Active optics and control architecture for a Giant Segmented Mirror - PowerPoint PPT Presentation

Active optics and control architecture for a Giant Segmented Mirror Telescope George Z. Angeli, Myung K. Cho, Mark S. Whorton

Overview • A feasible control architecture – How to separate and organize control functions • Supporting simulations – Proving it’s viability – With real, measured wind data

Physical configuration 1 New challenge – wind: Increased area Lower resonance frequencies Integrated aO and AO

Physical configuration 2

Control philosophy • Forced decoupling of control subsystems • Allows decentralization • Improves understanding of underlying concepts and processes • Simplifies control laws and cost functions • Supports detached design, implementation and troubleshooting of subsystems • Subsystems are still sophisticated MIMO (multiple-input-multiple-output) systems

Control architecture Parallel optical and mechanical feedback Main axes (tracking) control M1 phasing maintenance based based on WFS (0.5 Hz) on edge sensors (0.5 Hz) M2 rigid body motion control based on WFS (10 Hz) M2 facesheet control based on WFS (100 Hz) M1 low order shape control (aO) based on WFS (0.1 Hz)

Frequency separation of optical subsystems M1 M2 Shape Deformable Zernike modes 20 temp.avg. 3 M2 Rigid temp.avg. Body temp.avg. 2 Main Axes temp.avg. 0.01 0.1 1 10 100 Bandwidth [Hz]

Control configuration Sky motion, Turbulence Wind Control H atm A DM system Aberration reference x B DM C DM2 Optics R DM2 K(s) sec WFS ∫ Deformable M2 M sec M pri dynamics B wind C sec B sec R sec K(s) sec A ts x B pri C pri R pri K(s) pri ∫ Optical Phasing Telescope system reference B edge C edge R edge K(s) edge dynamics

Fundamental assumption • Primary mirror phasing maintenance possible with limited bandwidth loop – High order, high frequency M1 wind deformations well bounded – Structural interactions avoidable – Secondary rigid body control only with actuator-structure interaction Whorton et al . 4840-23 • Verifying simulations – Structural modeling for large scale deformations – Segment modeling for continuity check (no structural deformation)

Model for GSMT structural simulation • Structure − & & + & + = 1 T q 2 Z Ω q Ω q M Φ Bu m m m – Modal description (20 modes)     0 I 0 = + & x x u     – State-space representation − − 2 − 1 T Ω 2 Z Ω M Ω B     Cho et al. 4837-40 • Wind – Gemini South measurements – Open dome, slit facing wind – Wind velocity 2 10 1 10 • ~10 m/s @ dome 0 10 PSD [Pa 2 /Hz] • ~4 m/s @ M1 -1 10 • ~4 m/s @ M2 -2 10 measurement von Karman fit -3 10 -2 -1 0 1 10 10 10 10 Fre que ncy [Hz]

M1 deformation due to wind 20 RMS deformation [ µ m] 15 10 5 0 -20 20 0 10 0 X [m] -10 Y [m] 20 -20

Zernike expansion of M1 deformation 15 total wind wind on secondary wind on primary RMS Zernike coefficient [ µ m] 10 5 0 0 2 4 6 8 10 12 14 16 18 Zernike term

PSD of RMS M1 deformation µ m/ √ Hz] 2 10 PSD of RMS primary mirror deformation [ 1 10 0 10 -1 10 -2 -1 0 1 10 10 10 10 Frequency [Hz]

Residual M1 deformation Zernike terms removed up to #36: -1 10 32 nm RMS RMS error [ µ m] -2 10 -3 10 -2 -1 0 1 10 10 10 10 Frequency [Hz]

Model for segment control simulation [ ] ( ) ( ) ( ) 2 = + − D r p r r p r p 0 0 spatial 2.5 Wind 2 Structure function, √ D [Pa] – Same as for structural 1.5 simulation 1 – Cho et al., SPIE 4837-40 0.5 – Correlation length < 2m on M1 0 0 1 2 3 4 5 6 7 8 Sens or s pacing, d [m] “Segmented” Gemini mirror – Segment size 1.152 m edge-to- edge – Actuator stiffness 10 N/ µ m – No dynamics

Segment continuity control -2 10 PSD of Edge Sens or Nois e ( µ m/ √ Hz) -3 10 Actuator and sensor modes based on SVD: ( )   diag σ 0 = i T G U V   -4 10 0 0   -1 0 1 2 10 10 10 10 Frequency (Hz) n(s) d(s) estimator controller r(s) y(s) u(s) R=G † K(s) G 108 84 From initial phasing Band limited     1 proportional diag 0     + = T σ G V U   i 20   ( ) = K s 0 0   + 1 . 6 s 1

M1 deformation (ventilation gates open) µ m

RMS edge displacement (ventilation gates open) High wind Wind velocity: 0.2 Open loop ~10 m/s @ dome t Closed loop 0.18 ~4 m/s @ M1 0.16 ~4 m/s @ M2 RMS Edge Reading [ µ m] 0.14 110 nm RMS open loop 0.12 0.1 0.08 0.06 0.04 30 nm RMS closed loop 0.02 0 0 2 4 6 8 10 Time [s econd]

RMS edge displacement (ventilation gates closed) Low wind 0.2 Wind velocity: Open loop 0.18 Closed loop ~11 m/s @ dome t ~0.6 m/s @ M1 0.16 RMS Edge Reading [ µ m] ~4 m/s @ M2 0.14 0.12 0.1 0.08 0.06 0.04 12 nm RMS open loop 0.02 6 nm RMS closed loop 0 0 2 4 6 8 10 Time [s econd]

Conclusion • Wind load on a 30-meter class telescope is not trivial, but manageable with a distributed control architecture • Further studies necessary – Integrated structural, optical and control model to – Realize optical feedback – Evaluate performance – Balancing dome seeing and structural deformation effects to find the “optimum” wind inside the enclosure

Frequency bands of actuator groups MCAO 50 spatial & temporal avg. M2 Deformable Zernike modes 20 spatial & temporal avg. M1 spatial & temporal avg. Actuators temporal avg. 8 M2 Rigid spatial avg. Body 2 spatial & temporal avg. Main Axes 0.01 0.1 1 10 100 Bandwidth [Hz]