The Green Bank Telescope

Antenna Control (collimation and pointing)

Richard PrestageScientist / PTCS System Architect

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Telescope Structure and Optics

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Telescope Structure and Optics

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Telescope Structure and Optics

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Telescope Structure and Optics

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Telescope Structure and Optics

Challenges for large telescopes

The Astronomical Journal, February 1967

Telescope Construction

The Astronomical Journal, February 1967

NRAO/AUI/NSF 9

Quasi-Homologous design

The Advantage of Unblocked OpticsDynamic Range

Near sidelobes reduced by a factor >10 from conventional antennas

Gain & SensitivityThe 100 meter diameter GBT performs better than a 120 meter conventional antenna

Reduced Interference

Large Collecting AreaUnblocked Aperture

Sensitive to Low Surface BrightnessAngular Resolution

Sky Coverage & Tracking (>85%)Frequency CoverageRadio Quiet Zone

state-of-art receivers & detectorsmodern control softwareflexible scheduling

Characteristics of the GBT

Advantage of Unblocked Aperture

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Antenna Control / PTCS

• The GBT was designed that so that the telescope would perform as an “ideal” telescope for frequencies up to 15 GHz. To observe at frequencies above 15 GHz, we need to measure and correct for departures of the telescope from ideal behavior.

– Pointing– Collimation– Surface Accuracy

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Antenna Control / PTCS

• The optical and structural design was carefully selected in order to achieve certain scientific (observational) objectives, but this design acknowledged the influence of a variety of repeatable and non-repeatable factors that would degrade performance over the desired operating regime.

• The GBT Precision Telescope Control System is the combination of metrology systems, servos and control software which will deliver the pointing, collimation and surface accuracy required to operate the GBT at frequencies up to 115 GHz (wavelengths as short as 2.6 mm).

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Departures from Ideal (I)

• [Refraction]

• Misalignment of the antenna structure (e.g. non-perpendicularity of the Az and El axes)– May change (slowly) with time – e.g. effects due to non-

flatness of azimuth track.

• Deformations due to gravity (affects all three components). Most well behaved deformation; depends only on GBT elevation angle. The structure was designed so as to minimize the effect where possible, and the distortions have been modeled to some level of accuracy.

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Departures from Ideal (II)

• The effect of temperature change over time and location in the structure is to distort the optical alignment. Although the structure was designed to minimize these effects, they can still be substantial. While temperature effects are repeatable, the state of the structure (distribution of temperatures, whether the structure is in thermodynamic equilibrium) is not well known.

• Wind loading can cause structural loads that significantly distort the telescope (i.e. cause the optical properties to change). Again, the effects are repeatable, but the flow field will not be well known.

• Structural vibrations can be excited by wind or servo system drives. These vibrations can be significant, and have modal frequencies from 0.6Hz and up. The largest magnitude motions are in the feedarm assembly.

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PTCS System Philosophy: Original Intent

• Use sophisticated metrology system (specifically the laser rangefinders) to measure the absolute position, orientation and shape of the GBT optical elements in an appropriate coordinate system.

• “Division of Concerns” – i.e. collimation and pointing are independent

• Adopt a specific control strategy (e.g. move the subreflector to the position appropriate for the best-fit parabola at the required elevation)

• Potentially, use subsequent optical elements (e.g. subreflector) to correct for misalignments of preceding elements (i.e. primary).

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THIS APPROACH FAILED

• Metrology system was too complex and did not deliver required performance.

• Control algorithms not developed in parallel with metrology system

• System integration challenges were severely under-estimated

• Effort required to complete the system would be prohibitive

• This approach put on hold at end of CY 2003; never subsequently revisited

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Alternative Approach

• Sophisticated application of traditional astronomical approaches

• Astronomical measurements define reference positions of optical elements

• One result is that “division of concerns” is not achieved, i.e. pointing model depends on collimation model.

