Introduction

A deformable mirror (DM) is the central element of an adaptive optics system. The DM changes shape to correct for a phenomenon known as atmosphering seeing. Atmospheric seeing is the result of air masses of different densities (and thus of different refractive indices) causing uneven deformations in incoming wavefronts. The same phenomenon causes the stars to twinkle and is responsible for the "mirage" effect that can be seen over hot pavement. Atmospheric seeing is the main limiting factor when it comes to image quality for earth-based observatories. The use of adaptive optics allows the observatory to compensate for these wave deformations by changing the shape of the primary mirror in real time.

Traditional DMs are made with glass or metal faceplates deformed by a number of actuators. These mirrors are very precise, but generally very expensive. The advantage of liquid mirrors is that they provide a high quality, seamless optical surface while absolutely no mechanical polishing is required. The aim of this project is to take advantage of these properties to make a low cost DM.

In previous attempts to deform liquid mirrors, the goal was generally to modify the shape of the mirror as a whole and not make the kind of rapidly changing wavefront corrections needed for adaptive optics. These attempts focused on mercury, as was used in the first generation of rotating liquid mirrors at Laval.

The problem with using mercury as a DM is that because of its high density, a substantial force is needed to deform it. Using magnetic liquids (ferrofluids) represents a solution to this drawback.


What is a ferrofluid?

A ferrofluid is a colloidal suspension of magnetic (ferrimagnetic or ferromagnetic) nanoparticles with sizes in the range between 3 and 20 nanometers in a carrier liquid. Particles have no natural affinities for liquids. It is thus essential to introduce a stabilising agent (surfactant) to increase the solubility of particles in the chosen carrier liquid and also to ensure repulsive forces between particles to prevent agglomeration. In the presence of an external magnetic field, these magnetic particles align themselves with the field and the bulk of the liquid becomes magnetized. The surface of the liquid can thus be shaped according to the magnetic field geometry. We use this property to shape the liquid surface as we would do with conventionnal deformable mirrors.

Common ferrofluids look like motor oil and their reflectivity of ferrofluids is quite low (about 4%). This is not a problem for testing purpose of our deformable mirrors and for certain optical applications, but can be troublesome for applications that require a highly reflective surface. This is why ferrofluids need to be coated with a reflective liquid layer called a MeLLF. Unfortunately, MeLLFs are not compatible with commercial ferrofluids and we had to develop our own ferrofluids. This has been achived by a team of chemists working in our group under the supervision of Prof. Anna M. Ritcey.

The best carrier liquid for this specific application appeared to be ethylene glycol due to its relatively high surface tension and its low vapour pressure limiting the evaporation. The metallic silver particles are concentrated and sprayed on the surface of a few millimetres thickness of ferrofluid. The liquid mirror spread on the surface of the ferrofluid exhibits excellent reflectivity properties comparable to those previously reported for silver nanoparticles spread on water (see the Reflective liquids section). Furthermore, interferometry measurements indicate that the reflective film forms a smooth surface with a root mean square roughness of approximately λ/20.

Illustration showing the structure of the ferrofluid-mellf interface.

Photographs of a homemade ferrofluid coated with a MeLLF. The image at right
shows the liquid deformed by the presence of a magnetic field of a
permanent magnet located under the container.

More information on ferrofluids can be found here and a video on how a ferrofluid works here (QuickTime required).