Géraldine Guerri Post-doc ARC @ CSL 4 March 2011 First results of the CSL piston sensor breadboard and further application Géraldine Guerri Post-doc ARC @ CSL Liège Space Center, Angleur 4 March 2011

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 E-ELT (Europe) 42 m diameter 1000 segments GMT (USA) 25 m diameter 7 segments TMT (Europe) 30 m diameter 492 segments Géraldine Guerri 4 March 2011 2

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 March 2011 3

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 4 March 2011 4

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 Piston measurement Tip/tilt measurement Range: ± 1 mm Range: 100 µrad Accuracy: 0.5 µrad Accuracy: 50 nm Géraldine Guerri 4 March 2011 5

Cophasing sensor architecture PISTON TIP-TILT COARSE PHASING Cf JF Simar presentation FINE COPHASING Error < λ/2 Phase retrieval real-time algorithm (Baron et al., 2008) Shack-Hartmann Sensor or Phase diversity real-time algorithm (Mocoeur et al., 2008) Géraldine Guerri 4 March 2011 6

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 Focal plane image Phase retrieval algorithm ? Géraldine Guerri 4 March 2011 7

Phase retrieval algorithm Without Piston error Differential Piston errors can be determined from the intensity of peaks of the phase of the OTF OTF Modulus OTF Phase PSF 3 aperture pupil With Piston error Géraldine Guerri 4 March 2011 8

Piston sensor validation breadboard Géraldine Guerri 4 March 2011 9

Piston sensor breadboard optical setup Laser He-Ne λ= 633 nm P1 Creation of collimated beam O1 3 sub-apertures Pupil mask These mirors reflect only 2 sub-pupils over 3 MP M4 M5 Delay line to compensate the OPDs between the 2 paths M1 M2 M3 L1 O2 M6 CCD PZT S1 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 4 March 2011 10

Piston sensor experimental results Application of a piston ramp on a sub-aperture : Géraldine Guerri 4 March 2011 11

Piston sensor experimental results Metrological standards obtained from measurements : Results presented at SPIE conference « Astronomical Telescopes and Instrumentation 2010 » Measurement range [ -λ/2 , λ/2 ] Linearity >0.92 (best 0.96) Resolution < 20 nm Deviation around zero point < 10 nm Absolute error ±25 nm Géraldine Guerri 4 March 2011 12

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 4 March 2011 13

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 4 March 2011 14

Application Cophasing of 3 silicon bimorph mirors developed at ULB (Rodrigues et al., 2009) Géraldine Guerri 4 March 2011 15

Cophasing demonstrator principle 3 segments deformable mirror demonstrator Collimated beam (Φ=130mm) Piston sensor Illuminating system Géraldine Guerri 4 March 2011 16

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 4 March 2011 17

3 segment cophasing demonstrator Calibration tests in progress in Liège … Validation tests in Bruxelles very soon .. Illuminating system Piston sensor Géraldine Guerri 4 March 2011 18

Thanks for your attention Géraldine Guerri 4 March 2011 19

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 4 March 2011 20

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Sensor techniques selection 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 4 March 2011 22

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