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 Cantilevered feed arm is at focus of the parent parabola

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.

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.

Efficiency and Gain

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 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

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.

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

Summary of Requirements (GHz)

Structural Temperatures

Focus Model Results

Elevation Model Results

Azimuth Blind Pointing

Elevation Blind Pointing

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

Power Spectrum Servo resonance 0.28 Hz

Servo Error

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)

Effects of wind

Effects of Wind

“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 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.

Typical data – Q-band

Typical data - Q-band

Gravitational Deformations

Gravity model

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)

Summary

The End

Supplemental Material

Pointing Requirements Condon (2003)

Focus Requirements Srikanth (1990) Condon (2003)

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