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

2

Overview

• General GBT overview (10 mins)

• GBT antenna performance (20 mins)

3

GBT Size

4

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

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' 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.

8

Efficiency and Gain

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.

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

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 => 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 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

14

Summary of Requirements

(GHz)

15

Structural Temperatures

16

Focus Model Results

17

Elevation Model Results

18

Azimuth Blind Pointing

arcsec 2.7 Cos(E)]A [

19

Elevation Blind Pointing

arcsec 4.8 E][

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

21

Power Spectrum

Servo resonance 0.28 Hz

22

Servo Error

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

24

Effects of wind

1''

12

2

11

68

2)(

sec16.0)(

smsat

windwind

arcsm

swind

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.

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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 panel-panel errors (night-time), thermal gradients (day-time)

33

Summary

The End

35

Supplemental Material

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

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

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