Pointing and Focusing the GBT

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Pointing and Focusing the GBT

• K. T. Constantikes
• NRAO
• Green Bank

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Telescope Structure and Optics
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PTCS / High Frequency Environmental Envelope
s
s
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Scientific Requirements ½ to 1 Hour Stability
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Pointing Coefficients

• Feed arm tip moves 400mm as
• elevation changes from horizon to zenith
• 250 arcsec of elevation pointing
• 140 mm of focus shift
• gt 2 orders of magnitude improvement needed!

Entire pointing error budget is 0.32 mm
subreflector X translation, or 0.41 mm
subreflector Y translation, etc.
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Effects and Current Best Performance

• Gravity Model Fit
• Temp Gradient With and W/O Linear Model
• Track/Alidade Astronomy, Inclinometers
• Wind Estimated from Astronomical Data,
  Inclinometers
• Vibration Wind gusting, Max 20 MPH
• Servo Tracking Error per Axis
• Inertial Prelim from QD
• Hysteretic, Inertial, Bearing, Encoder, etc.
• Model Residuals from All-Sky and Single Source
  Tracking, 10/03 to 11/04
• Includes Symmetric Thermal Model, Azimuth
  Dependent Wind, Non-Parametric Astronomical
  Estimates of Track Effects

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Gravity, Structure Temperature Gradient, and Wind
Model

• Gravity terms, linear combinations of structure
  temperatures, and alidade-relative wind
  transformed to linearized space (T). 23
  temperature sensors
• Linear combinations of temperatures enforce
  symmetry conditions, e.g., mirroring about plane
  of telescope symmetry
• Robust least-squares fitting to linear model (m)
• Subsequent non-parametric estimate of residuals
  as a function of azimuth and elevation (f, g)
• Dataset contains 11000 separate astronomical
  observations from jack-scans in all-sky or
  single-source track modes

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Confidence and Significance of Gravity, Thermal,
Wind Model Coefficients (Bootstrap)
Large contribution of HFA thermal gradient to
focus
Vx2 Term N(0.25, 0.03)
T3F N(6.8, 0.3)
X wind V2 term (elevation) is significant and
stable Agrees with inclinometer
Track tilt (Cos) contribution to elevation error
T4E N(29.3, 1.2)
Cos(Az) Term N(3.4, 0.2)
Large alidade thermal term for elevation. 0.1 C
sensor accuracy is on the edge
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Correcting Focus for Thermal Distortions

• Corrected via robust linear regression of
  linearized features
• Temperature differences
• Gravity model
• Wind velocities in alidade-relative frame
• Include k-nearest neighbor estimates of az, el
  anomalies
• Current best model has 68 at lt 2mm

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Azimuth Track Effect on Elevation Axle Pose Side
to Side Motions
Acceleration in Y direction, rotations around X
Acceleration in X direction, rotations around Y
Measurement ? 0.6
Mean wind V2 in alidade frame
Blue and Red are same data, symmetry
condition applied
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Azimuth Track Effect on Elevation Axle Pose
Torsional Motions
12/12/04 Data, windy and gusty
11/28/04 Data, low wind
1 ? difference is 1.4 1 ? measurement 1.0
Measurement ? 0.6
?0 Inclinometer level, ?1 tempco of angle, Px,
Py are alidade relative V2wind, ?4 and ?5 are
track tilt coefficients. Bootstrap estimates of
tilt coefficient uncertainty 0.2. El encoder is
in X2 frame! Jaggies are real (repeatable).
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Comparison to Astronomical Estimate, Tilt
Stability over Time
Azimuth
Elevation
Track tilt has changed over 2 days
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Wind Pumped Vibration of Feed Arm WRT Elevation
Axle
High Q structure rejects wind gusts except over
narrow range of frequencies, mitigates wind
pumping.
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Servo Resonances From Half Power Track Experiment
0.04 (Limit cycle?) and 0.30 Hz (Position Loop
Peaking?)
RMS lt 1 Substantial track degradation since
this? Good track interval?
Structural vibration at start of scan
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Predicting Tracking Error From Inclinometers
8500 data points fit by linear combination of
two measurements (smoothed X2 and Y2
inclinometers) ! Time scales and direction
(elevation) suggest track bumps.
Samples (10 Hz)
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Vibration, Estimating Positions from Acceleration

• Inclinometer 1 apparent rotation 4.8 ?G 3 ?m
  motion _at_ 0.6 Hz
• 5 Hz sampling BW noise 0.3, 30 sec random walk
  100 ?m.
• Accelerometer 2 ?G/root-Hertz, or 10 sec random
  walk 400 ?m (5 Hz BW)
• Limiting BW to structural freqs extends interval
  x 5 (50 sec 100 ?m with inclinometer),
  disciplining with other constraints even better
  (e.g., QD, LRF)
• Currently no servos capable of correcting for 0.6
  Hz (lowest structural mode) and higher vibrations
  post-processing for actual position on sky
• Structural health monitoring via modal analysis

60 micron motions
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Acknowledgements, Etc.

• Prestage and Balser Astronomical data
  collections and reduction to pointing offsets
• Condon Scientific requirements on pointing,
  focus
• Much more info on instruments in next
  presentation