1. Adaptive optics Using Ferro-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 = λx f/D Characteristic length dc (correlation) dc~10cm Disc : Ø = λx f/dc D 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 Stability “Smooth surface” Domain of the RW Instability “Hedgehog surface” B>Bc B<Bc

9. 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) MHD Fundamentals Equations

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 Deformation of the fluid above the magnet Physical Experiment 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)