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The Green Bank Telescope: Overview and Antenna Performance
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  1. The Green Bank Telescope:Overview and Antenna Performance Richard Prestage GBT Future Instrumentation Workshop, September 2006

  2. Overview • General GBT overview (10 mins) • GBT antenna performance (20 mins)

  3. GBT Size

  4. GBT optics • 100 x 110 m section of a parent parabola 208 m in diameter • Cantilevered feed arm is at focus of the parent parabola

  5. GBT Capabilities • Extremely powerful, versatile, general purpose single-dish radio telescope. • Large diameter filled aperture provides unique combination of high sensitivity and resolution for point sources plus high surface-brightness sensitivity for faint extended sources. • Offset optics provides an extremely clean beam at all frequencies. • Wide field of view (10’ diameter FOV for Gregorian focus). • Frequency coverage 290 MHz – 50 GHz (now), 115 GHz (future). • Extensive suite of instrumentation including spectral line, continuum, pulsar, high-time resolution, VLBI and radar backends. • Well set up to accept visitor backends (interfacing to existing IF), other options (e,g, visitor receivers) possible with appropriate advance planning and agreement. • (Comparatively) low RFI environment due to location in National Radio Quiet Zone. Allows unique HI and pulsar observations. • Flexible python-based scripting interface allows possibility to develop extremely effective observing strategies (e.g. flexible scanning patterns). • Remote observing available now, dynamic scheduling under development.

  6. Antenna Specifications and Performance

  7. Efficiency and Gain

  8. Azimuth Track Fix • Track will be replaced in the summer of 2007. Goal is to restore the 20 year service life of the components. Work includes: • Replace base plates with higher grade material. • New, thicker wear plates from higher grade material. Stagger joints with base plate joints. • Thickness of the grout will be reduced to keep the telescope at the same level. • Epoxy grout instead of dry-pack grout. • Teflon shim between plates. • Tensioned thru-bolting to replace screws. • Outage April 30 to August 3, followed by one month re-commissioning / shared-risk observing period.

  9. Azimuth Track Fix Old Track Section New Bolts Extend Through Both Plates Transition Section Joints Aligned Vertically – Weak Design Screws close to Wheel Path Experienced Fatigue • New Wear Plates • Better Suited Material • Balanced Joint Design • Joints staggered with • Base Plate Joints New Higher Strength Base Plates

  10. Antenna Pointing, Focus Tracking and Surface Performance

  11. Precision Telescope Control System • Goal of the PTCS project is to deliver 3mm operation. • Includes instrumentation, servos (existing), algorithm and control system design, implementation. • As delivered antenna => 15GHz operation (Fall 2001) • Active surface and initial pointing/focus tracking model => 26GHz operation (Spring 2003) • PTCS project initiated November 2002: • Initial 50GHz operation: Fall 2003 • Routine 50 GHz operation: Spring 2006 • Project largely on hold since Spring 2005, but now fully ramping up again.

  12. Performance Requirements

  13. Summary of Requirements (GHz)

  14. Structural Temperatures

  15. Focus Model Results

  16. Elevation Model Results

  17. Azimuth Blind Pointing

  18. Elevation Blind Pointing

  19. Performance – Tracking Half-power in Azimuth Half-power in Elevation

  20. Power Spectrum Servo resonance 0.28 Hz

  21. Servo Error

  22. Performance – Summary Benign Conditions: (1) Exclude 10:00  18:00 (2) Wind < 3.0 m/s Blind Pointing: (1 point/focus) Offset Pointing: (90 min) Continuous Tracking: (30 min)

  23. Effects of wind

  24. Effects of Wind

  25. “out-of-focus” holography • Hills, Richer, & Nikolic (Cavendish Astrophysics, Cambridge) have proposed a new technique for phase-retrieval holography. It differs from “traditional” phase-retrieval holography in three ways: • It describes the antenna surface in terms of Zernike polynomials and solves for their coefficients, thus reducing the number of free parameters • It uses modern minimization algorithms to fit for the coefficients • It recognizes that defocusing can be used to lower the S/N requirements for the beam maps

  26. Technique • Make three Nyquist-sampled beam maps, one in focus, one each ~ five wavelengths radial defocus • Model surface errors (phase errors) as combinations of low-order Zernike polynomials. Perform forward transform to predict observed beam maps (correctly accounting for phase effects of defocus) • Sample model map at locations of actual maps (no need for regridding) • Adjust coefficients to minimize difference between model and actual beam maps.

  27. Typical data – Q-band

  28. Typical data - Q-band

  29. Gravitational Deformations

  30. Gravity model

  31. Surface Accuracy • Large scale gravitational errors corrected by “OOF” holography. • Benign night-time rms ~ 350µm • Efficiencies: 43 GHz: ηS = 0.67 ηA = 0.47 90 GHz: ηS = 0.2 ηA = 0.15 • Now dominated by panel-panel errors (night-time), thermal gradients (day-time)

  32. Summary

  33. The End

  34. Supplemental Material

  35. Pointing Requirements Condon (2003)

  36. Focus Requirements Srikanth (1990) Condon (2003)

  37. Surface Error Requirements Ruze formula: ε = rms surface error ηp = exp[(-4πε/λ)2] “pedestal” θp ~ Dθ/L ηa down by 3dB for ε = λ/16 “acceptable” performance ε = λ/4π