the green bank telescope: overview and antenna performance
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
![Page 1: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/1.jpg)
April 8/9, 2003 Green BankGBT PTCS Conceptual Design Review
Richard Prestage
GBT Future Instrumentation Workshop, September 2006
The Green Bank Telescope:Overview and Antenna Performance
![Page 2: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/2.jpg)
2
Overview
• General GBT overview (10 mins)
• GBT antenna performance (20 mins)
![Page 3: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/3.jpg)
3
GBT Size
![Page 4: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/4.jpg)
4
![Page 5: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/5.jpg)
5
• 100 x 110 m section of a parent parabola 208 m in diameter• Cantilevered feed arm is at focus of the parent parabola
GBT optics
![Page 6: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/6.jpg)
6
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.
![Page 7: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/7.jpg)
7
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 focif/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 azimuth17 / 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.
![Page 8: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/8.jpg)
8
Efficiency and Gain
![Page 9: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/9.jpg)
9
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.
![Page 10: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/10.jpg)
10
Azimuth Track Fix
New Wear Plates•Better Suited Material•Balanced Joint Design•Joints staggered with Base Plate Joints
New Bolts ExtendThrough Both Plates
New Higher StrengthBase Plates
Transition Section
Old Track Section
Joints AlignedVertically –
Weak Design
Screws close toWheel Path
Experienced Fatigue
![Page 11: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/11.jpg)
Antenna Pointing, Focus Tracking and Surface Performance
![Page 12: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/12.jpg)
12
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.
![Page 13: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/13.jpg)
13
Performance Requirements
Good Performance Acceptable Performance
Quantity 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
Surface efficiency ηs ~ 0.54 ε < λ/16 ηs ~ 0.37 ε < λ/4π
![Page 14: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/14.jpg)
14
Summary of Requirements
(GHz)
![Page 15: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/15.jpg)
15
Structural Temperatures
![Page 16: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/16.jpg)
16
Focus Model Results
![Page 17: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/17.jpg)
17
Elevation Model Results
![Page 18: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/18.jpg)
18
Azimuth Blind Pointing
arcsec 2.7 Cos(E)]A [
![Page 19: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/19.jpg)
19
Elevation Blind Pointing
arcsec 4.8 E][
![Page 20: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/20.jpg)
20
Performance – Tracking
arcsec 2.12 )/'10,/'2(,
)5,1(),(
)58,290(),(
mmt
El
t
Az
ElAz
ElAz
)m/'7.11,m/'8.10(t
El,
t
Az
)9.3,6.3()El,Az(
)37,99()El Az,(
Half-power in Azimuth Half-power in Elevation
)m/'4.11,m/'0.12(t
El,
t
Az
)8.3,0.4()El,Az(
)43,105()El Az,(
arcsec 2.12
![Page 21: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/21.jpg)
21
Power Spectrum
Servo resonance 0.28 Hz
![Page 22: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/22.jpg)
22
Servo Error
![Page 23: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/23.jpg)
23
Performance – Summary
mm 2.5 focus)(
arcsec 5pointing)(1
mm 1.5 focus)(
arcsec 7.2pointing)(2
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)
arcsec 12
![Page 24: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/24.jpg)
24
Effects of wind
1''
12
2
11
68
2)(
sec16.0)(
smsat
windwind
arcsm
swind
![Page 25: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/25.jpg)
25
Effects of Wind
![Page 26: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/26.jpg)
26
“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
![Page 27: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/27.jpg)
27
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.
![Page 28: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/28.jpg)
28
Typical data – Q-band
![Page 29: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/29.jpg)
29
Typical data - Q-band
![Page 30: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/30.jpg)
30
Gravitational Deformations
![Page 31: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/31.jpg)
31
Gravity model
![Page 32: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/32.jpg)
32
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)
![Page 33: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/33.jpg)
33
Summary
![Page 34: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/34.jpg)
The End
![Page 35: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/35.jpg)
35
Supplemental Material
![Page 36: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/36.jpg)
36
Pointing Requirements
2
2El
2Az
22
2
2
GHZ
arcsec 740
2ln4exp)(
f
g
Condon (2003)20.0 10%)( Usable
14.0 5%)( Good
s
s
f
f
![Page 37: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/37.jpg)
37
Focus Requirements
2/8/)95.0( Usable
4/16/)99.0( Good
4
Axial 2ln4exp2
asa
asa
a
a
sa
yg
yg
yg
Srikanth (1990)Condon (2003)
5.03.17
mm 3.2 mm 7 :band-Q
mm/3.7Scale Plate
3/16/)99.0( Good
6
Lateral 2ln4exp
"
"
2
f
x
xg
xg
s
lsl
l
l
sl
![Page 38: The Green Bank Telescope: Overview and Antenna Performance](https://reader035.vdocuments.us/reader035/viewer/2022081513/5681595d550346895dc69b0f/html5/thumbnails/38.jpg)
38
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” performanceε = λ/4π