Polarized Tsys vs Azimuth using cband receiver.

june, 2002

     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:
date az step cal onoff 
za samples
at constant az
07jun02 am 60 degrees (90-450) 2 1.5->5.5->1.5 (1 deg steps)
07jun02 pm 30 degrees (300-450) 5 1.5->5.5 (1 deg steps)
10jun02 am 30 degrees (90-330) 5 1.5->5.5 (1 deg steps)

The figures 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 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:

  • 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.
  • processing: x101/020707/cball.pro