An Update on the GBT Metrology System

1
An Update on the GBT Metrology System

• K. T. Constantikes
• NRAO
• Green Bank

2
GBT Metrology System

• New Approach
• Parametric corrections using astronomy, e.g.,
  Thermal Model
• Parametric corrections using direct measurements,
  e.g., elevation axle pose
• Combinations of angle, distance, and dynamical
  data, i.e. Quadrant Detector (QD), Laser Range
  Finder (LRF), Accelerometers for direct
  measurements of e.g. primary tilt
• Combination of holography (surface figure at one
  elevation, no temp gradient) and few (gt 6) direct
  measurements of primary to correct for smoothly
  varying FEA error (vs grav) and thermal errors
• Fiducial and relayable tipping structure
  coordinate system (including orientation) tied to
  primary rim
• Inclinometers on el axles tie all to topocentic
  frame, perhaps using a track map, perhaps direct
  alidade measurements
• No rangefinders on ground
• Fixed baseline range and angle measurements
• System study underway, i.e., can
  pointing/focus/collimation/efficiency be met with
  new instruments and notional geometry of
  metrology components and telescope?

3
Current Instrument Set
Structure Temperature
Quadrant Detector Illuminator Structure
Temperature Air Temperature
3-Axis Accelerometer
Structure Temperatures (4)
Structure Temperatures (3)
Air Temperature
Structure Temperatures (2)
Structure Temperatures (4) Air Temperature
Structure Temperatures (2) Air Temperature Quadran
t Detector 2-Axis Inclinometers (2) 3-Axis
Accelerometers (2) Elevation Encoder
Structure Temperatures (2)
Structure Temperatures (2)
Structure Temperatures (2)
Air Temperature
Azimuth Encoder
4
Communications Infrastructure
2 RS485 to RS232 Transceivers (4 drops) Active
Surface Actuator Control Room, Elevation Bearings
RS232 to 802.x Concentrator (8 ports) Receiver
Room
2 RS232 to 802.x Concentrators (16 ports) Active
Surface Actuator Control Room
2 RS232 to 802.x Concentrator (16 ports) Vertex
RS232 to 802.x Concentrator (8 ports) Alidade
Level
5
Additional Design Constraints have been Addressed

• RFI Mitigation and Testing
• Thermal, Thermal/Mechanical Design
• Maintainability
• Availability (System MTBF)
• Alignment/Calibration
• Design for Installation

6
Structural/Air Temperature Sensors
Environment Enclosure

• 0.15 C accuracy, -35 to 40 C
• 0.10 C interchangable thermistors
• 0.01 C resolution, 1 sec sampling
• 23 structure sensors
• 5 air sensors , forced convection cells, 5 sec
  time constant
• RS232 communications
• Automated testing (daily)
• Structure thermal distortions
• Vertical air lapse
• Laser rangefinder group index calculations

Thermistor in mounting slug
Forced Convection Cell
Mounted with VHB Tape and Delrin plate
RFI Enclosure
ESD Protection
7
Inclinometers

• 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
• Azimuth track maps
• Real time measure/correct Az/El
• Verify thermal effects
• Wind force spring balance
• Structural resonances

8
Inclinometers, Cont.
Elevation Bearing Casting
Accelerometer Cube
Three Point, Spherical Washer and Shim Leveled
Mount
X Inclinometer
Y Inclinometer
9
Accelerometers

• 3-axis, elevation bearings and receiver cabin
• MEMS torsion, capacitive readout, nickel
• 2 micro-G/root Hz
• 10 Hz sampling
• 1 x 1 x 0.1 G dynamic range
• 24 x 24 x 16 bit mixed signal ADC/microprocessor
• Structural resonances
• Receiver room vibration
• Feed Arm motions
• Vertical dynamics at El Bearings

10
Quadrant Detector (Version 3)
Optical Tube, 800 mm fl lens, detector and TIA
preamp assy.

• Sub-arcsec angle-angle measurements, 1000 FOV
• 5 Hz bandwidth, sampling at 10 Hz
• Instrument noise 0.2 arcsec (in lab)
• Good relative measurements on ½ hour time scales
• Degraded by turbulence, index gradients
  (19/100m/1K/m)
• Feed arm position/motion WRT elevation shaft
  (tipping structure coordinates)
• Structural resonances

