1. George Angeli 11 September, 2001 Current Concepts and Status of GSMT Control Systems

2. Introduction l Feasibility of GSMT depends on its controllability  Segment alignment maintenance  Wind effect compensation (structure correction) l Whatever is designed, it must be observable and CONTROLLABLE! l How does the boot get on the dinner table? Why do we care about the control system in this early phase of design?

3. Frequency Band Separation of Subsystems 0.0010.010.1110100 Bandwidth [Hz] Zernike modes ~100 ~50 ~20 ~10 2 aO (M1) AO (M2) Main Axes LGS MCAO Secondary rigid body temporal avg spatial & temporal avg spatial avg

4. MCAO l MCAO can be separated from telescope control  First MCAO sensor is behind the last telescope control actuator in the light path  MCAO is fed with a wavefront corrected up to 30-50 Zernike @ 20 Hz BW on tracking guide star

5. Active Optics l Initial phasing in open loop  Low spatial frequencies barely observable by edge sensors l Phasing maintenance in closed loop with edge sensors  Assumption: wind buffeting has negligible high spatial frequency effects on primary mirror Continuity maintenance system is static (no interference with structural resonances)

6. Phasing Maintenance l Static influence function l Edge detector / actuator modes by SVD

7. Control Configuration for Phasing Maintenance From phasing Edge sensors Actuator space Pseudo-inverse:

8. Adaptive Optics l Adaptive (deformable) secondary  Atmospheric correction r 0  0.5 m @ 1.2  ~7000 actuators for 30 m MMT 1200 actuator/m 2 on secondary@ 0.6 m GSMT 2200 actuator/m 2 on secondary@ 2 m  Telescope deformation correction max. 1800 actuators for 600 segments 570 actuators/m 2 on secondary  In the close future atmospheric correction is not feasible in the NIR (maybe in midIR)

9. Deformable Secondary l Face-sheet mass is negligible  No interaction with telescope structure l Face-sheet motion is over-damped  No local dynamics Secondary AO system is static l Wavefront correction with deformable secondary  Temporal average (0.1 Hz) off-loaded on primary

10. Frequency Band Separation of Wavefront Correction and Tracking

11. Control Configuration for Wavefront Correction and Tracking Measurement noiseWind, Gravity, HeatAtmosphere Offset due to: telescope aberration off-axis guide star

12. Physical Configuration

13. Computational Load l Static active optics l Deformable secondary  1 Hz bandwidth  10 Hz sampling rate  Reconstructor matrix [1800 x 3600] 2 sensors on each edge   230 GFLOP/s  20 Hz bandwidth  200 Hz sampling rate  Reconstructor matrix [1800 x 1000]   360 GFLOP/s Texas TMS320C64x 4.8 GFLOP/s Intel P4 1.4GHz 5.6 GFLOP/s

14. System Modeling and Simulation l Investigate telescope behavior l Validate design assumptions  Observability (sensor choices, placement)  Controllability (actuator choices, placement) l Allows modal-based feedback design (Linear Quadratic Gaussian, H , etc.)  Performance l Validate model reduction for simulation and control

15. Current Model l Modal based state space representation of the structure, based on FEA (20 modes)  Primary mirror as a surface fit on raft support nodes  Line-of-sight equation for rigid body motion of primary and secondary  Force actuators at weakened or completely opened degrees of freedom l Zernike representation of wavefront quality l Integrated structure FEA  Redefined base for OPD as a linear combination of Zernikes linked to structural modes

16. Primary Mirror Truss Structure

17. Structural Mode Shapes on the Primary Mirror

18. Zernike Content of the Structural Modes

20. Zernike Content of Secondary Rigid Body Motion

21. Wind Load (X Direction)

22. Wind Load (Y Direction)

23. Wind Load (Z Direction)

24. Future Path l Wavefront control (wind buffeting)  Segmented primary model (edge detectors, detector and actuator mode spaces)  Verification of ‘static phasing maintenance’ hypothesis l Primary control  Feedback design and simulation  Deformable secondary ‘surface fit’ model  Primary ‘surface fit’ model on actuator nodes  Wind load definition (on structure and primary)  Feedback design and simulation

25. Future Path (cont’d) l Tracking  Actuator definition  Nonlinear (large signal) and linearized (small signal) models  Gravitational load definition  Feedback design and simulation l Integration  Structural model integration  Optical model integration  Feedback integration and simulation

26. Modeling Issues l Structural model  Integrated structure versus interfaced subsystems  Boundary value problems l Optical model  Refined ‘fitted surface’ model with ray tracing and fitting each structural modes, i.e. building a new orthogonal basis for OPD which is characteristic to the telescope  Segmented mirror optical response l Load model  Wind power spectral density and spatial distribution  Wind-to-force conversion