The Green Bank Telescope

Frank Ghigo,National Radio Astronomy Observatory

7th 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

1 7-beam 18-26GHz - being constructed2 Mustang expansion to 100 pixels - in progress

3 Expand 7-beam 18-26GHz array to 64 or 1004 Expand 4-6GHz receiver to include Methanol line at 6.7GHz.5 Dual-beam, dual polarization 3mm receiver for spectroscopy and

VLBI6 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.

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

Reconfigurable Open

ArchitectureComputingHardware

Reconfigurable Open

ArchitectureComputingHardware

Front-endElectronics

Reconfigurable Open

ArchitectureComputingHardware

IF 0

Pixel 0

Pixel 1

Pixel 7Pixel 6

Pixel 5P ix el 4

Pixel 3Pixel 2

Front-endElectronics

IF 1

10 Gb Switch

10 GB X 210 GB X 2

10 Gb Switch

32 Core Pipeline Processor

Disk Controller

Disk Controller

Dual10 GbLinks

Fibers FromGBT

40 TB

40 TB

Up to 500 MB/sec to disks ,

sustained

Fibers To Jansky Lab Equipment

Room

Signal Processors

Signal Combiner /Accumulator

To existing GB Network infrastructure

The Green Bank Telescope

Frank Ghigo7th 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 planetsaround M-dwarfs

29 stars, 4 epochs per year.

1392 hours

~ 170 8-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 1966.

Effect of surface efficiency

⋅⋅⋅= surfpatap εεε€

εsurf = e−(4πσ / λ )2

= 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 43.12 GHz is ~63%– At 85 GHz: 40%– At 110 GHz: 25%|