Intro

    On 12aug22 azimuth swings at low elevation were done to determine the horizon mask for  geodetic vlbi observations.
Some rfi info from the azimuth swings can be found (here).

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Setup:

• The 12meter was set to xband.
• the setupwcalmap routines was used:
• Standard ifsetup xb
• cfr set to 8646  MHz.
• hardware winking cal used (25 Hz).
  
• 7 mock spectrometer were used:
  
• 172.032 MHz bw polA,B recorded
• cfr: 8219 8363 8505 8647 8789 8931 9073
• 142 MHz spacing
• 512 freq channels
• 2millisecond spectral sampling using 16 bit spectra

• The telescope pattern was:
• az swing at 1 deg/sec between -100 and 282 (and then reversing)
• swings were done at
  
• el=6deg CW
• el=10deg CCW
• el=15deg CW
• data taking started at 07:48
  
• each swing took 380 seconds
• we finished at 08:07
• During the observation the sun location was:
• 81 deg az +/-.5 deg
• elevation at start of swing:
• 6 deg swing: sun at 22.3  el
  
• 10 deg swing: sun at 23.7  el
  
• 15 deg swing: sun at 25.3 el
• The tcl command file user/p12m/x101/220812_azswing.tcl was used to take the data.
  

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Processing the data"

• Each azswing was read in with  the idl routine masazswinginp() with the hwc keyword set
• masgethwcal()  was used to  input the file
• input file
• separate out calon,caloff cycles
• average 16ms of each calon, off cycle spectra
  
• compute rms/mean for calOm ,calOff spectra
• linear fit to rms/mean.  flag outliers freq channels as rfi
• compute caldeflection (at 40ms intervals)
• do a 3rd order fit and use to correct total power for  electronic gain variations
• compute total  power (excluding rfi and correcting for electronic gain variation) at 40ms intervals.
  
• use the caloff total  power vs  azimuth for the analysis below.
• interpolate the 1 second azimuth positions the to 40ms total power samples
• select the data between -90 and 270 degrees azimuth
• We end up with 9000 (360/.04) samples  times 7 frequency bands (covering  total of 1 ghz)
  

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Looking at the data:

    The first set of plots show the Tsys vs azimuth for the 3 azimuth swings for each frequency band (.ps) (.pdf)

• Each page is a separate 172 MHz frequency band.
• the colors correspond to the elevations of each azimuth swing
• black: elevation  = 6deg
• red: elevation = 10 deg
• green: elevation = 15 deg
• The top Frame is polA, the bottom frame is polB
• The vertical axis is in deg K
• The purple vertical line is the azimuth of the sun during the observation
• there is no appreciable bump around the sun azimuth
  
• The sun was at an elevation of 22->25 deg during the test.
• The red spike at 260 deg az is rfi since it only shows up in the first freq band.
• All the other bumps, spikes appear in all freq bands so they are continuum radiation.
• The increase in Tsys when going from el = 15 deg to el=6deg is from the atmosphere (more info).

The 2nd plot is a blowup around the azimuth bumps seen on the previous plots (.ps) (.pdf)

•  For each azimuth of a bump, i moved the telescope to that azimuth and then took a photo in the direction the telescope was pointed.
  
• 
  

  frame	centerAz deg	photo  in az direction	notes
  top	-52	.jpg	The photo is centered at the dip in the middle(between the 2 sets of bushes)
  middle	105	.jpg	the photo is pointed at the large bump at 105 deg
  bottom	204	.jpg	The large spike (130 to 220K) is in the direction of tower 12

  
  

SUMMARY:

• 3 azimuth swings were done at elevations of 6,10, and 15 degrees
• 3 directions showed increase of Tsys when the elevation was 6 degrees
• az=-52  bushes   +/- 30 deg
• az= 105   bushes: -15/+55deg
• az=204   Tower 12 +/- 2 deg
• We are going to cut back the bushes so they should not be a problem in the future.
• A horizon mask should include tower 12 at az=204 deg
• A possible mask could be:
• az: 0-202, el =6
• az;202-206:, el=10
• az:206-360: el=6

processing: x101/220822/horazswing.pro