Polarized Tsys vs Azimuth using cband receiver.
The system temperature versus azimuth and zenith
angle was measured using the cband receiver at 5000 Mhz on 07jun02 8-9am,
07jun02 2:3 pm, and 10jun02 8-9am. The telescope would sit at an az, za
position, a number of 2 second cal on/offs were done, and then it was moved
to a new position. The sky would drift through the beam during the cal
on,offs which caused the system temperature to change as sources drifted
through the beam. The sampling was:
at constant az
||60 degrees (90-450)
||1.5->5.5->1.5 (1 deg steps)
||30 degrees (300-450)
||1.5->5.5 (1 deg steps)
||30 degrees (90-330)
||1.5->5.5 (1 deg steps)
show the variation of Tsys and TsysA-TsysB with the azimuth motion.
Fig 1-3 top: This is Tsys for polA (black) and polB (red) plotted versus
sample number. The blue vertical lines separate the constant azimuth position
with the za motion moving between 1.5->5.5 degrees. The bumps and dips
are sources drifting through the beam.
Fig 1-3 center: This is the polarized power (TsysA-TsysB) (to the accuracy
of the cal values). You no longer see the sources so they must not have
been polarized. There is a variation of polarized power with az.
Fig 4. This plots the difference TsysA-TsysB versus azimuth for all of
the data. The yellow curve is a sine wave fit. The amplitude of the sine
wave is .5 K, it has a period of 180 degrees, and the maximum aligns with
278 degrees (8 degrees north of west). The Stokes calibration measured
the probe rotation angle to be -1.23 degrees so this would move the 278
->277 (closer to east,west).
The opening in the bottom of the bowl is oriented east/west. There is currently
3 25 by 4 foot panels that are not installed so we can have access to the
theodolite at A09 (although it seems a little hard to get polarized power
out of this).
The platform triangle, cables, tiedown cables have a 120 degree symmetry
which does not fit with the 180,360 symmetry measured here.
The support cables under the dish run north south and in the past there
was a scalloping between these cables that caused a grating lobe. After
the latest adjustments i'm not sure how large this affect is.
The sun was up during these measurements and it moves east, west. The azimuth
dependence remains the same for the sun on the horizon and after transit.
It's not evident how scattering off of the structure would give the 180
deg period. The azimuth arm is ruled out since it moves with the feed.
The polarized power is fixed relative to the
platform or ground. The exact angle will depend on the angle that the receiver
probes are set at. Some things that come to mind are: