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

100 x 110 m section of a parent parabola 208 m in diameter Cantilevered feed arm is at focus of the parent parabola Unblocked Aperture

Subreflector and receiver room

Receiver turret

On the receiver turret

Inside the receiver room

GBT active surface system Surface has 2004 panels –average panel rms: 68  2209 precision 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 Surface Panel Actuators

Track Replacement - summer 2007

Present receivers

Future Receivers 17-beam 18-26GHz - being constructed 2Mustang expansion to 100 pixels- in progress 3Expand 7-beam 18-26GHz array to 64 or 100 4Expand 4-6GHz receiver to include Methanol line at 6.7GHz. 5Dual-beam, dual polarization 3mm receiver for spectroscopy and VLBI 63mm 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.

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

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)

GUPPI

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

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

Array Spectrometer design

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

The Megamaser Cosmology project Braatz, Condon, Greenhill, et al 200 hours NGC4258

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 hours ~ hr sessions

GBT_VLBI software

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

Mechanical adjustment of the panels.

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

OOF: out of focus “holography” Zernike polynomials

Auto-OOF corrections

“Traditional Holography”

Reduction of small-scale surface features

43 GHz Moon scans

Beam pattern improvement

Theoretical beam patterns

43 GHz Measured Aperture Efficiency

43 GHz comparisons

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