CMSC 2006Orlando

Active Alignment System for the LSST

William J. GresslerLSST ProjectNational Optical Astronomy Observatory (NOAO)

Scott Sandwith New River Kinematics

Page #2

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

Page #3

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

Page #4

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

Page #5

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

Page #6

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

Page #7

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)

Page #8

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

Page #9

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

Page #10

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

Page #11

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)

Page #12

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

Page #13

CMSC 2006 Orlando

Measurement Network Simulation

Page #14

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

Page #15

CMSC 2006 Orlando

Metrology System Uncertainty Analysis

Page #16

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

Page #17

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.)

Page #18

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

Page #19

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