|M.Sc Student||Yacobi Lee|
|Subject||Phasing Stellar Interferometer by Simulated Annealing|
|Department||Department of Physics||Supervisor||Dr. Erez Ribak|
|Full Thesis text - in Hebrew|
Modern large telescopes are already exceeding the maximum diameter that a single reflector can be polished to with sufficient accuracy for acceptable image quality needed in astronomical research. Instead of polishing a single reflector, telescopes are built as an array of small reflectors (segments), ordered and tuned to imitate the surface of a single reflector. Similarly to the requirement of accurate polishing, misplacements of each segment should not exceed a fraction of wave length equal to several nano-meters.
In most cases, a segmented array telescope is integrated with an active optics system, i.e. each segment is supported on a motorized stage that allows high resolution movements of the segment. In addition to the active optics system, a sensing system is added to provide feedback to allow the active optics system to position each segment in the correct place.
It was found in previous theoretical studies that when a segmented array is completely tuned, the image from the system reaches its maximum sharpness. Following these studies, several optimization algorithms were suggested to optimize the active optics system, by finding the maximum sharpness obtainable by the system. Here we concentrated on the Simulated Annealing algorithm, which is a stochastic algorithm considered to perform an efficient optimization search over a large parameter space. The Simulated Annealing process mimics an annealing process which a solid undergoes as it is cooled at a slow rate.
We implement phasing by Simulated Annealing on an optical stellar interferometer system. A stellar interferometer is a segmented array of reflectors, in which the segments are separated by a large distance. Increasing the distance between segments allows one to improve the optical resolution of the device without the need for additional segments. In stellar interferometry, information about the source is obtained from the details of the interference fringes contrast.
Our stellar interferometer is a small prototype Fiezau Interferometer designed to operate in a lab and to perform simulated observations. The prototype is composed of four separate spherical mirrors, where each mirror is attached to three motorized piezoelectric actuators. The actuators are capable of sub-wavelength steps that allow accurate positioning of each mirror with the three degrees of freedom of a spherical surface.
At the end of the phasing process we were able to locate white light interference fringes absent before the phasing. A white light interference pattern indicates a phasing error of at most several wavelengths.