CMSC 2006 Orlando Active Alignment System for the LSST William J. Gressler LSST Project National Optical Astronomy Observatory (NOAO) Scott Sandwith New.
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CMSC 2006Orlando
Active Alignment System for the LSST
William J. GresslerLSST ProjectNational Optical Astronomy Observatory (NOAO)
Scott Sandwith New River Kinematics
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CMSC 2006 Orlando
Introduction
• Large Synoptic Survey Telescope (LSST)– Optical Design/Layout– Operational Requirements
• Active Alignment System– System Definition– Alignment Requirements– Design Methodologies
• Spatial Analyzer (SA) Effort– SA Model Description– Technologies Reviewed– Performance Analysis
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CMSC 2006 Orlando
M2
M1/M3
Camera
LSST Optical System
• Modified Paul-Baker Design– f/1.23– 3.5 Degree FOV
• 3-Mirror Telescope– Unique 8.4m M1/M3– 3.4m M2
• Camera– 3 Refractive Lenses, 6 Filter
Bands– 63cm Detector
• 3 Billion Pixels/Image!• 15 Tbytes/night, 5 Pbytes/yr
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CMSC 2006 Orlando
LSST Comparison
Primary mirror diameter
Field of view(full moon is 0.5 degrees)
KeckTelescope
10 m
0.2 degrees
LSST
8.3 m 3.5 degrees
Product of areas measures survey capabilityEtendue = 319 m2deg2
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CMSC 2006 Orlando
Optics Subsystem Layout
• 3 Major Optical Subsystems– M1/M3 Provides Reference Optical Axis– M1/M3 & M2 Active Figure Control– M2 & Camera Hexapods for Rigid Body
• Telescope Survey Operational Cadence– Open Shutter 15sec Exposure– Close Shutter 2sec Readout– Repeat Sequence for 2nd Exposure– 5sec Slew to Next Field
• Maintain Alignment During Operation
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CMSC 2006 Orlando
Telescope Control System (TCS)
• TCS Delivers Best Possible Image to Camera– Multiple Inputs
• Operator• Enclosure• Mount• Sky Camera• Weather Station• Wavefront Sensing System• Active Alignment System
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CMSC 2006 Orlando
Camera Wavefront Sensing
• Wavefront Sensors w/in Camera Focal Plane– Baseline Curvature Sensors– Provides Mirror Figure Control & Rigid Body
Positioning– No Information While Shutter Closed
Guider Sensors (8 locations)
Wavefront Sensors (4 locations)
Potential Aux. Sensors (16 locations)
3.5 degree Field of View (63 cm diameter) Sensor Package
(9 per Raft)Raft (21 in FPA)
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CMSC 2006 Orlando
Active Alignment System Description
• Complementary to Focal Plane Wavefront Sensing– Supports Telescope Alignment
• Initial Site Installation/Mount Model Development• Re-Assembly after Repair, Recoating, etc.• Perform Start of Night Operational Setup
– Maintain Alignment of 3 Major Subsystems• M1/M3 Reference• M2 Position (5 DOF) Camera Position (5 DOF)Hexapods
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CMSC 2006 Orlando
Active Alignment System Requirements
LSST Alignment RequirementsBody
MotionDecenter Tilts Piston
M1/M3 Reference Optical Axis
M2 +/-10 microns +/-5 arcsec +/-10 microns
Camera +/-5 microns +/-2 arcsec +/-5 microns
• Define Subsystem Fiducials– Fiducials Define Optical
Axis– Locate on Telescope Mount,
M1/M3, M2, & Camera– Incorporate into Final
Factory Acceptance Testing
• Measure Fiducials to Maintain Subsystem Alignment
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CMSC 2006 Orlando
System Design Constraints
• Packaging/Line of Sight Issues– No Interference w/ Light Rays– See Fiducials for Measurements
• Operational Needs– Ease of Service / Calibration– No Heat/Vibration– Support Full Telescope Pointing (Zenith to Horizon)
• Operational Temperature Range -10C to +25C– Minimal Warm-up Time Allowed– High Altitude/Low Pressure
• Sufficient Measurement Speed – 30 Second Cadence• Incorporate into TCS for Closed-Loop Feedback• Provide Required Accuracies
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CMSC 2006 Orlando
Light Sources
• Measurement Light Sources Must Minimize Camera Science CCD Impact– >1m Preferred (also ~400nm & ~950nm)– Pointing system technology (wavelength)– Ranging system technology (wavelength)
Camera Focal Plane Transmission
(Ideal Filters, Optics, Atmos, QE)
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CMSC 2006 Orlando
System Development Approach
• Study Effort w/ NRK to Define Active Alignment System using SA Modeling
• Baseline Definition & Performance Prediction• Establish Handoff to Wavefront Sensing System• Uncertainty Analysis for Metrology Controlled
Optical Alignment System
• Review Various Technologies– Laser Tracker– Laser Radar– Videogrammetry
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CMSC 2006 Orlando
Measurement Network Simulation
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CMSC 2006 Orlando
Uncertainty Field Analysis
• Metrology Network Optimized (Range Weighted Optimization) Composite Points
• Uncertainty Fields Established for Each Composite Measured Target
• Uncertainty Estimate for Telescope Mirror/Camera Computed with Sets of Target Uncertainty Clouds in Over-Determined Circle, Planar, and Cylindrical Shape (Monte-Carlo) – Centering– Normal Direction Pointing– Focus Position
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CMSC 2006 Orlando
Metrology System Uncertainty Analysis
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CMSC 2006 Orlando
Tilt Uncertainty Analysis vs. Num of Pts
Tilt (Normal Vector) Uncertainty Analysis verses Number of Points[Default Tracker Uncertainties]
0.8
1.0
1.2
1.6
1.3
1.6
1.9
2.52.6
3.0
3.7
4.7
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
0 2 4 6 8 10 12
Number of Points on each Element
No
rm
al V
ecto
r U
ncertain
ty (
Arcseco
nd
s) [
1-sig
ma]
M1/M3
M2
Camera
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CMSC 2006 Orlando
Uncertainty Analysis Conclusions
• LSST SA Model Results– 3 Metrology Systems Analyzed: (Laser
Tracker, Laser Radar, & Videogrammetry)– Each System Capable of Meeting
Requirements for Relative Subsystem Position/Orientation
– No Perfect Solution (Source Issues, Total Measurement Time, etc.)
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CMSC 2006 Orlando
Planned Future Activity
• Continued System Development– Define Fiducial Geometries for Major Telescope
Subsystems– Engage Metrology Device Vendors
• Review Requirements• Explain Current Deficiencies & Needs
– Perform Measurements at Nearby AZ Telescope Sites (Similar Operating Conditions) w/ Existing Commercial Hardware
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CMSC 2006 Orlando
What the sky will look like with LSST
• Survey image shown is ~0.5 degree field from Deep Lens Survey Project
• Shows roughly ten times as many galaxies per unit area (vs. Sloan Digital Sky Survey)
• The LSST images will cover 50,000 times this area in 6 different optical bands (20,000 sq. degrees!)
• LSST will show changes in the sky by repeatedly covering this area - multiple times per month
• 250,000 Type 1a supernovae detected each year
QUESTIONS/COMMENTS
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