System temperature versus turret angle offset.

14apr02

see also Tsys vs turpos before/after lbn focus change(22aug02)
  The system temperature is a function of the turret offset from the nominal position above the tertiary.  It was measured moving the turret from turret position 20 degrees through 340 degrees in 5 degree steps firing the cal at each position. A sine wave of amplitude 20 turret degrees was then done about the nominal turret position with a period of 100 seconds (sampling once a second). These measurement were done in two separate epochs:
 
dec00: 20->340 deg  lbw(1405),lbn(1405),sbn(2380),cb(4500)
feb02: 20->340 deg sbw(2380),xb(8800)
mar02:20->340 deg sbh(3500)
dec00: 20 deg Sine Wave lbw(1405),lbn(1404),sbn(2380),cb(4500)
feb02: 20Deg Sine Wave lbw(1405),lbn(1405),sbn(2380),sbw(2380)
cb(4500),cb(5600),xb(8500),xb(9000),xb(9700)
mar02:20deg sin wave sbh(3500)
The plots show the results of the measurements. When looking at the plots, a turret degree is about .75 arc minutes on the sky.
  1. Fig 1 Top shows the Tsys versus absolute turret position. The solid lines are polA and the dashed lines are polB. The vertical lines are the feed positions for each receiver (that were thought to be the correct positions when the data was taken).
  2. Fig 1 Bottom has aligned each plot about the nominal turret position. At 0 degrees offset the horn is looking directly at the tertiary/secondary/sky. As it move away from the nominal position, the horn illumination pattern spills over the edge of the tertiary and starts to see the radiation bouncing around inside the dome. At 180 degrees offset the horn is positioned 4 feet above a metal plate (the service platform). The jumps in sbw may have been rfi.
  3. Fig 2 Top is the 20 degree sine wave The vertical scale is in kelvins (determined by the cal). Pol A and pol B have been averaged together. On both dates lbw, lbn, and sbw do not show any bumps as 0 degrees turret offset. Sbn, sbh,cband, and xband show bumps at turret offset 0 of 1 to 2  degK. This is surprising since turret offset 0 should be looking at the sky which is the coldest thing around.
  4. Fig 2 Bottom subtracts the minimum Tsys from each frequency and then plots it (with an offset) to blowup the bumps in the center. The 2 periods of the sine wave have been averaged together.
  5. Fig 3 fits a gaussian (plus offset, linear term, and quadratic term) to the sine waves for each receiver. The dashed lines are the fits. There is an offset for display purposes.
  6. Fig 4 top plots the amplitude of the fit versus frequency (in kelvins). The accuracy of this data depends on the cal values.
  7. Fig 4 center is the offset of the bump from the nominal turret position (used for the sine wave) in arc seconds. Sbw and cband seem to be close to the center while sbh and xband are off by 83 and -45 asecs. After this data was taken, the xband and sbh horns were surveyed into position with a theodolite. The red line shows the offset of the bump from the "correct" horn postion. For the 7 measurements, the mean offset is 2 asecs with an rms of 3 asecs. So the Tsys bump is centered on the optical axis.
  8. Fig 4 bottom plots the full width half max in arc minutes for the gauss fit to each bump. The width decreases linearly with increasing frequency.
From the measurements we cans say a few things about the bump.
  1. It's existence  is not a function of frequency. Sband narrow has the bump and sband wide does not have it (both at 2380 Mhz).
  2. The bump is centered on the paraxial ray of the optics.
  3. The width of the bump appears to decrease with increasing frequency.
  4. Looking at the focus curve data, the receivers that focus low (lbw,sbw,lbn) do not have a bump while those that focus higher (sbh,cband,xband) do have the bump. It may be that by defocussing the horn on the tertiary the minimums of the bump are getting filled in.
The type of horns and edge taper for the receivers are:
abbreviations: corHorn=corrugated horn,kildal=kildal designed feed,SSAMP=Stepped Septum Atlantic Microwave Polariazer,QROMT=quadRidgeOmt
 
receiver Bump horn edge Taper isolator polarizer turPosDeg
lbw no kildal ? no lin/QROMT
lbn no kildal ? yes cir/turnstile
sbw no kildal ? ? lin/QROMT
sbn yes corHorn ? no cir/turnstile 76.83
cband yes kildal ? yes cir/SSAMP 206.65
xband yes corHorn ? yes cir/SSAMP 162.34
Looking at the table, there is no single parameter that correlates with the bumps (I need to get the edge tapers for the various feeds).
    The focus curves for the various receivers do correlate with the bump. The sbn,sbh,xband and cband receivers focus higher than the lbw,lbn,sbw receivers. 
processing: x101/tur/tursw/turswproc.pro, turswplt.pro
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