Design and Implementation of a Fast-Steering Secondary Mirror System Maryfe Culiat Trex Enterprises July 25, 2007

Overview Benefits of a fast-steering secondary mirror (FSSM) FSSM system diagram Optical bench set-up Actuator analysis System parameters and implementation of design

Conventional fast-steering mirror technology is placed downstream from telescope FSM Atmospheric turbulence and mechanical jitter results in the loss of resolution in imagery Adaptive optics needed for correction Microscopic: Deformable mirror Macroscopic: Fast-steering mirror Tip-tilt easiest to correct, yet provides substantial return in image quality

Why use fast-steering mirrors at the secondary mirror position? Advanced technologies such as ultra light- weighted SiC allow for FSSM Benefits of having a fast-steering secondary mirror: 1. Tilt correction at the secondary keeps FSM close-coupled to the pupil 2. Simplified adaptive optics system; eliminates need for a pupil relay 3. Reduces need for another mirror surface FSSM

Understand closed-loop FSSM system Hands-on hardware integration Actuator trade study Obtain product information from vendors Using system parameters, down-select from vendor products Goals

Overview Benefits of a fast-steering secondary mirror (FSSM) FSSM system diagram Optical bench set-up System parameters Implementation of design Summary

Actuators Position Sensors Read camera Read FSSM position Offset coordinate transformation Command actuator Tracker Camera Fast Steering Secondary Mirror Computer Drive Electronics, Position Sensor Module Actuators Position Sensors A fast-steering secondary can provide rapid tip/tilt correction for line-of-sight stabilization

Actuators Position Sensors Read camera Read FSSM position Offset coordinate transformation Command actuator Tracker Camera Fast Steering Secondary Mirror Computer Drive electronics, Position Sensor Module Actuators Position Sensors A fast steering secondary can provide rapid tip/tilt correction for line-of-sight stabilization

Overview Benefits of a fast-steering secondary mirror (FSSM) FSSM system diagram Optical bench set-up Actuator analysis System parameters and implementation of design

Fast steering mirror Actuators Position sensors Drive electronics Computer Quad cell Spare fast-steering mirror allows for better understanding of closed-loop system

Tilt range of +/- 3mrad Small-signal bandwidth: ~ Hz Full stroke bandwidth: ~ Hz Integration of FSSM components

Overview Benefits of a fast-steering secondary mirror (FSSM) FSSM system diagram Optical bench set-up Actuator analysis System parameters and implementation of design

Piezoelectric actuators Voice coil actuators Two types of actuators are used in fast-steering mirror applications Source: Physik Instrumentewww.pi.wsSource: BEI Kimco Magneticswww.beikimco.com

Piezoelectric actuators are preferred over voice coil actuators for this application Voice coilPiezoelectric StrokeHighLow BandwidthLowHigh ResolutionLowHigh RepeatabilityLowHigh Thermal dissipation HighLow CostLowHigh Based on comparable products from BEI Kimco and Physik Instrumente

Overview Benefits of a fast-steering secondary mirror (FSSM) FSSM system diagram Optical bench set-up Actuator analysis System parameters and implementation of design

Parameters given: Mirror tilt range of at least ±2mrad SiC mirror – allows for reactionless Tripod drive Vendor products can be narrowed down given system parameters Source: Physik Instrumentewww.pi.wsSource: S. Walton

P – 3 actuators = $8,895 Physik Instrumente’s P fulfills performance specifications Source: Physik Instrumentewww.pi.ws Source: Physik Instrumentewww.pi.ws 90 µm stroke 1.8 nm resolution Integrated position sensor

Resistive film bonded to piezo stack Sub-nanometer resolution Low heat generation Indirect metrology Integrated strain gauge sensors offer high resolution and bandwidth Source: Physik Instrumentewww.pi.ws

Drive electronics: $16,025 – E-500, chassis w/ voltage supply = $2,327 – E-505, amplifier = $6,687 – E-509.S3, SGS module = $3,075 – E-516.I3, display module = $3,936 Drive electronics are integrated into a single package Source: Physik Instrumentewww.pi.ws

Interface between mirror and actuators; mounting details of flexures Continuation of design includes finite element analysis

Fast-steering secondary mirror technology allows for simplified adaptive optics system Piezoelectric actuators fulfill requirements for this application Continued analysis of system includes interface between mirror and actuators Conclusion

Acknowledgements Steve Walton Rich Holmes Don Bruns J.D. Armstrong This work has been supported by the National Science Foundation Science and Technology Center for Adaptive Optics, managed by the University of California at Santa Cruz under cooperative agreement No. AST Hilary O’Bryan, Scott Seagroves, Lisa Hunter Riki Maeda, Dennis Douglas, Daron Nishimoto James Deichmann, Jason Wong

Flexible tips M5 threading 20mm length Tilting angle of +/- 0.5 degrees Bending stiffness of 22 nm/rad 3 flexures = $459

Raw calculation of actuator stroke Assuming movement of only one actuator: Actuator stroke, y Distance of actuator from center of mirror, x Θ, maximum tilt angle Θ

Calculation of loaded resonant frequency Equations source: f o = resonant frequency of unloaded actuator (Hz) f o = 6 kHz k T = piezo actuator stiffness (N/m) k T = 10 7 N/m m eff = effective mass (kg) m eff = 3.5 g m’ eff = additional mass M + m eff m’ eff = 88.5 g f’ o, Loaded resonant frequency ≈ 1293 Hz Maximum operating frequency ≈ 431 Hz

Tracker camera FLIR SC6000 – InGaAs detector (near-IR) – Resolution: 320 x 256