• Less “clean”, but significantly simpler to implement, and has allowed GBT to achieve 90GHZ operation!

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Components of the PTCS (I)

• Pointing:– Antenna Control System, including “thermally neutral”

pointing model– Temperature Sensor System and “dynamic temperature

corrections”– “Inclinometry system” to measure and correct for

azimuth track irregularities– “Quadrant Detector” to correct for (non-gravitational)

motions of the antenna feed arm (collimation error treated as a pointing error).

• Collimation:– “Focus-tracking” – adjusting the position of the prime

focus / subreflector to be at the position appropriate for the observed primary mirror parabola as a function of elevation.

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Components of the PTCS (II)

• Surface Accuracy:– Photogrammetry to obtain initial actuator zero-points– FE model for initial gravitational deformation correction– “Out-of-focus holography” to correct for residual gravity

and thermal deformations– “With-phase holography” to correct for small-scale

surface errors

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Remainder of this talk

• High-level Antenna Control

• Collimation (“upstream” of pointing)

• Pointing

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• High-level Antenna Control

• Collimation (“upstream” of pointing)

• Pointing

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Main Drives

• Azimuth: 1 drive/wheel, 4 wheels per truck, 4 trucks• Elevation: 8 drives (bull gear/pinion)• 0.3” per bit, azimuth and elevation encoders• Analog velocity (tach) and torque (current) loops• Digital position loop 50Hz sampling (10 Hz parabolic

demand)• ~0.3 Hz closed loop bandwidth

(< ~0.6 Hz first structural mode !)• Current loop lag-compensated, velocity loop lead-lag

compensated, position loop type-II with nonlinear compensation for large angle motions

• < 1” spec tracking error for constant velocity• Max 20 Deg/min elevation, 40 Deg/min azimuth

Astronomical Coordinate Conversions

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Any of these coordinate systems may be used to control telescope.Use SLALIB to perform coordinate conversions

Astronomical Catalogs

• Possible input formats:– SPHERICAL - A fixed position in one of our standard

coordinate systems, e.g., RA/DEC, AZ/EL, GLON/GLAT, etc.

– EPHEMERIS - A table of positions for moving sources (comets, asteroids, satellites, etc.)

– NNTLE - NASA/NORAD two-line element sets for earth satellites.

– CONIC - Orbital elements for solar system objects.

• Can enter solar system objects (sun, moon, major planets) by name.

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Catalogs Examples

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Catalogs Examples

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Catalogs Examples

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Catalogs Examples

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Catalogs Examples

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Offsetting with respect to tracking center

• Powerful facilities for offsetting with respect to tracking center

– Can perform simple offsets, e.g. track in J2000 and perform a raster scan in (Az,El)

– Can perform circles, ellipses, daisy-petal scans, lissajous figures

– Can define arbitrary scan pattern as series of piecewise (position, velocity, acceleration, time) scan segments

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• High-level Antenna Control

• Collimation (“upstream” of pointing)

• Pointing

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Collimation

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• As telescope tips in elevation:• Feed-arm tilts

downwards• Surface deforms,

displaces and rotates to a new parabola

• Telescope becomes mis-collimated• Error in pointing• Loss of gain

Collimation

• Srikanth, King and Norrod (1994):– A geometrical optics analysis of the system for a given

position of the subreflector was used to obtain the phase distribution across the aperture.

– By representing the phase distribution as a plane wave tilted relative to the aperture plane of the telescope, the residual phase distortion in the wavefront can be found.

– This analysis was carried out at different locations of the subreflector until the residual phase distortion was a minimum.

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Collimation

– A diffraction analysis was carried out for the system at this position of the subreflector.

– For the telescope at zenith the loss in efficiency is completely recovered. At horizon the loss in efficiency is 2.4% at 50GHz.

– This is the result of the residual phase distortion as the feed is still laterally displaced from the secondary focus.

– The tilt in the phase distribution is compensated for by re-pointing the antenna.

