AUSAC Meeting

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Telescope Performance

• Pointing the telescope.
• Gain curves for all receivers
• Reflector alignment
• Recent high frequency results.
• Beam widths for all receivers.
• Things to work on.
• More info at http//www.naic.edu/phil
• -gt pointing
• -gt system performance

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Pointing the Telescope

• To point the telescope you use
• Pointing model
• Azimuth, za drive systems.
• Turret drive system.
• 6 Distomats (laser rangers)
• Tiedown control system.
• Temperature sensor on the platform.
• Tertiary drive system (installed but not in use).
• Beam widths of receivers range from
• 900 arc seconds 327 MHz
• 28 arc seconds at 10 GHz
• 1/8 inch motion at the horn is 5 arc seconds on
  the sky.
• Make sure that any errors are repeatable.

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The pointing model

• 13 term model in az and za with a za lookup
  table.
• Latest model created may04 after
• Alfa added (2000 lbs)
• New compressors on dome (4504 lbs)
• Kevlar cables added to stiffen feed tower.
• Dome side rollers tightened on 1 side, loosened
  on the other.
• Model history (errors in arc seconds).

Date Az err asec Za err asec Total asec
1998 3.82 2.37 4.49
1999 5.4 3.1 6.2
2000 5.68 3.45 6.65
2003 5.38 3.48 6.41
2004 4.20 2.41 4.84
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The distomats and tiedowns

• The distomats and tiedowns correct for
  temperature variation.
• 6 distomats (around the rim road) are used to
  measure the position of the platform.
• Tiedown cables connect each platform corner to a
  computer controlled jack in the bowl.
• Operation
• The distomats measure the average platform height
  every two minutes.
• The computers then drive the tiedowns so the
  average platform height remains at a fixed value.
• We keep the average height of the platform fixed.
  We do not keep the platform level.
• Plot shows platform height 05feb05.

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PL Platform hght vs hour
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Distomat failure

• The distomats fail if it is raining, or if the
  platform has tilted enough so that the targets
  are no longer in the distomat beam.
• On failure the computer switches to a temperature
  sensor on the platform. The differential
  temperature change from the last good reading is
  then used to control the tiedowns.
• A 5 deg F change will move the platform 1 inch.

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Tiedown failure

• Large platform tilts and higher temperatures
  cause a tiedown cable to go slack.
• On 27may04 tension in td 4 was lost during a
  calibration run
• 10Am, 84degF, za16.5 az155.
• The td4 jack still had about 8 inches of throw.
• The plot shows the pointing error became gt 85 arc
  seconds.
• The problem is magnified because the computer
  continues to move the 3 jacks (but only two of
  them are now moving the platform). This causes
  the platform to tilt even more.
• Day/evening observing is affected by this. Dome
  weight is the main culprit.

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Pnt err when td4 looses tension
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Pre 2004 gain Curves
10
Curvature of the hi freq gain curves

• The curvature in the high frequency gain curves
  comes from collimation and focus errors that are
  a function of az and za.
• A 1 lambda focus error is a 3db loss in gain. At
  10 Ghz 3cm is 3db
• Theodolite survey
• Jul03 after shimming

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2004 Measurement and Alignment Accomplishments

• Reflector alignment
• Tertiary Measured and adjusted Oct. 2004
• Initial RMS .9 mm. Final RMS .3 mm.
• Measurement Std. Dev. 25 microns
• Secondary Initially measured Nov. 2003
• Final measurments and adjustments May/June 2004
• Inital RMS 1.3 mm. Final RMS .6 mm.
• Measurement Std. Dev. lt 50 microns
• Primary Adjustments Nov. 2004 to Feb. 2005
• RMS from last measurement 1.6 to 1.8 mm.
• Measurement Std. Dev. .5 mm
• 11,232 points with error larger than 1.5 mm.
  adjusted
• Estimated setting RMS after adjustments lt 1.5
  mm.

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Tertiary Setting Errors Prior to
AdjustmentsData Taken 21 October 2004
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Tertiary Setting Errors After All Adjustments
Data Taken 26 October 2004
14
Secondary Setting Errors Prior to
AdjustmentsData Taken 11 November 2003

• 93 points out of 1629 targets missing or off scale

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Secondary Setting Errors After All
AdjustmentsData Taken 11 November 2003

• 11 points out of 1629 targets missing or off scale

16
Primary Surface ErrorsPhotos Taken
October/November 2001

• Histogram of 37889 points, 168 points off scale
• RMS of points less than 8mm., 333 points
  dropped 1.6mm.
• RMS of points less than 4 mm., 1393 points
  dropped 1.4mm.
• RMS of points less than 2mm., 7060 points
  dropped 1.0mm.
• RMS of points less than 1.5mm., 11234 points
  dropped .8mm.
• RMS of points less than 1.0mm., 18220 points
  dropped .5mm.

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Recent hi freq measurements

• Data taken 01dec04 thru feb05
• The plots contain
• Gain (K/Jy)
• Tsys (K)
• SEFD (Jy)
• Average beamwidth (Asecs)
• A single freq/rcvr is plotted versus za.
• Color/symbols are different sources.

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PL Gtsb cb5000
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PL Gtsb cbh7200
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PL Gtsb xb9000
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• System performance vs freq.
• Za range limited to 5-14 deg
• 4th order poly fit to data
• Table contains median values
• Scatter from optics.

Rcv Gain K/Jy Tsys K Sefd Jy
Cb4500 8.6 29.0 3.4
Cb4860 8.7 29.9 3.4
Cb5000 8.5 30.6 3.6
Cb5400 8.1 34.5 4.3
Cbh6600 6.0 26.8 4.5
Cbh6900 5.7 25.6 4.6
Cbh7200 5.1 24.1 4.6
Cbh7400 5.0 24.4 4.8
Xb8500 5.0 27.9 5.8
Xb8800 5.0 28.4 5.9
Xb9000 5.0 29.2 6.1
Xb9200 5.0 30.3 6.3
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Beam widths vs. wavelength

• HalfPowerBeamWidth K x lambda/Diameter
• HPBW is not sensitive to Cal errors or Flux
  errors.
• HPBW is sensitive to extended source size,
  focus and surface errors.
• The plots show HPBW measured during 2004 (11600
  points).
• The illumination lets you trade Tsys for Gain.
• Observations
• The dipole feeds have a larger illumination than
  the horns.
• SBN, CBH horns are under illuminating the
  dish/tertiary. This gives a smaller Tsys at the
  expense of the gain.
• The tertiary skirt would let us increase the
  illumination of the tertiary without a large
  increase in Tsys. It would also lower Tsys for
  those receivers that are over illuminating the
  tertiary.

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Things to work on

• The dome weight is causing the tiedown/distomats
  to fail. We need to
• Make the load cell tension measurements more
  robust
• When low tension in 1 tie down, disable tracking
  of average height.
• Get the tertiary drive system online. Use it to
  change the focal length
• Install the tertiary screen so that receivers
  that over illuminate the tertiary do not have
  such a large Tsys (of course this adds weight!!)
• Measure the pitch/roll/focus errors of the
  current configuration and then make a new model
  to try and correct the pitch, roll, focus errors.
• Continue monitoring the system performance of all
  receivers for maintenance purposes.