1. The Green Bank Telescope: Overview and Antenna Performance
Richard Prestage
GBT Future Instrumentation Workshop, September 2006

2. Overview General GBT overview (10 mins)
GBT antenna performance (20 mins)

3. GBT Size

5. 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

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.

7. Antenna Specifications and Performance
Coordinates
Longitude: 79d 50' " West (NAD83)
Latitude: 38d 25' " 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] = ’ / [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.

8. Efficiency and Gain

9. Azimuth Track Fix
Track will be replaced in the summer of 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.

10. 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

11. Antenna Pointing, Focus Tracking and Surface Performance

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.

13. Performance Requirements
Good Performance
Acceptable Performance
Quantity
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
|Δys| < λ/2
Surface efficiency
ηs ~ 0.54
ε < λ/16
ηs ~ 0.37
ε < λ/4π

14. Summary of Requirements
(GHz)

15. Structural Temperatures

16. Focus Model Results

17. Elevation Model Results

18. Azimuth Blind Pointing

19. Elevation Blind Pointing

20. Performance – Tracking
Half-power in Azimuth
Half-power in Elevation

21. Power Spectrum
Servo resonance 0.28 Hz

22. Servo Error

23. 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)

24. Effects of wind

25. Effects of Wind

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

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.

28. Typical data – Q-band

29. Typical data - Q-band

30. Gravitational Deformations

31. Gravity model

32. Surface Accuracy
Large scale gravitational errors corrected by “OOF” holography.
Benign night-time rms
~ 350µm
Efficiencies:
43 GHz: ηS = ηA = 0.47
90 GHz: ηS = ηA = 0.15
Now dominated by panel-panel errors (night-time), thermal gradients (day-time)

33. Summary

34. The End

35. Supplemental Material

36. Pointing Requirements
Condon (2003)

37. Focus Requirements
Srikanth (1990)
Condon (2003)

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” performance
ε = λ/4π