Adaptive optics

Using Ferro-fluidsMathieu De Goer-de Herve and Raphael-David Lasseri

(ENS Cachan)

Resolution in optics

• Two limitant factors :→ Diffraction

→ Atmosphere turbulences : speckle patterns.

Characteristic length DAiry disc : Ø=λxf/D

Characteristic length dc (correlation)dc~10cmDisc : Ø=λxf/dc

D

Ø

What solution?

• High places

• Adaptive optics

How?

● Mechanics deformation of a “usual” mirror using piezoelectric devices

How?● Mechanics deformation of a “usual” mirror using piezoelectric devices

● Shack-Hartmann wavefront sensor

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.

Field of the study

Rosenweig instability • Domain of Stability

“Smooth surface”

• Domain of the RW Instability

“Hedgehog surface”

B>Bc B<Bc

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

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)

Experiment VS Theory

Numerical approach • Deformation of the fluid above the

magnet

Physical Experiment • Ferrofluid sample subjected to a strong

magnet

Validation of the model

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

Applications to adaptive optics • Generic principle of adaptive optics

Some examples • Results using classic adaptive

Final Results (NGC 7469 Galaxy)

Liquid Mirror

• A well-know Patent (Ernesto Capocci -1856)