel shows residual phase vs time for two relatively
>>> clean subbands, one from each IF (2680 and 3320 MHz), after
>>> subtracting off the average phase vs freq. These are plotted vs scan
>>> number which is proportional to HA (-2h in scan 2, to +2h in scan 20).
>>>
>>> The third attached plot shows the residual phase vs scan for for all
>>> antennas overplotted; top panel is AC, bottom panel is BD. A few ants
>>> show more scatter than others in BD, and many seem to have a spike in
>>> scan 13, which I have not looked at too closely but would guess is
>>> some RFI event. If needed we could repeat this, pretty easy now that
>>> it's all set up.
>>>
>>> Cheers,
>>> Paul
>>>
>>> On 2022-03-28 13:13, George Moellenbrock via evlatests wrote:
>>>> Gang-
>>>>
>>>> I'm going to try to revive interest in a broadened, but still /~purely
>>>> //geometrical explanation/. In particular, I wonder if we appreciate
>>>> the true incongruities in the realized optical geometry as well as we
>>>> think we do, at least as regards the net effective rotation of the
>>>> feeds on the sky near the zenith. Apparently not quite, and more than
>>>> just simple Az axis tilts could be relevant (yet still without going
>>>> behind the feed to wires and software). I.e., where is the /primary/
>>>> boresight actually pointing?
>>>>
>>>> *Regarding Az tilts (for starters): *The properties of the symmetries
>>>> Rick has shown are entirely consistent with (differential) tilts in
>>>> the AZ axes by a few arcmin. E-W tilts cause the even symmetry (time
>>>> offsets in geometry calculation can do this to...). N-S tilts will
>>>> cause the odd symmetry. It was casually asserted early in the
>>>> conversation that the peculiar tilts (due to sag of pads, etc.) aren't
>>>> big enough for this. Is there a real quantitative basis for this
>>>> claim?
>>>>
>>>> The basic geometrical interpretation is really a matter of answering
>>>> the following question: "How is the antenna rotating around the
>>>> direction to the source in general, and especially nearer the
>>>> zenith?" Do we really think we know this near the zenith at levels
>>>> comparable to the scale of tilts required for the observed effect?
>>>> Rick correctly stated "parallactic angle is not a function of
>>>> polarization", but misapprehension of the realized parallactic angle
>>>> evolution (in geometric models used to correlate/calibrate) will have
>>>> opposite phase effect on the R and L polarizations. I.e., any
>>>> effective rotation of the feed on top of the assumed geometric model
>>>> of the system will advance the R phase and retard L's, or vice-versa.
>>>> This is what Rick's plots show, differentially with the refant. So,
>>>> does the correlator phase model include terms for the peculiar tilts
>>>> in each antenna? If not, then the peculiar tilts /must be
>>>> /introducing these effect at some level, i.e., at least some of the
>>>> observed effect is due to the Az axis tilts.
>>>>
>>>> *But, if you need more effect than mere Az tilts can supply, my main
>>>> /new /point is /primary boresight pointing accuracy/:* Beyond the
>>>> simple peculiar tilts, we are actually also assuming that the antennas
>>>> point precisely on their /primaries' /optical boresights toward the
>>>> source. Is this true? The (joint?) optimization of pointing, focus
>>>> and collimation is presumably respectably optimizing /net forward
>>>> sensitivity/ (and somehow averaged between R and L to fall between
>>>> between the squinted beams, which we presume /is/ the primary Stokes I
>>>> boresight). This broad optimization should tend toward, but by no
>>>> means necessarily guarantee, that the pointing /of the primary /(which
>>>> is what is driven by the motors) is precisely toward the source.
>>>> Indeed, is there any way to objectively guarantee (i.e., constrain)
>>>> the primary boresight in the optimizations we perform for the
>>>> optics? In particular, I wonder if the collimation optimization
>>>> does, in fact, push us distinctly (but subtly..., and enough?) /away/
>>>> from the primary's boresight? I.e., to what extent do we optimize
>>>> collimation by moving/pointing the feed horns themselves (at the few
>>>> arcmin level), cf just adjusting the _net_ pointing of the mechanical
>>>> optics (forward of the feed) to ~compensate and balance? A few
>>>> arcmin offset only negligibly affects the primary's forward gain, so
>>>> no leverage there... And it is interesting to note that a lot is
>>>> bootstrapped between and among bands in achieving the general combined
>>>> optimization of all feeds across the sky (i.e., everything pegged to
>>>> whatever uncompensated mechanical offsets exist in the X-band feed?).
