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The Green Bank Telescope:
Overview and Antenna Performance
Richard Prestage
GBT Future Instrumentation Workshop, September 2006
GBT PTCS Conceptual Design Review
April 8/9, 2003 Green Bank
Overview
• General GBT overview (10 mins)
• GBT antenna performance (20 mins)
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GBT Size
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4
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
5
GBT Capabilities
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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]
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.
=
12.4’ / [GHz]
7
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
New Wear Plates
•Better Suited Material
•Balanced Joint Design
•Joints staggered with
Base Plate Joints
Transition
Section
Joints Aligned
Vertically –
Weak Design
Screws close to
Wheel Path
Experienced Fatigue
New Higher Strength
Base Plates
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Antenna Pointing,
Focus Tracking and
Surface Performance
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.
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Performance Requirements
Good Performance
Quantity
Acceptable Performance
Target
Requires
Target
Requires
rms flux uncertainty due to
tracking errors
5%
σ2 / θ < 0.14
10%
σ2 / θ < 0.2
loss of gain due to axial
focus error
1%
|Δys| < λ/4
5%
|Δys| < λ/2
ηs ~ 0.54
ε < λ/16
ηs ~ 0.37
ε < λ/4π
Surface efficiency
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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
 [A Cos(E)]  2.7 arcsec
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Elevation Blind Pointing
 [E]  4.8 arcsec
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Performance – Tracking
Half-power in Azimuth
Half-power in Elevation
( Az, El )  (290 , 58  )
(Az, El )  (1 , 5  )
 2  1.2 arcsec
(Az, El )  (99 , 37  )
(Az, El )  (3.6 , 3.9 )


 Az El 
,

  (10.8 ' / m, 11.7 ' / m)

t

t


 Az El 
,

  ( 2 ' / m, 10 ' / m)
 t t 
 2  1.2 arcsec
(Az, El )  (105 , 43  )
(Az, El )  (4.0 , 3.8  )
 Az El 
,

  (12.0 ' / m, 11.4 ' / m)
 t t 
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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 10:00  18:00
(2) Wind < 3.0 m/s
Blind Pointing:
(1 point/focus)
 1 (pointing)  5 arcsec
 (focus)  2.5 mm
Offset Pointing:
(90 min)
 2 (pointing)  2.7 arcsec
 (focus)  1.5 mm
Continuous Tracking:
(30 min)
 2  1 arcsec
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Effects of wind
 s 
 1 ( wind )  0.16  1 
m s 
2
arc sec
 2 ( wind )  2  1 wind 
 8'' at s  6 m s 1
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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: ηS = 0.67 ηA = 0.47
90 GHz: ηS = 0.2 ηA = 0.15
•
Now dominated by panelpanel errors (night-time),
thermal gradients (day-time)
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Summary
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The End
Supplemental Material
35
Pointing Requirements
2


  
g (  )  exp  4 ln 2  
   



  GHZ 



 740 arcsec    
2
 2   22   Az
  El2
2 
f  
 
Good
( s  5%)  f  0.14
Usable ( s  10%)  f  0.20
Condon (2003)
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Focus Requirements
2

 ys  
  Axial
g a  exp  4 ln 2

  a  
 a  4
Good ( g a  0.99)  y s   a / 16   / 4
Usable ( g a  0.95)  ys   a / 8   / 2
2

 xs  
 
g l  exp  4 ln 2

  l  
 l  6
Lateral
Good ( g l  0.99)  xs   l / 16   / 3
Plate Scale  3.7" / mm
Q - band :   7 mm  xs  2.3 mm
   17.3"  f  0.5
Srikanth (1990)
Condon (2003)
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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
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