• A similar analysis for shaped Cassegrain antennas was performed by Battilana and Hills (1993).

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Focus Tracking

• The optimum position of the subreflector in X and Y measured empirically using astronomical observations at 2 GHz. – Track a bright calibrator– Step the telescope through a

range of X (Y) positions.– Perform a peak scan at each

position to determine peak amplitude for that focus setting

– Fit 5th order polynomial to peak amplitude as a function of focus setting for an individual elevation.

– Fit to A + B*cos(el) + C*sin(el)37

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Focus tracking curves

mm Sin(E) 21.86 - Cos(E) 11.18 9.56 Z

mm Sin(E) 9.96 Cos(E) 183.74 148.39- Y

mm Sin(E) 25.55 - Cos(E) 301.98 - 212.55 X

Axial Focus Measurement

Axial Focus

Ghigo et al. (2001)

Focus Accuracy Requirements

2/8/)95.0( Usable

4/16/)99.0( Good

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Axial 2ln4exp2

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mm/3.7Scale Plate

3/16/)99.0( Good

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Lateral 2ln4exp

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• High-level Antenna Control

• Collimation (“upstream” of pointing)

• Pointing

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Point Source Calibrators

Condon & Yin (2005)

PCALS 4.0: 7108 sourcesTwo-dimensional rms error < 0.2 arcsec

3 mm pointing calibrators

Data Collection

Gaussian Fits (Az, El, Focus)

Polarization (LCP – RCP)

Direction (Forward – Backward)

Jack Scan

All-sky ObservationsSingle Source Track

Up-Down at Night GravityNCP Source Temperature

Simple Pointing Model

Balser et al. (2002)

Azimuth Series ∆A Cos(E)

Elevation Series ∆E

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Gravity/Temperature Effects - Pointing

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Structural/Air Temperature Sensors

• YSI 083 thermistors• YSI 4800LC Thermistor Linearizing

Circuit• 0.15 C accuracy, -35 to 40 C• 0.05 C interchangable accuracy• 0.01 C resolution, 1 sec sampling• 19 structure sensors (soon 23)• 5 air sensors (forced convection

cells, ~ 5 sec time constant)

• Structure thermal distortions• Vertical air lapse

Temperature Sensors

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R.Side EL BearingBUS 15+440

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Structural Temperatures

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Algorithms

• Use existing GBT gravity pointing and focus models• Structure is linear: Thermal effects superpose• Temperature effect on focus, pointing assumed linear

in temperatures• No dependence on air or bulk temps, just differences• Simultaneously estimate gravity and temperature

model coefficients• Estimate coefficients using 9/11, 10/2, 11/10 data• Test models using 9/5, 11/20 data

Typical Terms (Elevation)

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Focus Model

Term Coefficient Min-Max Significance Parameter

M1 1.086 13.1 14.3 SR-PriM2 -0.697 6.2 -4.3 VFA-PriM3 3.981 15.6 62.0 HFAM4 -7.326 0.9 -6.8 BUS V1M5 -0.688 12.1 -8.3 BUS V2M6 -2.576 12.1 -31.2 BUS FM7 -180.630 0.0 0.0 Offset

M8 66.189 .7 43.1 sin term

M9 196.949 0.6 110.8 cos term

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Focus Model Estimation

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Focus Model Test

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Elevation Model

Term Coefficient Min-Max Significance Parameter

M1 -4.6455 1.2 -5.3 BUSM2 1.7830 15.6 -27.8 HFAM3 4.4488 5.9 26.4 VFAM4 -8.4477 1.6 -14.0 AlidadeM5 62.2218 0.0 +0.000 -IE,d(0,0)M6 -55.8624 0.7 -62.792 HZCZ,b(0,1)M7 -22.8268 0.9 -38.216 HZSZ,d(0,1)M8 2.4960 2.0 +2.169 -AW,c(1,0)M9 -1.3360 2.0 -1.750 AN,d(1,0)

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Elevation Model Estimation

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Elevation Model Test

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Thermal Compensation Results

• Significantly improved “static” gravity models.

