Adaptive optics

Description

Adaptive optics appeared as a necessity to correct or modulate the wave front, especially in astronomical observations

H.V. Babcock (1953) and V.P. Linnick (1957) independently proposed techniques to improve the telescope images based on deformable mirrors. These mirrors make real time corrections to the wave fronts and are coupled with a wave front sensor. Anyway, the systems proposed were too complicated and expensive to be manufactured at that time. Studies of A. Labeyrie (1970) proposed speckle interferometry as reconstruction method for the turbulance degraded images. After Labeyrie astronomers were preocupied with post-detections image processing techniques. The first optical system able to improve the contour of 2D images was built at ITEK by J. W. Hardy and his collaborators in 1977. More and more adaptive optics (AO) systems were realized at the beggining of 70's. Nowadays, systems with more than a thousand degrees of freedom are available.

The first active telescope, ESO 3.5 m, began to operate in 1989 at La Silla. Also, active optics is the "heart" of the primary mirror of the Keck telescope in Mauna Kea Hawaii since 1992. Adaptive optics systems can be used to build both primary and secondary mirror. However, taking into account the costs, only the secondary mirror is deformable.

The need for cheap systems lead to new materials such as: polymeric membranes and different types of fluids like OALM. Regarding micro- and nano- adaptive systems, one of the most popular modulating systems today is the DMD - Digital Micromirror Device. This device has two positions (closed/opened) and resolution from 640X480 pixels/17 microns to 1280X1024 pixels /17 microns at a speed of 20 microseconds and a bit depth of 24 bits. Many applications of adaptive optics are taking place in the field of medicine. Such a device based on liquid crystals as a wave front corrector, could be used as an implant into the crystalline lens in order to correct for the high order abberations and accomodation losses of the eye. Experiments have shown that a focusing power of 3 diopters can be achieved.

OALM - Adaptive optics systems with magnetic liquid Cercetari privind realizarea unui model de oglinzi adaptive cu lichide complexe magnetice, cu aplicatii in telescoapele spatiale. Project number 111/2004/ROSA (www.rosa.ro)

Description

With magnetic liquids ( or often called magnetic fluids) we can build adaptive mirrors for large telescopes. Such a telescope working in a large spectrum of light will “see” only in zenith direction ( the point directly above a given observation site on Earth).

Optical properties of magnetic liquids:

Magnetic fluids

Magnetic liquids (fluids) are quasi-homogeneous, magnetizable fluids that keep their properties even in non-uniform intense magnetic field.

Magnetic nanoparticles (CoFe2O4, Fe3O4, etc.) are the main component of these magnetic nanofluids. They are "integrated" in the structure of the dispersion medium (e.g. a base liquid, any organic or inorganic fluid) through superficial active substances called surfactants.

This means the suspension of magnetic nanoparticles has the same colloidal properties, even in strong magnetic fields.

The magnetic fluids were produced at the Laboratory of Magnetic Fluids, Center for Fundamental and Advanced Technical Research, Romanian Academy - Timisoara Division and magneticaly characterized by National Centre for Engineering of Systems with Complex Fluids.