[evlatests] Another Mystery Problem... {External}
rperley
rperley at nrao.edu
Sun Jul 17 16:21:57 EDT 2022
I guess I don't understand how any effect related to fringe stopping
could cause a progressive rotation of the polarized plane of emission
which increases with HA and image offset.
And with regard to beam polarization effects -- in the first place, the
observed offset in EPA is frequency independent, while the beam
polarization, to at least first order, will be much stronger at higher
frequencies than at low. And in second place -- the beam polarization
should be nearly independent of HA, while our observed effect certain is
not!
Rick
On 2022-07-17 09:24, Benjamin Hugo wrote:
> Hi Steven
>
> We could certainly try this, but the HA clock (first order rotation on
> the
> plane astrometry) has been fixed quite a while ago. The astrometry
> these
> days don't show even arcsecond offsets as a function of radial
> distances
> (aside from global offsets still seen on some of the older data due to
> gain
> calibrator coordinate errors / sub-arcsecond tropospheric errors).
>
> However, we could still re-fringestop the short spacings. I will try
> this
> coming week. Due to the cadence of the samples we will not be able to
> do
> this fast enough for the long spacings not to induce large smearing
> errors
> at the edge of the moon.
>
> It may also have something to do with the rotating leakage patterns of
> the
> beam (which rapidly deteriorates from 20'-30' onwards). We can
> certainly
> expect >~10% level leakage contributions at this point. At least as is
> shown in L-band.
>
>
>
>
>
> On Fri, Jul 15, 2022 at 11:09 PM Steven Myers <smyers at nrao.edu> wrote:
>
>> Some off-the-top-of-my-head thoughts.
>>
>> Looks like the EVPA changes are about half the HA changes suggesting
>> it
>> may be related to how the hour angle clocks. Maybe is the uv-plane
>> rotation
>> sign thing? This might explain why point sources at phase center dont
>> show
>> this. Might also be amusing to shift the phase center when making
>> these
>> (and the point source) images to see if something changes.
>>
>> -S
>>
>> > On Jul 15, 2022, at 2:49 PM, rperley via evlatests <
>> evlatests at listmgr.nrao.edu> wrote:
>> >
>> > Caveat Emptor! This email has nothing to do with the EVLA, or even with
>> any NRAO instrument. It deals with a problem we are having with lunar
>> observations taken with the MeerKAT radio telescope. I'm giving this
>> long
>> summary here in hopes that somebody out there can explain what is
>> going
>> wrong.
>> >
>> > As many are aware, Bryan, Eric and I have for some time been using
>> observations of planetary bodies to establish the correct position
>> angle of
>> the plane of polarization of 3C286. VLA observations of Mars have
>> been
>> taken from X through Q bands (and resulted in a 2013 publication).
>> More
>> recently, we have extended VLA the observations downwards to L, S and
>> C
>> bands, using Venus and the moon as targets. All the results indicate
>> a
>> slight (few degrees) reduction in the EPA of 3C286 as frequencies
>> decline
>> below 5 GHz.
>> >
>> > In conjunction with Ben Hugo and Oleg Smirnov, we have very recently
>> obtained considerable MeerKAT data, to extend the effort down to 580
>> Mhz
>> (and also as a teaching tool for learning how to implement and use
>> data
>> from linearly polarized systems). Observations of the moon were taken
>> at
>> their UHF, L, and S bands. Due to the particular choice in HA
>> coverage, we
>> asked for, and received, additional observations of 3C286 through
>> transit
>> (+/- 2 hours from transit -- this is all they can get) at these three
>> bands
>> -- these were need to sort out some orientation and phasing issues and
>> to
>> allow proper polarimetric calibration.
>> >
>> > The results of this calibration (all done in AIPS) are very good *for
>> the sidereal calibration sources*. Lovely polarimetric images of the
>> large
>> fields surrounding the lunar calibrator sources (J1733-1304 at UHF,
>> J1833-2103 at L, and J2052-3640 at S) have been made. In particular,
>> the
>> polarized galactic foreground shows up beautifully for the first two
>> sources, listed above. These, plus images of the 3C286 data convince
>> me
>> that we've got the polarization calibration and imaging of these
>> sources
>> correct.
