Richard Prestage

1
The Green Bank TelescopeOverview and Antenna
Performance

• Richard Prestage
• GBT Future Instrumentation Workshop, September
  2006

2
Overview

• General GBT overview (10 mins)
• GBT antenna performance (20 mins)

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GBT Size
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(No Transcript)
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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

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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.

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Antenna Specifications and Performance
Coordinates Longitude 79d 50' 23.406" West (NAD83) Latitude 38d 25' 59.236" North (NAD83)
Optics Off-axis feed, Prime and Gregorian foci f/D (prime) 0.29 (referred to the 208 m parent parabola) f/D (Gregorian) 1.9 (referred to the 100 m effective aperture)
FWHM beamwidth 720/? GHz 12.4 /? GHz
Declination limits - 45? to 90?
Elevation Limits 5? to 90?
Slew rates 35? / min azimuth 17? / min elevation
Surface RMS 350 ?m average accuracy of individual panels 68 ?m
Pointing accuracy RMS (rss of both axes) 4 (blind) 2.7 (offset)
Tracking accuracy 1 over a half-hour (benign night-time conditions)
Field of View 7 beams Prime Focus 100s 1000s (10 FOV) Hi Freq Gregorian.
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Efficiency and Gain
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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.

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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
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Antenna Pointing, Focus Tracking and Surface
Performance
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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 gt 15GHz operation (Fall
  2001)
• Active surface and initial pointing/focus
  tracking model gt 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.

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Performance Requirements
Good Performance Good Performance Acceptable Performance Acceptable Performance
Quantity Target Requires Target Requires
rms flux uncertainty due to tracking errors 5 s2 / ? lt 0.14 10 s2 / ? lt 0.2
loss of gain due to axial focus error 1 ?ys lt ?/4 5 ?ys lt ?/2
Surface efficiency ?s 0.54 e lt ?/16 ?s 0.37 e lt ?/4p
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Summary of Requirements
(GHz)
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Structural Temperatures
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Focus Model Results
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Elevation Model Results
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Azimuth Blind Pointing
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Elevation Blind Pointing
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Performance Tracking
Half-power in Azimuth
Half-power in Elevation
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Power Spectrum
Servo resonance 0.28 Hz
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Servo Error
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Performance Summary
Benign Conditions (1) Exclude 1000 ? 1800
(2) Wind lt 3.0 m/s
Blind Pointing (1 point/focus) Offset
Pointing (90 min) Continuous Tracking
(30 min)
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Effects of wind
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Effects of Wind
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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

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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.

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Typical data Q-band
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Typical data - Q-band
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Gravitational Deformations
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Gravity model
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Surface Accuracy

• Large scale gravitational errors corrected by
  OOF holography.
• Benign night-time rms
• 350µm
• Efficiencies
• 43 GHz ?S 0.67 ?A 0.47
• 90 GHz ?S 0.2 ?A 0.15
• Now dominated by panel-panel errors (night-time),
  thermal gradients (day-time)

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Summary
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The End
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Supplemental Material
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Pointing Requirements
Condon (2003)
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Focus Requirements
Srikanth (1990) Condon (2003)
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Surface Error Requirements
Ruze formula e rms surface error ?p
exp(-4pe/?)2 pedestal ?p D?/L
?a down by 3dB for e ?/16 acceptable
performance e ?/4p