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Telescope Performance

Telescope Performance. Pointing the telescope. Gain curves for all receivers Recent high frequency results. Beam widths for all receivers. Things to work on. More info at: http://www.naic.edu/~phil -> pointing -> system performance. Pointing the Telescope.

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Telescope Performance

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  1. Telescope Performance • Pointing the telescope. • Gain curves for all receivers • Recent high frequency results. • Beam widths for all receivers. • Things to work on. • More info at: http://www.naic.edu/~phil • -> pointing • -> system performance NAIC Visiting Committee Meeting February 15-16, 2005

  2. 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. NAIC Visiting Committee Meeting February 15-16, 2005

  3. 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 (450*4 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). NAIC Visiting Committee Meeting February 15-16, 2005

  4. 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. NAIC Visiting Committee Meeting February 15-16, 2005

  5. PL Platform hght vs hour NAIC Visiting Committee Meeting February 15-16, 2005

  6. 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. NAIC Visiting Committee Meeting February 15-16, 2005

  7. 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, za=16.5 az=155. • The td4 jack still had about 8 inches of throw. • The plot shows the pointing error became > 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. NAIC Visiting Committee Meeting February 15-16, 2005

  8. Pnt err when td4 looses tension NAIC Visiting Committee Meeting February 15-16, 2005

  9. Pre 2004 gain Curves NAIC Visiting Committee Meeting February 15-16, 2005

  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 NAIC Visiting Committee Meeting February 15-16, 2005

  11. Recent hi freq measurements • Latest reflector adjustments: • Tertiary: 12oct to 30oct04 • Secondary: 7jun to 29jun04 • Primary: nov04 to jan05 • 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. NAIC Visiting Committee Meeting February 15-16, 2005

  12. PL Gtsb cb5000 NAIC Visiting Committee Meeting February 15-16, 2005

  13. PL Gtsb cbh7200 NAIC Visiting Committee Meeting February 15-16, 2005

  14. PL Gtsb xb9000 NAIC Visiting Committee Meeting February 15-16, 2005

  15. System performance vs freq. • Za range limited to 5-14 deg • 4th order poly fit to data • Table contains median values • Scatter from optics. NAIC Visiting Committee Meeting February 15-16, 2005

  16. 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. NAIC Visiting Committee Meeting February 15-16, 2005

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  21. 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. NAIC Visiting Committee Meeting February 15-16, 2005

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