>> >
>> > But, this is not true for the images of the moon taken with the MeerKAT
>> data. The imaging shows a progressive rotation of the apparent plane
>> of
>> polarization, increasing with increasing observation hour angle. At
>> HA =
>> 0, there is no significant offset in the observed EPA. AS the HA
>> increases
>> (or decreases), and increasingly large offset is seen -- reaching -32
>> degrees (!) at the largest HA used (-4.5 hours).
>> >
>> > This is not a rotation on the sky. It is an apparent rotation of the
>> parallactic angle (orientation of the antenna w.r.t. the celestial
>> frame).
>> Yet it cannot due to an error in the calculation of the parallactic
>> angle
>> of the moon. Even if the parallactic angle used in the imaging were
>> wrong,
>> the error is nowhere large enough to explain what we see.
>> >
>> > *Key Facts*:
>> >
>> > 1) This only affects MeerKAT observations of the Moon. VLA and DRAO
>> observations of the moon do not see this. MeerKAT observations of
>> sidereal
>> objects show no such effects.
>> > 2) It is seen in both CASA and AIPS, identically.
>> > 3) It is independent of band -- all three bands give identical offsets.
>> > 4) It is not related to the actual parallactic angle. (See the
>> following table).
>> > 5) It is dependent on the HA of the observations.
>> > 6) We cannot test any dependency on declination, as all three
>> observations were taken with the moon at nearly the same declination
>> (-23.5
>> at S, -25.6 at L, and 20.0 at UHF).
>> >
>> > To illustrate, I attach two images: One is from L-band, and shows the
>> apparent EPA at HA = -5. The other is from the UHF band, taken when
>> HA =
>> 0. The orientations are correct. (The poorer I image is due to it
>> being a
>> single snapshot. The polarized emission in the center is from
>> reflected
>> terrestrial emission).
>> >
>> > Below is a short table showing how the offset depends on the HA. The
>> offset was determined with the AIPS program 'MARSP' -- with the very
>> high
>> SNR and large image, the values are (statistically) very accurate --
>> to
>> about 0.1 degree.
>> >
>> > Band HA(deg) Par Ang Observed Offset (deg)
>> > ---------------------------------------------------------------
>> > UHF +9 144 +3.2
>> > UHF -1 -173 -1.0
>> > UHF -10 -140 -2.6
>> > UHF -17 -128 -10.2
>> > UHF -20 -125 -10.5
>> > UHF -26 -120 -14.1
>> > L -29 -108 -14.7
>> > UHF -30 -116 -17.3
>> > UHF -35 -114 -19.3
>> > UHF -40 -114 -21.8
>> > UHF -44 -114 -23.1
>> > L -46 -114 -22.6
>> > UHF -47 -114 -24.4
>> > L -58 -114 -26.9
>> > L -69 -114 -30.2
>> > L -78 -114 -32.5
>> > ----------------------------------------------------------
>> >
>> > The near-constancy of the ParAng for HAs from -1 to -5 is a consequence
>> of the telescope latitude (-30) and lunar declination (-23). The VLA
>> has
>> the same situation for observations of a source near +20 dec.
>> >
>> > Although the relation between the EPA deviation and the HA is clearly
>> monotonic, it is not quite linear, with much slow rate of change near
>> zero
>> HA.
>> >
>> > So, the effect is not subtle, and we are completely confident it is not
>> due to any ionospheric rotation. It certainly is not real (i.e., due
>> to
>> the moon itself). The physics of lunar radio emission is well known.
>> Careful looks in to the imaging software show that the ParAng used is
>> correct (or at least close).
>> >
>> > So what is causing this? We (those listed above) are completely
>> stumped. Can anybody offer us the explanation?
>> >
>> > Rick
>> >
>> <Moon-L-HA=-5.png><Moon-UHF-HA=0.png>_______________________________________________
>> > evlatests mailing list
>> > evlatests at listmgr.nrao.edu
>> > https://listmgr.nrao.edu/mailman/listinfo/evlatests
>>
>>
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