[evlatests] Another Mystery Problem...
rperley
rperley at nrao.edu
Fri Jul 15 16:49:45 EDT 2022
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
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