[evlatests] Another Mystery Problem... {External}
Benjamin Hugo
bhugo at sarao.ac.za
Sun Jul 17 09:24:25 EDT 2022
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
>
>
--
*Benjamin Hugo*
Jnr Software Developer
Skype: benna.cn
Ph: [+27] 071 629 3858
South African Radio Astronomy Observatory (SARAO)
Tel: *+27 21 506 7300*
2 Fir Street, Black River Park (North Gate entrance)
Observatory, Cape Town, 7925
Website: www.sarao.ac.za
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