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First steps of the development of a cophasing sensor for synthetic aperture optics applications

First steps of the development of a cophasing sensor for synthetic aperture optics applications. Géraldine GUERRI. Post-Doc ARC @ CSL. Ground-Based Large telescopes projects : Space telescopes projects : JWST : 18 segments 6.5m aperture, 25 kg/m² density

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First steps of the development of a cophasing sensor for synthetic aperture optics applications

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  1. First steps of the development of a cophasing sensor for synthetic aperture optics applications Géraldine GUERRI Post-Doc ARC @ CSL

  2. Ground-Based Large telescopes projects : Space telescopes projects : JWST : 18 segments 6.5m aperture, 25 kg/m² density Increasing demand for larger apertures : 20m diameter, 6 kg/m² density Framework : Extremely Large Telescopes (ELT) GMT (USA) 25 m diameter 7 segments 30 m diameter 492 segments TMT (Europe) E-ELT (Europe) 42 m diameter 1000 segments

  3. 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

  4. Measurement the relative positioning of each subaperture : determination of piston and tip-tilt errors Cophasing sensor Piston : Translation along the optical axis (λ or nm) • 2 phasing regimes to consider : • 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) Increase sensor complexity

  5. 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 At longer term use of integrated optical components Sensor requirements • Tip/tilt measurement • Piston measurement

  6. 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

  7. 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 • Curvature sensor • Dispersed fringe sensor, Phase shifting interferometer • Phase retrieval/Phase diversity algorithm • Trade-off criteria : • best compliance with the requirements • sensor maturity • breadboard feasibility within a short term

  8. Cophasing sensor : methods selection

  9. 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 Phase retrieval algorithm

  10. 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 OTF Phase OTF Modulus PSF 3 sub-aperture pupil With Piston error

  11. Algorithm validation Phase retrieval algorithm numerical validation • Test of the sensor linearity Valeurs des pistons introduits (nm) ----------------------------------------------- p1 : -50 p2 : 0 p3 :100 Différence de piston calculées (nm) ------------------------------------------------ p1-p2 : -50 p1-p3 : -150 p2-p3 : -100 • Algorithm Computation time (MATLAB) : 0.4s

  12. Phase retrieval demonstrator set-up Window of known thickness Collimating Lens f=50mm Beam expander Focusing Lens f=300mm CCD Camera Laser diode λ=633nm Pinhole Pupil mask Implementation in laboratory in progress ….

  13. 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

  14. Tests of the preliminary sensor performances in open & closed loop Study and design of a compact and space-compatible sensor with fibered and integrated optics Implementation, validation and performance assessment of this cophasing sensor Outlook

  15. Thanks for your attention

  16. Différence de piston calculées (nm) ------------------------------------------------ p1-p2 : -50 p1-p3 : -150 p2-p3 : -100

  17. PHOTO Phase retrieval demonstrator breadboard • Shack Hartmann Sensor : • 101 x 101 MicroLens • λ/10 resolution • CCD Camera Atmel : • 2048x2048 pixels • 7.4 µm x 7.4 µmpixels • 10 bits dynamics Implementation in progress ….

  18. 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 Measuring steps

  19. Valeurs des pistons introduits (nm) ----------------------------------------------- p1 : -50 p2 : 0 p3 :100

  20. Today’ s astronomy needs extremely large telescope (High FOV, high resolution) with huge diameter >30m Technological solutions Large segmented telecopes Multiple aperture telescopes Framework

  21. - 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 Project presentation

  22. Validation of two algorithms : Dispersed speckle piston sensor .. in progress Problems with sensor linearity Real-time phase retrieval algorithms Numerical simulations

  23. Lightweight space deformable mirror : project work plan • Critical issue : the manufacturing of the sub-system dedicated to cophasing and the wavefront sensor of the mirror

  24. 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

  25. Shack-Hartmann Wavefront sensor available at CSL Analytical and real time Phase retrieval algorithm Cophasing sensor selection Tip-Tilt measurement

  26. Z Y X Piston – Tip/Tilt definition Piston : Change of poistion along the Z axis (λ or nm) Tip : Rotation of the surface around the Y axis (rad or arsec) Tilt : Rotation of the surface around the X axis (rad or arsec)

  27. Review of the state of art of cophasing sensor Sensor type Trade-off criteria

  28. 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 Plan

  29. 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 • Multiple aperture piston/tip/tilt/more (DWARF) • Multiple aperture piston with extended scenes Complexity is transferred from hardware to software

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