>>>> Not to mention the likely relevance of sub-reflector rotation tricks
>>>> (at higher-freq bands) to the net geometry of the optics, which speaks
>>>> to the level at which we need to account for loss of the rigidity
>>>> implicit in the simple geometric description..... In short,
>>>> optimization of the whole signal path is almost certainly not
>>>> optimization of each optical path component in isolation, and in
>>>> particular, */the drive motors are probably effectively moving the
>>>> primary for a point on the sky that is _not_ precisely the target
>>>> source coordinates, such that the net rotation around the direction to
>>>> the target isn't quite the right one/**.* And this will be most
>>>> noticeable nearer the zenith, of course, in the relative R-L phase/,/
>>>> in a manner very like the simple Az tilts. The main drawback to this
>>>> explanation is that we might have expected more band-dependence of the
>>>> effect, unless we are dominated mainly by the bootstrapping from one
>>>> band, or something else systematic (per antenna, not band) about the
>>>> effective boresight directions of (aging) VLA antennas.....
>>>>
>>>> (I pose the above based on some ongoing off-and-on (mostly off,
>>>> lately) experience studying similar questions for ALMA, where, in
>>>> fact, they have /deliberately/ chosen to translate (rather than tilt,
>>>> as designed) the subreflector to reach off-axis feeds (on 2 feed
>>>> circles). This means they are deliberately /moving off the primary
>>>> boresight/. And since we don't really know the subreflector zero
>>>> points in tip/tilt and translation, I don't think we really know how
>>>> far off the boresight ALMA antennas actually are... So, they've
>>>> unintentionally compromised the feed orientation calculation in
>>>> calibration for a (measured, memo'd) very small net loss in forward
>>>> sensitivity.)
>>>>
>>>> *Regarding OTF pointing updates:* Also, I think we expect blind
>>>> pointing to be poor near the zenith, by which I mean we don't expect
>>>> our ordinary optimizations to be very good.... I wonder if the
>>>> amplitudes (in particular, the /relative/ R/L amp) might give a clue
>>>> about how far from the nominal pointing we have wandered (on top of
>>>> the offsets introduced ~deliberately through nominal optimizations
>>>> described above). Also, manually tweaking up the pointing on top of
>>>> the model at, say, HA ~ -1h might actually be an effectively arbitrary
>>>> "correction" to a point decidedly off-source for the primary boresight
>>>> nearer the zenith....
>>>>
>>>> *Regarding measuring cross-hand phase directly: *Examining truly
>>>> measured cross-hand phases will definitely be interesting. Note that
>>>> this will be a measurement relative to the (simple) parallactic angle
>>>> calculation used to make the sky nominally stationary in rotation.
>>>> This calculation does not include all of the inhomogeneities described
>>>> above (true axes tilts, effective optical path offsets), and is also
>>>> subject to the coordinate system chosen for the parang calculation. I
>>>> think both AIPS* and CASA have traditionally used the geocentric
>>>> latitude (not geodetic) for the parang calculation, which effectively
>>>> behaves like a ~10.7 arcmin tilt to the North. This creates a several
>>>> degree position angle error near the zenith (twice this in R-L phase)
>>>> of the odd symmetry, and is conveniently nulled by the difference
>>>> measurements Rick has shown so far (owing to the fact that VLA
>>>> antennas are nominally mounted on the earth in parallel). Indeed, it
>>>> is the scale of this alone that keeps me scratching my head about just
>>>> the ordinary tilts of a few arcmin being enough to cause at least some
>>>> of the effect Rick observes. So, don't be surprised if the actual
>>>> cross-hand phase (effectively, of the refant) looks worse!
>>>>
>>>> (*I'd welcome Eric's correction on this point, if I'm wrong about
>>>> this.)
>>>>
>>>> *Regarding 'over-the-top': *I think over-the-top might ~decouple Az
>>>> tilts from internal (feed-forward) optics, since the net primary
>>>> boresight pointing error is probably different for over-the-top, but I
>>>> haven't thought very carefully about this.... Hmmm, I think net feed
>>>> rotation is in the opposite direction for over-the-top, so I don't
>>>> think you get the same thing for the Az tilt effect--won't it reverse
>>>> the sign of your differential phases? If only a sign reversal, then
>>>> that test tends to point to Az tilt as the culprit. But there are
>>>> probably also different boresight pointing effects, so you'll sorta
>>>> measure the relative scale of those... And bending wires can also
>>>> still contribute....
>>>>
>>>> Cheers,
>>>>
>>>> George
>>>>
>>>>
>>>>
>>>> On 3/27/22 21:25, 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
>>>>>>> _______________________________________________
>>>>>>> evlatests mailing list
>>>>>>> evlatests at listmgr.nrao.edu
>>>>>>> https://listmgr.nrao.edu/mailman/listinfo/evlatests
>>>>>> -- ------------------------------------------------------------------
>>>>>> 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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