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Topics in Adaptive Optics TOPICS IN ADAPTIVE OPTICS Edited by Robert K. Tyson Topics in Adaptive Optics Edited by Robert K. Tyson Published by InTech Janeza Trdine 9, 51000 Rijeka, Croatia Copyright © 2011 InTech All chapters are Open Access distributed under the Creative Commons Attribution 3.0 license, which allows users to download, copy and build upon published articles even for commercial purposes, as long as the author and publisher are properly credited, which ensures maximum dissemination and a wider impact of our publications. After this work has been published by InTech, authors have the right to republish it, in whole or part, in any publication of which they are the author, and to make other personal use of the work. Any republication, referencing or personal use of the work must explicitly identify the original source. As for readers, this license allows users to download, copy and build upon published chapters even for commercial purposes, as long as the author and publisher are properly credited, which ensures maximum dissemination and a wider impact of our publications. Notice Statements and opinions expressed in the chapters are these of the individual contributors and not necessarily those of the editors or publisher. No responsibility is accepted for the accuracy of information contained in the published chapters. The publisher assumes no responsibility for any damage or injury to persons or property arising out of the use of any materials, instructions, methods or ideas contained in the book. Publishing Process Manager Dejan Grgur Technical Editor Teodora Smiljanic Cover Designer InTech Design Team First published January, 2012 Printed in Croatia A free online edition of this book is available at www.intechopen.com Additional hard copies can be obtained from [email protected] Topics in Adaptive Optics, Edited by Robert K. Tyson p. cm. ISBN 978-953-307-949-3 free online editions of InTech Books and Journals can be found at www.intechopen.com Contents Preface IX Part 1 Atmospheric Turbulence Measurement 1 Chapter 1 Optical Turbulence Profiles in the Atmosphere 3 Remy Avila Chapter 2 Atmospheric Turbulence Characterization and Wavefront Sensing by Means of the Moiré Deflectometry 23 Saifollah Rasouli Part 2 Imaging and Laser Propagation Systems 39 Chapter 3 Direct Imaging of Extra-Solar Planets – Homogeneous Comparison of Detected Planets and Candidates 41 Ralph Neuhäuser and Tobias Schmidt Chapter 4 AO-Based High Resolution Image Post-Processing 69 Changhui Rao, Yu Tian and Hua Bao Chapter 5 Adaptive Optics for High-Peak-Power Lasers – An Optical Adaptive Closed-Loop Used for High-Energy Short-Pulse Laser Facilities: Laser Wave-Front Correction and Focal-Spot Shaping 95 Ji-Ping Zou and Benoit Wattellier Part 3 Adaptive Optics and the Human Eye 117 Chapter 6 The Human Eye and Adaptive Optics 119 Fuensanta A. Vera-Díaz and Nathan Doble Chapter 7 Adaptive Optics Confocal Scanning Laser Ophthalmoscope 151 Jing Lu, Hao Li, Guohua Shi and Yudong Zhang VI Contents Part 4 Wavefront Sensors and Deformable Mirrors 165 Chapter 8 Advanced Methods for Improving the Efficiency of a Shack Hartmann Wavefront Sensor 167 Akondi Vyas, M. B. Roopashree and B. Raghavendra Prasad Chapter 9 Measurement Error of Shack-Hartmann Wavefront Sensor 197 Chaohong Li, Hao Xian, Wenhan Jiang and Changhui Rao Chapter 10 Acceleration of Computation Speed for Wavefront Phase Recovery Using Programmable Logic 207 Eduardo Magdaleno and Manuel Rodríguez Chapter 11 Innovative Membrane Deformable Mirrors 231 S. Bonora, U. Bortolozzo, G. Naletto and S.Residori Preface Advances in adaptive optics technology and applications move forward at a rapid pace. The basic idea of wavefront compensation in real‐time has been around since the mid 1970s. The first widely used application of adaptive optics was for compensating atmospheric turbulence effects in astronomical imaging and laser beam propagation. While some topics have been researched and reported for years, even decades, new applications and advances in the supporting technologies occur almost daily. This book brings together 11 original chapters on topics related to adaptive optics, written by an international group of invited authors. In Section 1, Dr Avila and Dr Rasouli support this volume with 2 chapters on optical turbulence aberration profiles, measurement, and characterization. Dr Avila’s chapter concentrates on a description of the turbulent atmosphere and parameters necessary to characterize it, such as index of refraction fluctuations, scintillation, and wind profiles. Dr Rasouli’s chapter describes turbulence characterization and wavefront sensing by exploiting the Talbot effect and moiré deflectometry. In Section 2, specific applications and solutions are addressed. Dr