Patrick Moore' s Practical Astronomy Series Springer-Verlag London Ud. Other titles in this series Telescopes and Techniques Chris Kitchin The Art and Science of CCD Astronomy David Ratledge (Ed.) The Observer's Year Pa trick Moore Seeing Stars Chris Kitchin and Robert W. Forrest Photo-guide to the Constellations Chris Kitchin The Sun in Eclipse Michael Maunder and Pa trick Moore Software and Data for Practical Astronomers David Ratledge Amateur Telescope Making Stephen F. Tonkin Observing Meteors, Comets, Supernovae and other Transient Phenomena Neil Bone Astronomical Equipment for Amateurs Martin Mobberley Transit: When Planets Cross the Sun Michael Maunder and Pa trick Moore Practical Astrophotography Jeffrey R. Charles Observing the Moon Peter T. Wlasuk Deep-Sky Observing Steven R. Coe AstroFAQs Stephen F. Tonkin The Deep-Sky Observer's Year Grant Privett and Paul Parsons Field Guide to the Deep Sky Objects Mike lnglis Choosing and Using a Schmidt-Cassegrain Telescope Rod Mollise Astronomy with Small Telescopes Stephen F. Tonkin (Ed.) Solar Observing Techniques Chris Kitchin Observing the Planets Peter T. Wlasuk Light Pollution Bob Mizon Using the Meade ETX Mike Weasner Practical Amateur Spectroscopy Stephen F. Tonkin (Ed.) More Small Astronomical Observatories Pa trick Moore (Ed.) Observer's Guide to Stellar Evolution Mike lnglis Lee Macdonald With 46 Figures , Springer British Library Cataloguing in Publication Data Macdonald, Lee How to observe the sun safely. - (Patrick Moore's practical astronomy series) 1. Sun - Observers' manuals I. Title 523.7 ISBN 978-1-85233-527-4 ISBN 978-1-4471-0053-9 (eBook) DOI 10.1007/978-1-4471-0053-9 Library of Congress Cataloging-in-Publication Data Macdonald, Lee, 1973- How to observe the sun safely / Lee Macdonald. p. cm. - (Patrick Moore's practical astronomy series, ISSN 1617-7185) Inc1udes index. 1. Sun-Observers' manuals. 2. Sun. I. Title. 11. Series. QB521 .M23 2002 523.7-dc21 2002070456 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 reprographie reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency. Enquiries concerning reproduction outside those terms should be sent to the publishers. Patrick Moore's Practical Astronomy Series ISSN 1617-7185 ISBN 978-1-85233-527-4 © Springer-Verlag London 2003 Originally published by Springer-Verlag London Berlin Heidelberg in 2003 The use of registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant laws and regulations and therefore free for general use. 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 responsibiIity or liabiIity for any errors or omissions that may be made. Observing the Sun, along with a few other aspects of astronomy, can be dangerous. Neither the publisher nor the authors accept any legal responsibility or liability for personal loss or injury caused, or alleged to have been caused, by any information or recommendation contained in this book. Typeset by EXPO Holdings, Malaysia 58/3830-543210 Printed on acid-free paper SPIN 10842242 Contents Introduction ................................ vii Acknowledgements ........................... ix Introducing the Sun . . . . . . . . . . . . . . . . . . . . . .. 1 The Sun's Place in Space .................... 2 How the Sun W orks . . . . . . . . . . . . . . . . . . . . . .. 3 Solar Activity ............................ , 5 The Sun's Influence on the Earth. . . . . . . . . . .. 10 2 Equipment for Observing the Sun ........... 15 Telescopes for Solar Observing .............. 16 Mounts ................................. 22 Observing the Sun by Projection ............ 23 Solar Filters .............................. 30 Other Observing Methods .................. 34 Observing the Sun with the Naked Eye and Binoculars ......................... 35 3 What We Can See on the Sun .............. 37 Setting up for Solar Observing .............. 37 The Sun's Surface ......................... 44 Faculae ................................. 54 Flares ................................... 55 4 Solar Drawings and Position Measurements .. 57 Drawing Using the Projection Method ........ 58 Deriving Sunspot Positions ................. 65 What We Can Learn from Drawings ......... 71 Detailed Drawings ........................ 72 Co-operation with Other Observers .......... 74 5 Measuring Solar Activity ................... 77 The Mean Daily Frequency ................. 78 The Relative Sunspot Number .............. 80 Recording and Reporting of Results .......... 84 Observing Faculae and White-Light Flares .... 90 Observing Naked-Eye Sunspots ............. 93 vi How to Observe the Sun Safely 6 Observing the Sun in Hydrogen-Alpha ........ 95 Equipment for H-Alpha Observing ........... 99 Prominences and Filaments. . . . . . . . . . . . . . . . 108 Flares .................................. 116 7 Solar Photography - Part 1: Photography with Film ............................. 119 Equipment .............................. 121 Photographic Techniques .................. 131 Taking Pictures .......................... 137 Photography in Hydrogen-Alpha ............ 140 8 Solar Photography - Part 2: Digital Techniques ............................ 145 Equipment for Digital Photography. ......... 148 Taking Digital Images ..................... 152 Handling and Processing Digital Images ...... 155 Appendix A Equiprnent Suppliers ............ 167 Appendix B Solar Observing Organisations .... 169 Appendix C Further Reading ................ 171 Index ..................................... 