The Green Bank Telescope
Antenna Control (collimation and pointing)
Richard Prestage
Scientist / PTCS System Architect
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
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Challenges for large telescopes
The Astronomical Journal, February 1967
Telescope Construction
The Astronomical Journal, February 1967
Quasi-Homologous design
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NRAO/AUI/NSF
Characteristics of the GBT
Large Collecting Area
Unblocked Aperture
Sensitive to Low Surface Brightness
Angular Resolution
Sky Coverage & Tracking (>85%)
Frequency Coverage
Radio Quiet Zone
state-of-art receivers & detectors
modern control software
flexible scheduling
The Advantage of Unblocked Optics
Dynamic Range
Near sidelobes reduced by a factor >10 from conventional antennas
Gain & Sensitivity
The 100 meter diameter GBT performs better than a 120 meter conventional antenna
Reduced Interference
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
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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
•
•
•
•
•
•
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Astronomical Coordinate Conversions
Any of these
coordinate
systems may be
used to control
telescope.
Use SLALIB to
perform
coordinate
conversions
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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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• Collimation (“upstream” of pointing)
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Collimation
• As telescope tips in
elevation:
• Feed-arm tilts
downwards
• Surface deforms,
displaces and rotates to
a new parabola
• Telescope becomes miscollimated
• Error in pointing
• Loss of gain
34
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)
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Focus tracking curves
X  212.55 - 301.98 Cos(E) - 25.55 Sin(E) mm
Y  - 148.39  183.74 Cos(E)  9.96 Sin(E) mm
Z
9.56  11.18 Cos(E) - 21.86 Sin(E) mm
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Axial Focus Measurement
Axial Focus
Ghigo et al. (2001)
Focus Accuracy Requirements
2

 ys  
  Axial
g a  exp  4 ln 2

  a  
 a  4
Good ( g a  0.99)  y s   a / 16   / 4
Usable ( g a  0.95)  ys   a / 8   / 2

 x
g l  exp  4 ln 2 s

 l
 l  6



2



Lateral
Good ( g l  0.99)  xs   l / 16   / 3
Plate Scale  3.7" / mm
Q - band :   7 mm  xs  2.3 mm
   17.3"  f  0.5
• Pointing
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Point Source Calibrators
PCALS 4.0: 7108 sources
Two-dimensional rms error < 0.2 arcsec
3 mm pointing calibrators
Condon & Yin (2005)
Data Collection
All-sky Observations
Single Source Track
Up-Down at Night  Gravity
NCP Source  Temperature
Gaussian Fits (Az, El, Focus)
Polarization (LCP – RCP)
Direction (Forward – Backward)
Jack Scan
Simple Pointing Model
Balser et al. (2002)
Azimuth Series ∆A Cos(E)
Elevation Series ∆E
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
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Temperature Sensors
BUS 15+440
R.Side EL Bearing
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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
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Typical Terms (Elevation)
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Focus Model
Term
Coefficient
Min-Max
Significance
Parameter
M1
M2
M3
M4
M5
M6
M7
1.086
-0.697
3.981
-7.326
-0.688
-2.576
-180.630
13.1
6.2
15.6
0.9
12.1
12.1
0.0
14.3
-4.3
62.0
-6.8
-8.3
-31.2
0.0
SR-Pri
VFA-Pri
HFA
BUS V1
BUS V2
BUS F
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
M2
M3
M4
M5
M6
M7
M8
M9
-4.6455
1.7830
4.4488
-8.4477
62.2218
-55.8624
-22.8268
2.4960
-1.3360
1.2
15.6
5.9
1.6
0.0
0.7
0.9
2.0
2.0
-5.3
-27.8
26.4
-14.0
+0.000
-62.792
-38.216
+2.169
-1.750
BUS
HFA
VFA
Alidade
-IE,d(0,0)
HZCZ,b(0,1)
HZSZ,d(0,1)
-AW,c(1,0)
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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Azimuth Track Effects
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 tilt
Wheel out-of-round (not stable wrt azimuth)
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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
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Janu
ary
6,
200
Inclinometers, Cont.
Accelerometer
Cube
Elevation Bearing
Casting
Three Point,
Spherical
Washer and
Shim Leveled
Mount
Y Inclinometer
January 6,
X Inclinometer
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Before and after track repair
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Blind Pointing Model
•
Azimuth model (cross-elevation): 11 terms
–
–
–
–
–
•
Elevation model: 11 terms
–
–
–
–
–
•
Encoder error: 2”.2 large angle, 1”.2 fine-cycle
Truck wheel out-of-round: ~0”.4 max-min
Un-modeled but suspected effects
–
•
•
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
–
–
•
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)
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
Jul6226, 2007
Residuals
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Tracking Performance
• Offset beam to half-power point of
bright calibrator. Then:
Quasar
Beam
• Assuming all sources of antenna temperature variation are
due to tracking errors, provides upper limit on 1-d tracking
error.
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Tracking Performance
65
Dynamic Pointing Issues
Relative motion between
the feedarm and the
dish will cause pointing
(collimation) errors.
Can be driven by servo
system and/or winds.
Major natural frequencies
of the structure are 0.6
and 0.8 Hz. Largest motion
is in the cross-elevation
direction.
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Quadrant detector system
View from receiver room:
LED Illuminator
detector
LED
Two-dimensional PSD, 4mm
Detector
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Measurement of feed arm motion
• Collimated LED source above receiver cabin
• Two-dimensional detector below middle of the dish
• 70mW LED
• 800mm f.l. telescop
• Calibrated by U.Va
grad student Paul Ries
FAST Visit to NRAO (July 2010)
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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:
• 27% Improvement in peak intensity and beam shape
• Future goal: closed-loop adjustment of subreflector
Paul Ries (UVa)
FAST Visit to NRAO (July 2010)
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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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