Space active optics: performance of Marie a deformable Laslandes mirror for. Devilliers, Arnaud Liotard, Céline Lopez, Frédéric Chazallet

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1 Space active optics: performance of Marie a deformable Laslandes mirror for in-situ wave-front Emmanuel correction Hugot, Marc in Ferrari, space Claire telescopes Hourtoule, Christian Singer, Christophe Devilliers, Arnaud Liotard, Céline Lopez, Frédéric Chazallet International Conference of Space Optics 2012

2 Context Earth or Universe observation from Space Telescope aperture increase & weight constraint Large monolithic lightweight M1 Large deployable segmented M1 Structure stability: Thermo elastic and gravity deformations => Loss of M1 optimal shape during operations => Optical aberrations in the optical train => Instrument performance degradation Mass: 11 T 3,3 T 6,2 T Active optics to the rescue: Active Mirror to compensate for M1 deformation Mechanism with 5 Degrees Of Freedoms for realignement ICSO 2012 MADRAS 2

3 CXCI example! CNES technological demonstrator for compact lightweight large telescopes Lightweight M1 M1 deformation M2 Misalignment M1/M2 5 DOF for Rigid Body Motion compensation Focal plane Active mirror in pupil relay Alternative solution to the classical ultra-stable concept ICSO 2012 MADRAS 3

4 MADRAS project LAM/Thales active mirror technological demonstrator ( project) Correction in a pupil relay => Compensation of M1 deformation with a small and light Deformable Mirror Specifications Study case: expected deformation in 3m monolithic space telescopes DM dimensions: Diameter: ~ 100 mm, Weight < 5 kg Correction of 9 Zernike modes Precision of correction: each mode: < 5 nm rms global WFE: < 10 nm rms Design wrt space constraints Mode Amplitude max (nm rms WFE) Coma3 200,0 Astig3 150,0 Sphe3 50,0 Tref5 30,0 Astig5 30,0 Tetraf7 30,0 Tref7 30,0 Pentaf9 30,0 Tetraf9 30,0 Shapes of the different modes to correct ICSO 2012 MADRAS 4

5 MADRAS prototype Multimode Deformable Mirror Mechanical structure Zerodur Invar Total diameter: 130 mm Total height: 80 mm 130 g 2 kg 24 piezoelectrical actuators Stroke: 20µm (80V) 24 x 25 g Mirror design optimized with FEA Overall system design for a space environment => mechanical strength, vacuum/thermal behavior ICSO 2012 MADRAS 5

6 FEA design Optimization of the mirror s geometry for each specified modes Minimization of the residual wave front Minimization of the level of stress in Zerodur Optimization based on the system s Influence Functions (IF) ICSO 2012 MADRAS 6

7 Interferometrical results Measurement of the optical surface deformation with a Fizeau interferometer Same approach than with FEA IF measurements Specified mode correction on the measured base Deduction of the correction capabilities Residual wave front obtained for the correction of each mode at their maximum amplitude FEA Interferometer Validation of the opto mechanical concept Correspondance between simulations and measurements Efficient correction performance => goal for the active correcting loop ICSO 2012 MADRAS 7

8 Active loop testing: design Test of the correcting system in representative conditions Telescope simulator: WFE injection Active correcting loop: WFE correction PSF imaging: visualization before and after correction Interferometry: surface deformation monitoring ICSO 2012 MADRAS 8

9 Calibration Interaction Matrix (IM): Influence Functions measurement with WFS => Command Matrix (CM) CM~IM 1 Independent gestion of Tip, Tilt, Focus: add of 3 virtual IF Filtering of noisy Eigen Mode ICSO 2012 MADRAS 9

10 Mode correction Injection and correction of each specified mode Astm3&5, Tref5&7, Tetraf7&9: correction within the 5 nm rms specifications Pentaf9: 7 nm rms residuals, due to symmetry mismatch Coma3, Sphe3: 6 8 nm rms residuals, due to Command Matrix => can be improved Residual wave front obtained for the correction of each mode at their maximum amplitude FEA Interferometer Loop performance ICSO 2012 MADRAS 10

11 Injected WFE 150 nm rms Example: Astigmatism3 correction PSF before correction Commands Residual WFE 3.3 nm rms PSF after correction Mirror deformation (interf) 76 nm rms Residues expected from FEA 2.5 nm rms ICSO 2012 MADRAS 11

12 Global WFE correction Expected WFE = combination of specified Zernike modes System linearity is verified => sum of the individual performance Active loop performance: measured residual WFE [nm rms] Residues expected from interferometric measurements [nm rms] Worst case Representative case Example of random WFE correction ICSO 2012 MADRAS 12

13 Dead actuator study A dead actuator = free actuation point Simulation: projection of the WFE on the 23 remaining IF Impact depends on the actuator location and on the WFE No dead actuator 1 dead actuator (avg) Errorbar: 1 dead act (min and max) Several dead actuators Correction precision < 10 nm rms up to 2 dead actuators Residual wave front for each specified mode (FEA results) 24 actuators for 17 modes to correct => redundancy Expected performance vs number of dead actuators ICSO 2012 MADRAS 13

14 Conclusion Demonstrator of an active mirror for space Compensation of large lightweight mirror s deformation in space Performance experimental demonstration => Correction of the expected WFE with a mean precision of 8 nm rms System advantages Light (4 kg) and compact (80 mm height, 130 mm diameter) Only 24 actuators, with redundancy Possibility to change the actuator technology Next steps End of MADRAS project => correcting mirror at TRL 4/5 Qualification in space environment (2013) => TRL5 Wave front analysis studies ICSO 2012 MADRAS 14