Diffraction-limited Imaging With Large And Moderate Telescopes

Diffraction-Limi ted Imaging with Large and Moderate Telescopes This page intentionally left blank Swapan K. Saha Indian Institute of Astrophysics Bangalore, India Diffraction-Li m i ted Imaging with Large and Moderate Telescopes World Scientific N E W J E R S E Y • L O N D O N • S I N G A P O R E • B E I J I N G • S H A N G H A I • H O N G K O N G • TA I P E I • C H E N N A I Published by World Scientific Publishing Co. Pte. Ltd. 5 Toh Tuck Link, Singapore 596224 USA office: 27 Warren Street, Suite 401-402, Hackensack, NJ 07601 UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. DIFFRACTION-LIMITED IMAGING WITH LARGE AND MODERATE TELESCOPES Copyright © 2007 by World Scientific Publishing Co. Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the Publisher. For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA. In this case permission to photocopy is not required from the publisher. ISBN-13 978-981-270-777-2 ISBN-10 981-270-777-8 Printed in Singapore. Lakshmi - Diffraction-Limited.pmd 1 7/13/2007, 2:32 PM April 20, 2007 16:31 WSPC/Book Trim Size for 9in x 6in In memory of my wife, KALYANI vi lec April 20, 2007 16:31 WSPC/Book Trim Size for 9in x 6in Preface Diffraction-limited image of an object is known as the image with a resolution limited by the size of the aperture of a telescope. Aberrations due to an instrumental defect together with the Earth’s atmospheric turbulence set severe limits on angular resolution to ∼ 100 in optical wavelengths. Both the sharpness of astronomical images and the signal-to-noise (S/N) ratios (hence faintness of objects that can be studied) depend on angular resolution, the latter because noise comes from the sky as much as is in the resolution element. Hence reducing the beam width from, say, 1 arcsec to 0.5 arcsec reduces sky noise by a factor of four. Two physical phenomena limit the minimum resolvable angle at optical and infrared (IR) wavelengths − diameter of the collecting area and turbulence above the telescope, which introduces fluctuations in the index of refraction along the light beam. The cross-over between domination by aperture size (∼ 1.22λ/aperture diameter, in which λ is the wavelength of light) and domination by atmospheric turbulence (‘seeing’) occurs when the aperture becomes somewhat larger than the size of a characteristic turbulent element, that is known as atmospheric coherence length, r0 (e.g. at 10- 30 cm diameter). Light reaching the entrance pupil of a telescope is coherent only within patches of diameters of order r0 . This limited coherence causes blurring of the image, blurring that is modeled by a convolution with the point-spread function (PSF), which