Schmidt and Dr Neuhäuser show how adaptive optics is used on large telescopes, such as the array of four 8.2 meter telescopes known as the Very Large Telescope (VLT), to directly image faint objects such as brown dwarfs and extrasolar planets orbiting nearby stars. Professor Rao and colleagues present a chapter describing high resolution image post‐ processing needed to compensate residual aberrations not corrected by the real‐time adaptive optics. This section concludes with a chapter by Professors Zou and Wattellier, who study the adaptive optics ability to compensate wavefronts and spot shapes caused by thermal effects in a high peak power laser. Section 3 is devoted to adaptive optics and the human eye. Dr Doble provides a chapter overview of the structure and optical aberrations of the eye with parts devoted to adaptive optics retinal imaging and vision testing. Professor Lu and colleagues present a chapter showing the operation of an adaptive optics scanning laser ophthalmoscope, with results for retinal imaging including super‐resolution and real‐ time tracking. X Preface The final section, Section 4, concentrates on the subsystems of adaptive optics, namely the wavefront sensors and deformable mirrors. The first chapter, by Mr. Akondi, provides various solutions for improving a Shack‐Hartman wavefront sensor. These include such topics as improved centroiding algorithms, spot shape recognition, and applying a small dither to the optics. A chapter by Dr Li and colleagues is an extensive investigation of the measurement error associated with a Shack‐Hartmann sensor. A chapter by Dr Magdaleno and Dr Rodriguez provides a detailed design of a wavefront reconstructor using field‐programmable gate array technology that can bridge the gap to high‐speed operation for the calculational burden of advanced large telescopes. The final chapter, by Dr Bonora and colleagues, describes a number of novel designs of micro‐electro‐mechanical systems (MEMS) deformable mirrors. The novel designs include resistive actuators, photo‐controlled deformations, and a solution to the “push‐pull” problem of conventional electrostatic MEMS deformable mirrors. Robert K. Tyson, PhD Associate Professor Department of Physics and Optical Science University of North Carolina at Charlotte Charlotte, North Carolina USA Part 1 Atmospheric Turbulence Measurement 10 Optical Turbulence Profiles in the Atmosphere Remy Avila Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México Centro de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México México 1. Introduction Turbulence induces phase fluctuations on light waves traveling through the atmosphere. The main effect of those perturbations on imaging systems is to diminish the attainable angular resolution, whereas on free-space laser communications the turbulence drastically affects system performances. Adaptive Optical (AO) methods are aimed at reducing those cumbersome effects by correcting the phase disturbances introduced by atmospheric turbulence. The development of such methods would not have seen the light without research of the turbulent fluctuations of the refractive index of air, the so called Optical Turbulence (OT). It is necessary to study the statistical properties of the perturbed wavefront to design specific AO systems and to optimize their performances. Some of the useful parameters for this characterization and their impact on AO are the following: The Fried parameter r0 (Fried, 1966), which is inversely proportional to the width of the image of point-like source (called ”seeing”), leads to the determination of the spatial sampling of the wavefront for a given degree of correction (Rousset, 1994). r0 is the dominant parameter in the calculation τ of the phase fluctuation variance. The coherence time 0 (Roddier, 1999), during which the wavefront remains practically unchanged, is needed to determine the temporal bandwidth of an AO system and the required brightness of the reference sources used to measure the θ wavefront. The isoplanatic angle 0 (Fried, 1982), corresponding to the field of view over which the wavefront perturbations are correlated, determines the angular distance between the corrected and the reference objects, for a given degree of correction. The parameters described in the last paragraph depend on the turbulence conditions encountered by light waves along its travel through the atmosphere. The principal physical 2 ( ) quantities involved are the vertical profile of the refractive index structure constant CN h , which indicates the optical turbulence intensity, and the vertical profile of the wind velocity V(h), where h is the altitude above the ground. 2 ( ) V( ) The profiles CN h and h can be measured with balloons equipped with thermal micro sensors
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