Practical Astronomy Springer-Verlag Berlin Heidelberg GmbH Other titles in this series The Observational Amateur Astronomer Patrick Moore (Ed.) Telescopes and Techniques: An Introduction to Practical Astronomy C. R. Kitchin Patrick Moore (Ed.) , Springer Cover photograph by courtesy of SCS Astro ofWellington, Somerset, England ISBN 978-3-540-19900-7 ISBN 978-1-4471-0387-5 (eBook) DOI 10.1007/978-1-4471-0387-5 British Library Cataloguing in Publication Data Modern Amateur Astronomer. - (Practical Astronomy Series) I. Moore, Patrick II. Series 522 Library of Congress Cataloging-in-Publication Data The modern amateur astronomer / Patrick Moore (ed.). p. cm. --(Practical astronomy) Includes index. ISBN 978-3-540-19900-7 1. Astronomical instruments. 2. Spherical Astronomy. 3. Telescopes. I. Moore, Patrick. II. Series. QB86.M63 1995 95-32987 522--dc20 CIP Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms of the licences issued by the Copyright Licensing Agency. Enquiries concerning reproduction outside those terms should be sent to the publishers. © Springer-Verlag Berlin Heidelberg 1995 Originally published by Springer-Verlag Berlin Heidelberg New York in1995 The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made. Typeset by Editburo, Lewes, East Sussex, England Printed by the Alden Press, Osney Mead, Oxford 34/3830-5432lO Printed on acid-free paper Contents Introduction Patrick Moore Vll 1 Optical Principles R. W. Forrest . . . . . . . . . . . . . . . . . . . 1 2 Buying a Telescope Patrick Moore . . . . ....... 17 3 Commercially Made Telescopes John Watson ......... . ....... 21 4 Observatories and Telescopes Denis Buczynski ...... ......... 31 5 Making Your Own Telescope John Watson .......... 51 6 Auxiliary Equipment M. Mobberley . . . . .. 69 7 Electronics and the Amateur Astronomer Maurice Gavin .. 95 8 First Steps in Astronomical Calculations Gerald North ................. 109 9 Astronomical Spectroscopes C. R. Kitchin . . . . . . . . . . . . . . . . . . 127 10 Astrophotography Robin Pearce .................. 143 vi Contents 11 Astronomical Societies Patrick Moore 161 Index ....... . 165 Introduction Astronomy has always been one of the few sciences in which the amateur can make valuable contributions. Indeed, not so very long ago some of the world's leading astronomers were amateurs - the classic example being the third Earl of Rosse who, alone and unaided, built what was then the largest telescope in the world, and from 1845 used it to make fundamental discoveries. This is still true today, if to a somewhat more limited extent. There are of course amateur theorists and cos mologists, but all in all the main amateur contribution at the present time comes from the observers. It is quite correct to say that the average amateur knows the night sky a great deal better than the average professional, who spends his observing time looking at a television screen, and who very seldom puts his eye to an eyepiece! Yet there has been a tremendous change in quite recent times. Forty years ago, the average amateur astronomer was equipped with a modest telescope, and carried out almost all of his work visually. Invaluable observations were made of Solar System bodies, and of variable stars; some amateurs made their reputations by hunting for comets and novre, with remarkable success. Now, much of this has altered. The telescope remains the essential tool, but the modern amateur can make use of techniques which would have been completely beyond his reach (or his personal capability) a few decades ago - and he can use modern equipment which did not even exist in the 'pre-electronic' age. Sophisticated photogra phy has come into use, plus electronic devices; for exam ple, there are many amateurs who own CCDs (charge coupled devices, which form the heart of a very sensitive vii Introduction television camera - more about this in Chapter 7) and are fully able to produce work of professional standard. There are many books which introduce the subject of practical astronomy. There are also many which cater for a far higher technical level. However, there is a gulf between these two types, and I hope that this gulf will be filled by the two present books - this one and its com panion, The Observational Amateur Astronomer. In both books every chapter has been written by an author who is really experienced in his chosen field. Each chapter has been written from a personal per spective, and in editing this book I have been careful to leave the writers' ideas, preferences, and writing styles alone. The same topics may be touched upon by more than one author, often from a slightly different perspec tive. This is quite deliberate. I expect that readers of this book will have some prior knowledge of amateur astronomy, but there is nothing here which will puzzle any enthusiast who has taken the trouble to master the basic facts. Remember, too, that amateur work is warmly wel comed by the professionals. Amateurs carry out research which the professional does not want to do, has no time to do, or literally cannot do. There is always something new to find out. Patrick Moore, 1995 R. W. Forrest Why learn about the optics of a telescope? The answer is simple: an appreciation of some of the optical principles that underlie astronomical telescopes will help serious observers make the best use of an instrument, and make reasoned judgements as to its capabilities for fulfilling particular observing aims. An understanding of how a telescope (or other optical instrument) works is central to an understanding of what it can do. Optical Theor~_ __ ~ Let's begin with the meanings of some fundamental terms. Telescopes using lenses as their principal compo~ nents are commonly called refractors. Those utilising mirrors are reflectors. When both lenses (other than an eyepiece) and mirrors are essential to the performance the telescope is termed catadioptric. The lens of a refractor is called its objective or object glass (often abbreviated OG). The first mirror of a reflector is the primary and subsequent mirrors are termed secondary, and then tertiary, etc. The aperture of a telescope is the diameter of the light beam that it collects, and is usually the diameter of its objective or primary mirror. Bigger telescopes allow fainter stars to be studied and show finer detail, which is why the aperture is so important that references to a telescope always include it - e.g., a '200~mm Schmidt Cassegrain' means a telescope of aperture 200~mm. The function of any optical system is to gather The Modern Amateur Astronomer • . ------------~~~ Figure 1.1 light from two stars af angular separation () arriving at the e telescope. has been exaggerated and will rarely exceed 1°. together light rays emanating from every single point of the object being imaged and to cause them all to con verge to a corresponding unique point within the instrument. Different points of the object are mapped to different image points; all the points that go to construct the image make up the focal plane of the system. Despite its name, the focal plane may be curved, but is generally required to be as flat as possible because this is most convenient for detectors such as photographic films and CCDs. In astronomical imaging the object points can all be regarded as lying at the same infinite distance, because the objects being viewed are all so far away. The light rays from each point consequently travel in parallel lines and the distinction between each point is the direction from which the rays arrive (Figure 1.1). The distance between the points at which the rays from two given directions converge in the focal plane defines the focal length. For a simple lens this is the same as the distance behind the lens at which the image is formed, or for a single mirror the distance from the mirror surface to the image. If this is f millimetres, and e the angle between two objects is radians, then they will be imaged a distance d apart in the focal plane given by d = fe (see Figure 1.2) In the more familiar (to most of us) angular units