1. Adaptiveoptics UsingFerro-fluids Mathieu De Goer-de Herve and Raphael-David Lasseri (ENS Cachan)

2. Resolution in optics • Two limitant factors : → Diffraction → Atmosphere turbulences : speckle patterns. Characteristic length D Airy disc : Ø=λxf/D D Characteristic length dc (correlation) dc~10cm Disc : Ø=λxf/dc Ø

3. What solution? • High places • Adaptive optics

4. How? • Mechanics deformation of a “usual” mirror using piezoelectric devices

5. How? • Mechanics deformation of a “usual” mirror using piezoelectric devices • Shack-Hartmann wavefront sensor

6. Ferrofluids • General definition: Colloidal suspensions of magnetic nanoparticles conferring super-paramagnetic properties to the fluid. When a magnetic field is applied to the system their magnetic moments tend to align along the applied field, leading to a net magnetization.

7. Field of the study

8. Rosenweig instability • Domain of the RW Instability “Hedgehog surface” • Domain of Stability “Smooth surface” B>Bc B<Bc

9. MHD Fundamentals Equations Approximations : We will neglect the influence of the capillary forces in this stable domain, 3 Major Factors: - Gravity • Magnetostrictive Pressure • Laplace Forces When the Equilibrium is reached -> Bernouilli Generalised Equation (1)

10. Numerical Resolution • Finite Element Method applied to model the interaction of the field and the fluid. Field of a cylindric shaped magnet Height of the ferrofluid sample (From Top)

11. Experiment VS Theory Numerical approach Physical Experiment • Deformation of the fluid above the magnet • Ferrofluid sample subjected to a strong magnet

12. Validation of the model

13. Classic methods • Mechanics deformation of a “usual” mirror using piezoelectric devices

14. Applications to adaptive optics • Generic principle of adaptive optics

15. Some examples • Results using classic adaptive Final Results (NGC 7469 Galaxy)

16. Liquid Mirror • A well-know Patent (Ernesto Capocci -1856)