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First steps of the development of a cophasing sensor for synthetic aperture optics applications Géraldine GUERRI Post-Doc ARC @ CSL Cophasing sensor for synthetic aperture optics applications 19 February 2009 Framework : Extremely Large Telescopes (ELT) • Ground-Based Large telescopes projects : E-ELT (Europe) 42 m diameter 1000 segments GMT (USA) 25 m diameter 7 segments TMT (Europe) 30 m diameter 492 segments • Space telescopes projects : – JWST : 18 segments 6.5m aperture, 25 kg/m² density – Increasing demand for larger apertures : 20m diameter, 6 kg/m² density Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Large lightweight telescope in space • Technological need : large diameter deployable lightweight cheap space mirrors • Critical questions : • How manufacturing this kind of mirror ? • How controling the mirror wavefront error ? • How aligning coherently the sub-apertures between each other? • My work and CSL concern : development a demonstrator breadboard of a cophasing sensor for space segmented mirrors made with 3 or 7 segments Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Cophasing sensor • Measurement the relative positioning of each subaperture : determination of piston and tip-tilt errors Piston : Translation along the optical axis (λ or nm) Tip/ Tilt : Rotation of the sub-pupil perpendicular to the optical axis (rad or arsec) • 2 phasing regimes to consider : – Coarse phasing in open loop – Fine phasing in closed loop : error < λ/2 Increase sensor complexity Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 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 Range: 100 µrad Accuracy: 50 nm Accuracy: 0.5 µrad • At longer term use of integrated optical components Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 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 Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Review of the state of art of cophasing sensor • Survey of 15 different principles : Pupil plane detection sensor Focal plane detection sensor Slope measurement : Shack-Hartmann sensor, Pyramidal sensor Dispersed fringe sensor, Phase shifting interferometer Curvature sensor Phase retrieval/Phase diversity algorithm • Trade-off criteria : • best compliance with the requirements • sensor maturity • breadboard feasibility within a short term Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Cophasing sensor : methods selection Coarse phasing PISTON TIP-TILT Dispersed fringe sensing (CSL : Roose et al, 2006) Shack-Hartmann Sensor (Shack & Platt, 1971) Fine cophasing Shack-Hartmann Sensor Phase retrieval real-time algorithm or Error < λ/2 (Baron et al., 2008) Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI Phase diversity real-time algorithm (Mocoeur et al., 2008) 19 February 2009 Phase retrieval algorithm • Phase errors extracted from one simple focal image • The problem to solve is highly non linear • Classical Phase retrieval algorithm are iterative and time consuming (~ 60 FFT computations) • (Baron et al., 2008) : For fine cophasing (Piston < λ/2), analytical and real-time solutions exists (only one FFT computation) • Based on Optical Transfert Function (OTF) Computation Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Numerical validation of the phase retrieval algorithm for piston estimation 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 sub-aperture pupil With Piston error Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Phase retrieval algorithm numerical validation • Algorithm validation • Test of the sensor linearity 300 Différence de piston calculées (nm) -----------------------------------------------p1-p2 : -50 p1-p3 : -150 200 Valeur de piston obtenue (nm) Valeurs des pistons introduits (nm) ----------------------------------------------p1 : -50 p2 : 0 p3 :100 100 0 -100 -200 p2-p3 : -100 -300 -400 -300 -200 -100 0 100 200 300 400 Valeur de piston introduite (nm) • Algorithm Computation time (MATLAB) : 0.4s Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Phase retrieval demonstrator set-up Collimating Lens Window of known thickness f=50mm Laser diode Beam expander Focusing Lens f=300mm λ=633nm CCD Camera Pinhole Pupil mask Implementation in laboratory in progress …. Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Future prospects • Experimental feasibility tests of the PR method • Optimisation of the PR algorithm • Study and design of a system to introduce various and precise piston values • Implementation of the coarse piston sensor • Design and implementation of the tip-tilt measurement Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Outlook • Tests of the preliminary sensor performances in open & closed loop • Study and design of a compact and spacecompatible sensor with fibered and integrated optics • Implementation, validation and performance assessment of this cophasing sensor Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Thanks for your attention Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Différence de piston calculées (nm) -----------------------------------------------p1-p2 : -50 p1-p3 : -150 p2-p3 : -100 Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Phase retrieval demonstrator breadboard Shack Hartmann Sensor : CCD Camera Atmel : • 2048x2048 pixels • 101 x 101 MicroLens • 7.4 µm x 7.4 µm pixels • λ/10 resolution • 10 bits dynamics PHOTO Implementation in progress …. Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 ULB Tip tilt controller AMOS Tip Tilt actuator Thales Piston Sensor CSL Microlenses ULB Shack-Hartman system DSP controller CSL Camera + source ULB S/W Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Measuring steps • Piston measurement : – Phase retrieval (PR) setup – Large amplitude piston : central fringe identification from visibility estimation – Small amplitude piston : accurate phase measurement by PR • Tip-tilt measurement : – Shack-Hartmann Wavefront Sensor Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Valeurs des pistons introduits (nm) ----------------------------------------------p1 : -50 p2 : 0 p3 :100 Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Framework • Today’ s astronomy needs extremely large telescope (High FOV, high resolution) with huge diameter >30m • Technological solutions – Large segmented telecopes – Multiple aperture telescopes Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Project presentation - How to build : large diameter deployable mirrors lightweight cheap – Collaboration between CSL, SCMERO Laboratory (Brussels University), AMOS & Thales – The goal of the project is to develop a demonstrator with 3 (7 design goal) segments λ/10 mirror – One of the critical issues is the control of the WFE of the system Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Numerical simulations • Validation of two algorithms : – Dispersed speckle piston sensor .. in progress Problems with sensor linearity – Real-time phase retrieval algorithms Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Lightweight space deformable mirror : project work plan Management, reporting and support WP 0000 CSL Overal Project Management and reporting WP 0100 CSL ULB Management and Reporting WP 0200 ULB Thales Management and Reporting WP 0300 Thales AMOS Management and Reporting WP 0400 AMOS AIV and testing WP 4000 CSL AIV WP 4100 AMOS Functional and performance testing WP 4200 CSL Deformable mirror preliminary design WP 1000 ULB Deformable mirror Breadboard detailled design WP 2000 CSL Procurement and bread board manufacturing WP 3000 AMOS Review the state-of-the-art of cophasing methods and selection Review the state -of-the-art in piezo actuator control methods and selection Wafer technology review and preliminary concept WP 1100 CSL WP 1200 WP 1300 Thales ULB Detailled design of the cophasing and WFS sub systems WP 2100 CSL Manufacturing of the cophasing and WFS sub systems WP 3100 CSL Preliminary opto mechanical concept WP 1400 AMOS Detailled design of the piezo control sub systems WP 2200 Thales Wafer detailled design WP 2300 ULB Opto mechanical detailled design WP 2400 AMOS Manufacturing of the piezo control sub systems WP 3200 Thales Manufacturing of Wafer subsystem WP 3300 ULB Manufacturing of Opto mechanical subsystem WP 3400 AMOS Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI Rigid mode control and control strategies overview WP 1500 ULB Rigid mode control and control strategies detailled design WP 2500 ULB Control algorithm development WP 3500 Guidelines and Recommenda tions for Future Work WP 5000 ULB Test Results and Synthesis Report WP 4300 ULB Establish Guidelines WP 5100 XXX Critical issue : the manufacturing of the sub-system dedicated to cophasing and the wavefront sensor of the mirror ULB 19 February 2009 Cophasing sensor selection Piston measurement • Focal-Plane WFS are very appealing: – Single/multi- aperture, simple hardware – Real-time algorithms exists (Baron et al., 2008 Mocoeur et al., 2008) – Performance experimentaly demonstrated at ONERA Complexity is transferred from hardware to software Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Cophasing sensor selection Tip-Tilt measurement • Shack-Hartmann Wavefront sensor available at CSL • Analytical and real time Phase retrieval algorithm Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Piston – Tip/Tilt definition Piston : Change of poistion along the Z axis (λ or nm) Z Tip : Rotation of the surface around the Y axis (rad or arsec) Tilt : Rotation of the surface around the X axis (rad or arsec) Y X Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Review of the state of art of cophasing sensor Sensor type Pupil plane detection sensors Parameter/ Method Shack-Hartmann Sensor Curvature Sensor Dispersed fringe sensing Dual Wavelength instantaneous Phase-shifting