First results of the CSL piston sensor breadboard and further application

1. First results of the CSL piston sensor breadboard and further application Géraldine Guerri Post-doc ARC @ CSL Liège Space Center, Angleur4 March 2011

2. Framework : Extremely Large Telescopes (ELT) • On the ground : • In space : • JWST : 18 segments, 6.5m aperture, 25 kg/m² density • Increasing demand for larger apertures : 20m diameter, 3 kg/m² density GMT (USA) 25 m diameter 7 segments 30 m diameter 492 segments E-ELT (Europe) TMT (Europe) 42 m diameter 1000 segments Géraldine Guerri

3. Large lightweight space telescope • Technological need : • Critical issues : • manufacturing • wavefront error control • sub-aperture coherent alignement • CSL concern : • Developpement of demonstrator breadboard of a cophasing sensor for space segmented mirrors made with 3 or 7 segments • large diameter • deployable • lightweight • cheap space mirrors Géraldine Guerri

4. What is a cophasing sensor ? • Measure the relative positioning of each subaperture : determination of piston and tip-tilt errors Piston : Translation along the optical axis (λ or nm) • 2 phasing regimes : • Coarse phasing in open loop • Fine phasing in closed loop : error < λ/2 Tip/Tilt : Rotation of the sub-pupil perpendicular to the optical axis (rad or arsec) Géraldine Guerri

5. Sensor requirements • Cophasing of 3 to 7 sub-apertures • Separate measurement of piston and tip/tilt • Low weight and Compacity • Real-time correction • Reduced hardware complexity • Linearity, High range and accuracy • Tip/tilt measurement • Piston measurement Range: ± 1 mm Accuracy: 50 nm Géraldine Guerri

6. Cophasing sensor architecture Géraldine Guerri

7. Why phase retrieval technique ? • Phase error extracted from one focal image • Baron et al., 2008 : For fine cophasing (error < λ/2), analytical and real-time solutions exists Only ONE FFT computation needed Object Phase retrieval algorithm Focal plane image ? Géraldine Guerri

8. Phase retrieval algorithm Differential Piston errors can be determined from the intensity of peaks of the phase of the OTF Without Piston error OTF Phase OTF Modulus PSF 3 aperture pupil With Piston error Géraldine Guerri

9. Piston sensor validation breadboard Géraldine Guerri

10. Piston sensor breadboard optical setup P Creation of collimated beam Laser He-Ne λ= 633 nm P1 O1 3 sub-apertures Pupil mask These mirors reflect only 2 sub-pupils over 3 M4 M5 MP Delay line to compensate the OPDs between the 2 paths M1 M2 M3 O2 L1 M6 CCD S1 PZT Miror + PZT : Introduction of a piston error on 1 sub-aperture Beamsplitter : re-formation of the pupil with 3 sub-apertures Piston sensor components Géraldine Guerri

11. Piston sensor experimental results • Application of a piston ramp on a sub-aperture : Géraldine Guerri

12. Piston sensor experimental results • Metrological standards obtained from measurements : • Results presented at SPIE conference « Astronomical Telescopes and Instrumentation 2010 » Géraldine Guerri

13. Feedbacks from experimental tests • Dependance of the phase measurements accuracy on : • the wavefront error of each beam until the common path (<λ/10 rms) • the set-up stability (vibration and drift during the measurement) • the PSF pattern (‘‘fringe’’) contrast • The beam coherence • The image quality of the imaging lens Géraldine Guerri

14. Future prospects • Experimental feasibility tests of the Phase Retrieval technique with a 7 sub-apertures system • Study and design of a system to introduce various and precise piston values • Design and implementation of the coarse piston sensor (cf JF Simar PhD) • Design and implementation of the tip-tilt measurement Géraldine Guerri

15. Application • Cophasing of 3 silicon bimorph mirors developed at ULB (Rodrigues et al., 2009) Géraldine Guerri

16. Cophasing demonstrator principle 3 segments deformable mirror demonstrator Collimated beam (Φ=130mm) Piston sensor Illuminating system Géraldine Guerri

17. Optical simulation of the cophasing demonstrator • Development of an end-to-end simulation (Matlab, ASAP) Illuminating system Piston sensor Illuminating system • 1 pupil mask with 3 sub-apertures • 1 beamsplitter (90/10) • 1 pupil imaging camera • 1 imaging lens • 1 focal image camera • 1 computer • 1 He-Ne Laser (λ=633 nm) • 1 Microscope objective • 1 Pinhole (Φ=15 µm) • 1 Off-axis parabola Piston sensor Géraldine Guerri

18. 3 segment cophasing demonstrator • Calibration tests in progress in Liège … • Validation tests in Bruxelles very soon .. Illuminating system Piston sensor Géraldine Guerri

19. Thanks for your attention Géraldine Guerri

20. 3 segment cophasing demonstrator 3 segments mirror Illuminating system Piston sensor • 1 pupil mask with 3 sub-apertures • 1 beamsplitter (90/10) • 1 pupil imaging camera • 1 imaging lens • 1 focal image camera • 1 computer • 1 He-Ne Laser (λ=633 nm) • 1 Microscope objective • 1 Pinhole • 1 Off-axis parabola • Calibration tests in progress in Liège… • Validation tests in Bruxelles very soon Géraldine Guerri

21. Géraldine Guerri

22. Work plan Survey of state of the art of cophasing sensor Sensor techniques selection Validation by numerical simulations Experimental validation Feasibility demonstrator of the cophasing of 3 sub-apertures with standard optical components Study and Design of a space-compatible breadboard Géraldine Guerri

23. Géraldine Guerri