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The Green Bank Telescope: Overview and Antenna Performance. Richard Prestage GBT Future Instrumentation Workshop, September 2006. Overview. General GBT overview (10 mins) GBT antenna performance (20 mins). GBT Size. GBT optics. 100 x 110 m section of a parent parabola 208 m in diameter

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the green bank telescope overview and antenna performance

The Green Bank Telescope:Overview and Antenna Performance

Richard Prestage

GBT Future Instrumentation Workshop, September 2006

overview
Overview
  • General GBT overview (10 mins)
  • GBT antenna performance (20 mins)
gbt optics
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
gbt capabilities
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.
azimuth track fix
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.
azimuth track fix10
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

precision telescope control system
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.
performance tracking
Performance – Tracking

Half-power in Azimuth

Half-power in Elevation

power spectrum
Power Spectrum

Servo resonance 0.28 Hz

performance summary
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)

out of focus holography
“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
technique
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.
surface accuracy
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)
focus requirements
Focus Requirements

Srikanth (1990)

Condon (2003)

surface error requirements
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