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The Green Bank Telescope

The Green Bank Telescope. Frank Ghigo, National Radio Astronomy Observatory 7 th US VLBI Technical Meeting, Haystack, Nov 2009. Panoramic View of Green Bank Telescopes. National Radio Quiet Zone. Unblocked Aperture. 100 x 110 m section of a parent parabola 208 m in diameter

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The Green Bank Telescope

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  1. The Green Bank Telescope Frank Ghigo, National Radio Astronomy Observatory 7th US VLBI Technical Meeting, Haystack, Nov 2009

  2. Panoramic View of Green Bank Telescopes

  3. National Radio Quiet Zone

  4. 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

  5. Subreflector and receiver room

  6. Receiver turret

  7. On the receiver turret

  8. Inside the receiver room

  9. GBT active surface system • Surface has 2004 panels • average panel rms: 68  • 2209 precision actuators

  10. 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

  11. Track Replacement - summer 2007

  12. Present receivers

  13. Future Receivers • 7-beam 18-26GHz - being constructed • Mustang expansion to 100 pixels - in progress • Expand 7-beam 18-26GHz array to 64 or 100 • Expand 4-6GHz receiver to include Methanol line at 6.7GHz. • Dual-beam, dual polarization 3mm receiver for spectroscopy and VLBI • 3mm many beam array (single polarization)

  14. CICADA Configurable Instrument Collection for Agile Data Acquisition FPGA based data acquisition and processing Uses CASPER tools and hardware Umbrella program for organizing FPGA projects Purchase/obtain boards, software, development systems 3 ROACH, 2 BEE2, 5 iBOB, 6 ADC, 3 ADC-2, 10 GbE switches, servers, etc. 14

  15. CICADA Projects Pulsar machines Green Bank Ultimate Pulsar Processing Instrument (GUPPI) West Virginia University Pulsar Processing Instrument (WUPPI) Coherent dedispersion machine GUPPI-2 Event capture machines Used for transient events Spectrometers KFPA backend (for 7-beam K-band receiver) Replacement for GBT Spectrometer 15

  16. GUPPI specs • Two IF inputs (RCP,LCP), up to 800 MHz bw each • Each input sampled at 1.6 GS/s • 8-bit samples • 128 to 4096 channel spectra • Auto- and cross-correlation spectra computed • Averaged to 50usec per output spectrum • Output rate (4096 size spectra): • (1/50usec)*4*4096 = 330 Mbytes/sec • Data flows to storage (spigot mode) • Data folded at pulsar period (timing mode)

  17. GUPPI

  18. GUPPI Status • Currently available GUPPI Modes • 128 to 4096 channels • 2 polarizations • 100 to 800 MHz bandwidth • Full Stokes or Total Intensity Only • Decimation in frequency and time in CPU • Records up to ~250 MB/S continuously • Disk space is a considerable problem • Fully integrated into the GBT Observing System

  19. GUPPI-2 Uses GUPPI to feed a 9-machine cluster of GPU’s via 4 10 GbE ports. 128, 256, or 512 coarse channels will be supported 100 MHz processed per GPU at “reasonable” DM’s. Only modification to GUPPI is a new output FPGA personality and software to control the nodes

  20. K band Feedhorn Array Spectrometer 7 pixels, dual polarization 21 ROACH boards, 14 ADC's Needs to be in receiver room 3 GHz bandwidth Cross correlation between polarizations on a single pixel “Zoom” mode for multiple narrow windows in a bandpass Fast dump mode for pulsar use on a few polarizations Normal dump times of 1 ms 20

  21. Array Spectrometer design

  22. The Green Bank Telescope VLBI usage: About 15% of GBT time goes to VLBI projects. Including 2 Large projects; Also HSA, globals, EVN Frank Ghigo 7th US VLBI Technical Meeting, Haystack

  23. The Megamaser Cosmology projectBraatz, Condon, Greenhill, et al200 hours NGC4258

  24. Radio Interferometric Planet Search (RIPL)Geoff Bower, Alberto Bolatto, et al; U.C. Berkeley Search for planets around M-dwarfs 29 stars, 4 epochs per year. 1392 hours ~ 170 8-hr sessions

  25. GBT_VLBI software

  26. Improving the surface for High-Frequency Performance: • Surface • Mechanical adjustments • Photogrammetry • FEM (finite element model) • OOF (“out of focus” holography) model - global • AutoOOF - correct thermal errors short term • “Traditional” holography

  27. Mechanical adjustment of the panels.

  28. Surface efficiency -- Ruze formula  = rms surface error Effect of surface efficiency John Ruze of MIT -- Proc. IEEE vol 54, no. 4, p.633, April 1966.

  29. OOF: out of focus “holography” Zernike polynomials

  30. Auto-OOF corrections

  31. “Traditional Holography”

  32. Reduction of small-scale surface features

  33. 43 GHz Moon scans

  34. Beam pattern improvement

  35. Theoretical beam patterns

  36. 43 GHz Measured Aperture Efficiency

  37. 43 GHz comparisons

  38. Aperture Efficiency at high frequencies • For illumination pattern efficiency of 75% • And surface rms of 225u • Peak Aperture efficiency at 43.12 GHz is ~63% • At 85 GHz: 40% • At 110 GHz: 25%|

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