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The Green Bank Telescope

The Green Bank Telescope. Richard Prestage NASA/NRAO Joint Institute 19 th July 2006. Outline of talk. Overview of the GBT, and how it came to be built Brief outline of unique active control systems A few science highlights. Chronology of the GBT.

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The Green Bank Telescope

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  1. The Green Bank Telescope Richard Prestage NASA/NRAO Joint Institute 19th July 2006

  2. Outline of talk • Overview of the GBT, and how it came to be built • Brief outline of unique active control systems • A few science highlights NRAO/AUI/NSF

  3. NRAO/AUI/NSF

  4. NRAO/AUI/NSF

  5. Chronology of the GBT • A next-generation, large single dish had been desired for many years and recommended by 1980 Radio Decade Review panel • Study group on a new, large dish, was at work at NRAO during 1988 • 300 Foot collapsed unexpectedly on 15 November 1988 while in routine use • Workshop on design of a new telescope held on 2-3 December 1988 • Senator Byrd (WVa) offers to help in 1989 • $75M appropriated for new telescope in June 1989 • Contract awarded in December 1990 to Radiation Systems, Inc. for $55M ($20M for NRAO systems) • Dedicated: August 25, 2000; Commissioning: 2001/2002; Fully Operational: 2003

  6. Original Schedule - 1989 NRAO/AUI/NSF

  7. GroundbreakingMay 1, 1991 NRAO/AUI/NSF

  8. Track foundation, Oct. 1991 NRAO/AUI/NSF

  9. Track sections - 1992 NRAO/AUI/NSF

  10. Alidade - Oct 1994 NRAO/AUI/NSF

  11. Hoisting the elevation axle - May 1995 NRAO/AUI/NSF

  12. Elevation gear assembly – March 1996 NRAO/AUI/NSF

  13. Dish backup structure – June 1996 NRAO/AUI/NSF

  14. Horizontal Feedarm – May 1997 NRAO/AUI/NSF

  15. Horizontal arm and backup structure – Dec 1997 NRAO/AUI/NSF

  16. Final sections of backup structure – June 1998 NRAO/AUI/NSF

  17. Subreflector and tip of feed arm – April 1999 NRAO/AUI/NSF

  18. Installing the surface panels – Nov 1999 NRAO/AUI/NSF

  19. Dedication – August 2000 NRAO/AUI/NSF

  20. What makes the GBT special? • Size • Unblocked main aperture • Precision Control System • Active Surface • Metrology • Frequency coverage • National Radio Quiet Zone location NRAO/AUI/NSF

  21. The GBT is large….. • Largest fully-steerable telescope in the world • At 17.3 Million Pounds (7856 metric tons), probably the largest moving structure on land. • Despite size and mass, built to extremely high precision NRAO/AUI/NSF

  22. Unique design – unblocked aperture NRAO/AUI/NSF

  23. Conventional optics with symmetric (blocked) feed supports Effelsberg 100 m Telescope NRAO 140 Foot Telescope NRAO/AUI/NSF

  24. Unblocked Aperture • 100 x 110 m section of a parent parabola 208 m in diameter • Cantilevered feed arm is at focus of the parent parabola NRAO/AUI/NSF

  25. Advantages of an unblocked aperture Reduces systematic responses, that are often the ultimate limitation in sensitivity: • No blockage of incident signal • Reduced scattering sidelobes • Reduced spectral standing waves • Less RFI pickup NRAO/AUI/NSF

  26. GBT Precision Control System NRAO/AUI/NSF

  27. Performance Metrics • Telescope performance can be quantified by two main quantities: • 1. Image quality / efficiency: • PSF / Strehl ratio (optical) • Beam shape / aperture efficiency (radio) • 2. Ability to point it in the right direction NRAO/AUI/NSF

  28. Image quality - optical NRAO/AUI/NSF

  29. Image quality/efficiency - radio Aperture efficiency η = Ae/A Ruze formula η = η0 exp[(-4πε/λ)2] ε = rms surface error “acceptable” performance: ε = λ/4π NRAO/AUI/NSF

  30. Image quality/performance requirements for GBT • total wavefront error ~1/15th of the wavelength of observations: • L-band (21cm) – 1cm or ~ ½ inch • W-band (3mm) – 200µm, the thickness of a human hair! • telescope pointed on the sky to ~1/10th of its beamwidth to avoid lost sensitivity or inaccurate results. • L-band (21cm) – 1 arcmin (diameter of Venus) • W-band (3mm) – 1 arcsec (small telescope stellar image) – 8m optical telescope performance! NRAO/AUI/NSF

  31. Scientific Requirements NRAO/AUI/NSF

  32. Small telescopes…. • Small optical telescopes (λ << D) • geometrical optics/aberration theory. • Two-mirror telescopes ~ perfect images given atmosphere • Small radio telescopes (satellite dishes/Direct TV) (λ not << D) • Need to use diffractive optics • Beam pattern in focal plane becomes an Airy disk • Can still build ~perfect implementations of chosen optical design NRAO/AUI/NSF

  33. Challenges for large telescopes • Manufacturing to required tolerances (100m diameter primary accurate to 1 part in a million) • Accurate alignment • Gravitational deformations • Thermal deformations • “non-repeatable” effects – wind, servo errors, etc. NRAO/AUI/NSF

  34. Challenges for large telescopes The Astronomical Journal, February 1967

  35. Solutions… • innovative design/construction • Calibration measurements • Real-time monitoring/dynamic adjustments (Potential alternative: use laser rangefinders to measure absolute position of all optical elements and correct appropriately. Not yet demonstrated.) NRAO/AUI/NSF

  36. Telescope Construction The Astronomical Journal, February 1967

  37. Homologous design NRAO/AUI/NSF

  38. Homologous design NRAO/AUI/NSF

  39. GBT active surface system • Surface has 2004 panels • average panel rms: 68 m • 2209 precision actuators • Designed to operate in: • open loop from look-up table • closed loop from laser metrology system NRAO/AUI/NSF

  40. Mechanical adjustment of the panels. NRAO/AUI/NSF

  41. Surface Panel Actuators One of 2209 actuators. • Actuators are located under each set of surface panel corners • Actuator Control Room • 26,508 control and supply wires terminated in this room

  42. Current FEM Model NRAO/AUI/NSF

  43. FEM corrections work well to 20GHz rms after active surface correction < 0.5mm at 50 deg. NRAO/AUI/NSF

  44. Next steps - holography NRAO/AUI/NSF

  45. NRAO/AUI/NSF

  46. Traditional (phase-reference) holography • Dedicated receiver to look at (usually) a terrestrial transmitter (at low elevation) or geostationary satellite. • Second dish (or reference antenna) provides phase reference. • Measure amplitude and phase of (near or far)-field beam pattern. • Fourier transform to determine amplitude and phase of aperture illumination. NRAO/AUI/NSF

  47. Alternative – “phase-retreival” holography • There are many advantages to traditional holography, but also some disadvantages: • Needs extra instrumentation • Reference antenna needs to be close by so that atmospheric phase fluctuations are not a problem • S/N ratio required limits sources to geostationary satellites, which are at limited elevation ranges for the GBT (35°-45°) • Alternative: measure power (instead of phase and amplitude) only, recover phase by modeling. NRAO/AUI/NSF

  48. Zernike polynomials z2: phase gradient (pointing shift) z5: astigmatism z8: coma aperture plane NRAO/AUI/NSF

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