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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 : E-ELT (Europe) 42 m diameter 1000 segments GMT (USA) 25 m diameter 7 segments TMT (Europe) 30 m diameter 492 segments • In space : – JWST : 18 segments, 6.5m aperture, 25 kg/m² density – Increasing demand for larger apertures : 20m diameter, 3 kg/m² density First results of CSL piston sensor breadboard Géraldine Guerri 4 March 2011 2 Large lightweight space telescope • Technological need : • Critical issues : large diameter deployable lightweight cheap space mirrors » 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 First results of CSL piston sensor breadboard 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 Tip/ Tilt : Rotation of the sub-pupil perpendicular to the optical axis (rad or arsec) First results of CSL piston sensor breadboard Géraldine Guerri – Fine phasing in closed loop : error < λ/2 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: 50 nm Accuracy: 0.5 µrad First results of CSL piston sensor breadboard Géraldine Guerri 4 March 2011 5 Cophasing sensor architecture PISTON TIP-TILT COARSE Cf JF Simar presentation PHASING Shack-Hartmann Sensor FINE Phase retrieval real-time algorithm or COPHASING Error < λ/2 First results of CSL piston sensor breadboard (Baron et al., 2008) Géraldine Guerri Phase diversity real-time algorithm (Mocoeur et al., 2008) 4 March 2011 6 Why phase retrieval technique ? • Phase error extracted from one focal image Focal plane image Object Phase retrieval algorithm ? • Baron et al., 2008 : For fine cophasing (error < λ/2), analytical and real-time solutions exists Only ONE FFT computation needed First results of CSL piston sensor breadboard Géraldine Guerri 4 March 2011 7 Phase retrieval algorithm Differential Piston errors can be determined from the intensity of peaks of the phase of the OTF Without Piston error PSF OTF Modulus OTF Phase 3 aperture pupil With Piston error First results of CSL piston sensor breadboard Géraldine Guerri 4 March 2011 8 Piston sensor validation breadboard First results of CSL piston sensor breadboard Géraldine Guerri 4 March 2011 9 Piston sensor breadboard optical setup P Laser He-Ne λ= 633 nm Creation of collimated beam P1 O1 These mirors reflect only 2 subpupils over 3 3 sub-apertures Pupil mask MP M1 M2 M4 M5 M3 L1 Delay line to compensate the OPDs between the 2 paths O2 M6 S1 PZT Miror + PZT : Introduction of a piston error on 1 sub-aperture First results of CSL piston sensor breadboard Beamsplitter : reformation of the pupil with 3 sub-apertures Géraldine Guerri CCD Piston sensor components 4 March 2011 10 Piston sensor experimental results • Application of a piston ramp on a sub-aperture : First results of CSL piston sensor breadboard Géraldine Guerri 4 March 2011 11 Piston sensor experimental results • Metrological standards obtained from measurements : [ -λ/2 , λ/2 ] Measurement range Linearity >0.92 (best 0.96) Resolution < 20 nm Deviation around zero point < 10 nm Absolute error ±25 nm • Results presented at SPIE conference « Astronomical Telescopes and Instrumentation 2010 » First results of CSL piston sensor breadboard 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 First results of CSL piston sensor breadboard 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 First results of CSL piston sensor breadboard Géraldine Guerri 4 March 2011 14 Application • Cophasing of 3 silicon bimorph mirors developed at ULB (Rodrigues et al., 2009) First results of CSL piston sensor breadboard Géraldine Guerri 4 March 2011 15 Cophasing demonstrator principle 3 segments deformable mirror demonstrator Collimated beam (Φ=130mm) Piston sensor First results of CSL piston sensor breadboard 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 He-Ne Laser (λ=633 nm) • 1 pupil imaging camera • 1 Microscope objective • 1 imaging lens • 1 Pinhole (Φ=15 µm) • 1 Off-axis parabola First results of CSL piston sensor breadboard Piston sensor Géraldine Guerri • 1 focal image camera • 1 computer 4 March 2011 17 3 segment cophasing demonstrator • Calibration tests in progress in Liège … Illuminating system Piston sensor • Validation tests in Bruxelles very soon .. First results of CSL piston sensor breadboard Géraldine Guerri 4 March 2011 18 Thanks for your attention First results of CSL piston sensor breadboard Géraldine Guerri 4 March 2011 19 3 segment cophasing demonstrator 3 segments mirror Illuminating system • 1 He-Ne Laser (λ=633 nm) • 1 Microscope objective • 1 Pinhole • 1 Off-axis parabola Piston sensor • Calibration tests in progress in Liège… • Validation tests in Bruxelles very soon • 1 pupil mask with 3 sub-apertures • 1 beamsplitter (90/10) • 1 pupil imaging camera • 1 imaging lens • 1 focal image camera • 1 computer First results of CSL piston sensor breadboard Géraldine Guerri 4 March 2011 20 First results of CSL piston sensor breadboard Géraldine Guerri 4 March 2011 21 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 First results of CSL piston sensor breadboard Géraldine Guerri 4 March 2011 22 First results of CSL piston sensor breadboard Géraldine Guerri 4 March 2011 23