5? Accuracy, 1.5? Repeatability, 0.1?
Resolution X-Z Translation Stage, Computer Control
High Intensity Green LED Illuminator
11
QD V3 Performance
65 Hour calibration, 225 points (dithered) per
affine transform estimate, 161 iterations 70mm
dynamic range at 18m ( 800), air strongly mixed
with fans Horizontal zero point 1 ? 0.18,
scale 1 ? 0.06 Vertical zero point 1 ?
0.85 (dominated by vertical index gradient),
scale 1 ? 0.1 Compare to PSD nonlinearity spec
of 0.05 1 ? measurement noise 1.1 at 10 Hz
(Telescope configuration SNR, fan
turbulence) Equivalent to 1.4 on sky _at_ 10
Hz Stationary telescope performance 0.2 to
0.4 on sky _at_ 10 Hz (100 s linear detrend, clear
day) 1700/ azimuth slew introduces 2 of
structural resonance modes.
12
Quadrant Detector (Version 4)
Interim Configuration 4 channel BP filter and
True RMS to DC Conversion 1x 24 bit sigma-delta,
3x 16 bit sigma-delta 512 Hz carrier modulation,
10 Hz sampling 7ppm local clock RS232 status,
control, and data Time transfer over
RS232/Ethernet Final Configuration Tunable
filters (SCF) for three channels Three
illuminator modulation frequencies Autocollimation
and angle-angle measurements
QD V4 with 500 mm fl catadioptric lens, temp
control, TIA, 4x4mm PSD
Analog Processor and Data Acquisition
13
IR Thermography
Conduction into BUS ribs and hoops clearly
visible, hot band in hoop direction ?
Ripple due to conduction into BUS?
2C cooler in Rcvr Room Shadow
14
Optical Telescope
TE cooled micro-lens array full frame
CCD mechanical shutter 765x510, 9? pitch 500 mm
F/4 3.8 IFOV 0.8 x 0.5 FOV 0.1 s min
exposure
USB 2.0 to Ethernet
Power Supply
Fiber Media Converter
Primary use Star tracking, i.e. orientation in
inertial frame. Interpolate to 1, limiting
magnitude better than 14 with 1 s
exposure Implementation will include 5 tip-tilt
stage, focus servo. Alternative to Inclinometers
when dynamic range gt 1 WRT gravity. Expect
(95) 2 stars gt Mag 11 in FOV. NOT for use
as Pointing/Tracking Telescope
15
Next Generation Laser Rangefinder
PA
Detector Bias Supply
IF Subsystem
TEC Controller, LD Power Supply
Fiber Optic Switches
DSP
VGA
Clock
Synthesizers
TE Cooler, Bias-T, Pigtailed Visible LD
Detectors, Xmit and Rcvr Optics
16
Next Generation Laser Rangefinder, Cont.
1 kHz Offset _at_ 150 MHz, 16 kHz BW
Fiber Coupled JFET TIA Detector
Xmit Aperture, 1cm
24 Bit Digitized I/Q
Rcvr Aperture, 1 cm
17
Rangefinder Improvements

• Frequency diverse (100-300 MHz), absolute range
  (incommensurate wavelengths)
• Fiber optically coupled optics
• MEMS chopping, zero points, xmit to rcvr coupling
  _at_ 100 Hz
• Fiber reference loop
• Multiplexing multiple remote heads from one EO
  package
• No longer share a single aperture- no need for
  polarization decoupling
• Xmit and rcvr optics are small
• No phase uncertainty associated with photon
  centroid on detector (group delay uncertainty in
  detector)
• RFI Mitigation No bare detectors or radiators,
  can use optical cutoff tube and fiber
• Visible LD
• TE cooled to mitigate lasing wavelength changes
  (secondary group index error)
• Mitigate eye hazard
• Easy alignment
• Looks cool.
• Might be marginally worse (?) for path loss, Mie
  scattering?
• Easy to convert to NIR if needed

18
Rangefinder Improvements, Cont.

• Diverged beam ( 5 mrad) mitigates pointing
  problems, small scale turbulence
• Fiber coupled optics could be mounted on existing
  pointing heads with fiber wrap
• Much lower cost and volume Telecom/OEM
  subsystems
• Design Goals Instrument noise less than group
  index fluctuation/uncertainty (0.3-1 ppm)
• Current Measured Performance (single phase
  measurement) 23? on 20m path, 10Hz, 5 mrad
  beam, baseband13µ _at_ 1kHz offset
• Instrument noise dominated by AM, shot noise
  limited (at detector diode) _at_ 1 kHz offset
• Measurement noise dominated by index fluctuations
  _at_ 10 Hz
• Disciplined measurement (e.g., structural modes)
  can have much lower measurement noise if needed

19
Acknowledgements, Etc.

• PTCS Instrumentation Team
• K. Constantikes, J. Ray, JD Nelson, J. Cromer,
• J. Shelton, R. McCullough, M. Stennes