• Focus peformance ~< 3 mm (excludes midday) during ~30 mm thermal focus shift.

• Elevation performance ~<3” 1s , <1”/hour (excludes midday) during ~ 30” thermal pointing shift.

• Azimuth performance ~<3” 1s , <1”/hour (excludes midday).

• Unanticipated dominance of horizontal feed arm influence.

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Orthogonal inclinometers on elevation bearing castings

Measure change in pose of elevation axle on moving telescope

Overconstrained track-alidade interface induces alidade distortion (twist)

Local tiltWheel out-of-round (not stable wrt azimuth)

Azimuth Track Effects

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January 6, 2005

Inclinometers

• AG gas-damped capacitive readout type from Wyler Zeromatic

• 2-axis (horizontal plane), both elevation bearings

• 0.1” short-term accuracy, 0.01” resolution

• ~1 sec damping, 17 Hz resonance

• 5 Hz sampling rate, 0.3” noise at 5 Hz

60January 6, 2005

Inclinometers, Cont.

Accelerometer Cube

X InclinometerY Inclinometer

Elevation Bearing Casting

Three Point, Spherical Washer and Shim Leveled Mount

Before and after track repair

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Jul 26, 2007

• Azimuth model (cross-elevation): 11 terms– 4 thermal terms, linear combos of structural temperature sensors– 3 gravity and geometry terms– 1 hysteresis term: Encoder running friction and backshaft windup– 2 track effect terms (alidade distortion estimate influence coefficients, and

local track tilt)– 1 local focus correction term (purely empirical)

• Elevation model: 11 terms– 4 thermal terms– 2 temperature dependent elastic modulus terms (feed arm only)– 3 gravity and geometry terms– 1 hysteresis term: Encoder running friction and backshaft windup– 1 track effect term (elevation encoder rotation in topocentric frame, fixed)

• Un-modeled but known effects– Encoder error: 2”.2 large angle, 1”.2 fine-cycle– Truck wheel out-of-round: ~0”.4 max-min

• Un-modeled but suspected effects– Subreflector position calibration: NB correlation of local focus offset w/

azimuth error• Traditional track tilt terms (AN/AW) replaced with track map• Encoder coupling misalignment does not appear to be

significant

Blind Pointing Model

Residuals

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Tracking Performance

• Offset beam to half-power point of bright calibrator. Then:

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Quasar

Beam

• Assuming all sources of antenna temperature variation are due to tracking errors, provides upper limit on 1-d tracking error.

Tracking Performance

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Dynamic Pointing Issues

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Relative motion betweenthe feedarm and thedish will cause pointing(collimation) errors.

Can be driven by servosystem and/or winds.

Major natural frequenciesof the structure are 0.6and 0.8 Hz. Largest motionis in the cross-elevationdirection.

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Quadrant detector system

LED Illuminator

Detector

detector

LED

View from receiver room:

Two-dimensional PSD, 4mm

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Measurement of feed arm motion

FAST Visit to NRAO (July 2010)

• 70mW LED• 800mm f.l. telescope• Calibrated by U.Va grad student Paul Ries

• Collimated LED source above receiver cabin• Two-dimensional detector below middle of the dish

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System calibrated with half-power tracks

FAST Visit to NRAO (July 2010)

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Correction of MUSTANG 90 GHz image:

FAST Visit to NRAO (July 2010)

Paul Ries (UVa)

• 27% Improvement in peak intensity and beam shape• Future goal: closed-loop adjustment of subreflector

Summary of Current Performance

• Blind Pointing~ 4” two-dimensional rms blind pointing

• Offset Pointing~1.5 – 2” one-dimensional offset pointing

• Tracking~ 1” rms under benign night-time conditions

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