interferometer for close-loop control "Classical" phase retrieval "Real-time" phase retrieval "Classical" phase diversity Piston measurement ? NO (except for E-ELT) NO (except for E-ELT) YES YES YES YES YES Tip-tilt measurement ? YES NO MULTI YES 3 DOF for each 61 segments : 183 YES #DOF YES 3DOF for each segment YES YES but with restrictions with combined piston >11 >11 High Pyramid Sensor YES Keck narrowband Algorithm (Chanan et al., 2000) N/A YES Gradient-based iterative Analytic Least-Square Gradient-based method approach iterative method Algorithm N/A N/A N/A Data processing Slope integration Poisson's equation Slope integration FFT 2D Time to compute 100N FFT 2D on the zone of interest for N diversity focal images High FFT 2D Speed Fast : up to 950 Hz (depends on configuration) Slow : up to 5 Hz Fast : 400 Hz Low 4.44 Hz Time to compute between 20 and 60 FFT Time to compute 1 2D on the zone of FFT 2D on the zone of interest interest #Apertures High High High 12 High High TBC already ? 12 FFT 2D Monochromatic or Polychromatic Monochromatic or Polychromatic Visible or NIR Polychromatic 2 superluminescent diodes at 834.6 nm & 859.6 nm High flux needed H/W detector CCD Avalanche photodiode (bulky & expensive) or CCD CCD 1024x1024 CCD 4 CCD Optical complexity Medium up to 1500λ Medium |Piston| < λ/2 ; High for tilt Medium Range Medium |Piston| < λ/2 ; High for tilt Linearity Close to 1 TBD TBD 0.98 High Simple Piston: ± 7.2 µm ; Tip- [-λ/2 ; λ/2] for piston and tilt: ± 250 µrad tilt 0,99 for Piston ; 0.93 for Tilt TBD Simple λ/2 < Piston < λ/2 ; |Tilt| < 0.3λ 0,96 for piston ; 1,15 for Tilt Measurement accuracy From λ/100 to λ/1000 less than 15 nm rms λ/30 rms 90 nm better than λ/13 rms Repeatability < λ/200 rms TBD TBD TBD TBD Piston : 0.75 nm for 5.105 photoe- ; Tilt : Piston: 0.48 nm rms ; 1.21 nm for 3.3 104 Tip-tilt: 74 nrad rms photoe- TBD Piston : 0.75 nm for 5.105 photoe- ; Tilt : 1.21 nm for 1.6 105 photoe- Particularities Cylindrincal microlenses on the E-ELT sensor to measure piston steps Restricted to spherical shapes that allow operation Excellent for phase without gigantic beam reconstruction but time expander consuming Need for diluted noncentrosymmetric pupil ; Small phase Excellent for phase aberrations reconstruction but time assumption (<2π rad) consuming Fine Phasing H/W source Trade-off criteria Focal plane detection sensors Type of achievable phasing : Cophasing (fine) or coherencing coarse Type of telescopes that can be controlled 1 µm to 16 µm Standard CCD Flux of 105 e-/frame White light source + 1 narrow band filter (λc = 650 nm, Δλ = 40 nm) High flux needed Standard CCD ; Flux of 105 e-/frame Both Both Both Coarse Phasing Fine Phasing Both All Segmented Segmented Segmented Segmented Multi-aperture ; Segmented Multi-aperture Maturity Well know and manufactured in mass ; The most used sensor Preliminary laboratory Preliminary laboratory results and upcoming results and upcoming on-sky tests on the VLT on-sky tests on the VLT & LBT Instrument controlled by this method or envisaged to be Keck, VLT/NACO, Gemini North and South, Subaru, VLT/MACAO, MMT, Palomar, Lick, E- Gemini South/NICI, ELT, Grantecan Grantecan, E-ELT ? LBT, WHT ?, E-ELT ? JWST ; Keck Main references Platt & Shack, 1971 Ragazzoni, 1996 Fogale Nanotech: F. Baron Thesis, Shi et al., Applied Optics Wilhem et al., Applied ONERA, 2005 ; Baron 2004 Optics 2008 et al., JOSA 2008 Cophasing sensor for synthetic aperture optics applications Roddier, 1988 Preliminary laboratory results and upcoming Testbed results ; On sky on-sky tests on the Laboratory tests in test on Keck VLT progress Géraldine GUERRI E-ELT JWST ; Point-source and extended scenes observations Laboratory tests in progress Defocus generator + Standard CCD Simple if only 2 phase diversity images are needed [-λ/2 ; λ/2] for piston and tilt 0.99 for piston and tilt TBD Piston : 0.75 nm for 2.105 photoe- ; Tilt : 1.21 nm for 1.1 104 photoe- Both Multi- Aperture ; Segmented Laboratory tests in progress Darwin, SOTTISE (Earth Observation) JWST ; Earth observation F. Baron Thesis, F. Baron Thesis, ONERA, 2005 ; ONERA 2005 ; Baron Delavacquerie et al., et al., JOSA 2008 2008 19 February 2009 Plan • • • • • • Framework and project presentation State of the art of the cophasing sensors Sensor selection Numerical simulations of the selected sensors Sensor Feasibility demonstration breadboard Future propects Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009 Cophasing sensor selection Piston measurement • Focal-Plane WFS are very appealing: – Single/multi- aperture, simple hardware – Real-time algorithms exists (Baron et al., 2008 Mocoeur et al., 2008) – Performance experimentaly demonstrated at ONERA Complexity is transferred from hardware to software – Multiple aperture piston/tip/tilt/more (DWARF) Cophasing sensor for synthetic aperture optics applications Géraldine GUERRI 19 February 2009