175 Introduction The Sun is the brightest and most accessible object in the sky, and it has much to offer the amateur astronom er with modest equipment. On most days it shows sunspots and other features that displaya wealth of fine detail and change their appearance strikingly from day to day. But observing the Sun can be dangerous. NEVER look at the Sun through a telescope or other optical aid, even for a brief instant. The Sun's intense radiation, amplified and focused by a telescope, will almost certainly cause eye injury and could weIl lead to complete blindness. Do not attempt any solar observing until you have read and understood the safety precautions and observing advice set out in Chapter 2 of this book - even if you think you have the correct equipment. Be especially wary about using filters to observe the Sun. If you have a filter that makes the Sun look dark, it is not necessarily safe, as it is largely the Sun's invisible radiation that is harmful to the eye. But provided you use the correct techniques, such as projecting the solar image onto a screen or using a specially designed, quality solar filter that fits over the telescope aperture, it is quite easy to observe the Sun safely. One of the joys of solar observing is that useful observations are possible even with very small tele scopes - such as the small refractors, Schmidt Cassegrains and Maksutov telescopes that are readily available off the shelf. In fact, due in part to the fact that the Sun has more than enough light, a small telescope will actually give better results than a large one! Observing the Sun is also not affected by light pollution - a major advantage for the many amateur astronomers whose view of the night sky is obscured by the glow of streetlights and security lighting. The Sun can be observed from the town just as successfully as from the country. As I shall explain in Chapter 1, solar activity affects the Earth in a number of important ways. For this viii How to Observe the Sun Safely reason, our nearest star is studied intensively by professional astronomers and is being monitored round the dock. The Sun is observed using space based observatories as well as from the ground, and both the level of research and the equipment required to carry it out are far beyond the amateur's means. Therefore solar observing does not offer the potential for discoveries or major scientific contributions like some other branches of amateur astronomy, such as variable star observing or supernova hunting. But monitoring solar features and keeping careful records of them is still a useful contribution. Throughout the world many amateur astronomers systematically moni tor the Sun and send their observations to solar observing organisations for analysis. Monitoring levels of sunspot activity is particularly useful, as it continues a long series of observations made with small telescopes over many years, and more observers are always welcome. In my view, solar photography is also useful, as it has considerable educational value. Professional solar images tend only to show small parts of the Sun or show our nearest star at invisible wavelengths, where its appearance is radically different from that in visible light. Amateur images, on the other hand, portray the Sun more realistically and so are more meaningful to the wider public. Recent years have seen some major advances in equipment for the solar observer. Filters for white-light observing have improved, allowing better photography and visual observing. Hydrogen-alpha filters for viewing the chromosphere and prominences are now more affordable than ever, allowing amateurs with modest budgets to have views - and take photographs - once reserved for well-off individuals or professionals. Im provements to photographic films have made it easier to take good solar photographs. In the last few years, the rise of digital imaging has opened up yet more horizons. Digital cameras of the type obtainable from a camera store, intended for everyday photography, can be used to take excellent solar photographs, and the images can be processed and enhanced using ordinary image proces sing software on a horne computer. For all that, not much has been written about amateur solar observing in recent years. Books on general astronomy and even those specifically about the Sun te nd to relegate practical solar observing to a single chapter, and so the techniques involved are not Introduction ix described in enough detail. The few books on the subject that have appeared have generally been either too technical or too specialised for the beginn er. This book attempts to fill the current gap in the literat ure. I have written it primarily for the amateur who knows the basics of astronomy and wants to know how to go about observing the Sun. All the time I have emphasised what is possible using commercially avail able equipment that is easy to get hold of in most parts of the world. For this reason, I have deliberately eschewed so me specialised topics, such as observing the Sun's radio emissions, which requires horne-made equipment and a fair amount of technical know-how. Neither have I discussed in much detail the Sun-related topics of eclipses and the aurora. Both are major fields in astronomy by themselves and some good books on them have already been published. Throughout the book my emphasis has been on practical solar observing - what you can do with ordinary equipment, provided you take the proper safety precautions. I have tried to avoid unnecessary theory and have not attempted detailed scientific explanations, as these are available elsewhere. Rather, this book is intended as a basic guide to give the amateur a taste for observing our ever-changing nearest star, in the ho pe that he or she will explore further. Acknowledgements I would like to thank Derek Hatch and Eric Strach for allowing me to use some of their fine solar images in the text, and Harold Hill for his masterly drawings of solar prominences. Thanks are also due to David Boyd for supplying photographs of his solar observing equipment and to Geoff Elston, Director of the Solar Section of the British Astronomical Association, for putting me in touch with some of the contributors. Last, but by no means least, I am grateful to John Watson, Astronomy Editor at Springer-Verlag, for giving me the opportunity to write about observing the Sun and for all his help in seeing the work through the press. Lee T. Macdonald Newbury, Berkshire June 2002 The Sun is important to astronomers for two reasons. The first is that it is Earth's only natural "power station", producing the light and heat essential to life on our planet. Without the Sun, the Earth would be more or less a frozen ball of rock, with no atmosphere, no weather, no life and no people. The Sun can also be harmful to us, because it emits huge quantities of radiation that would be fatal to humans and allliving matter, were our planet not protected from it by a thick atmosphere and powerful magnetic field. But intense bursts of solar activity can still harm communications and electrical power systems, so we need to keep a constant watch on the Sun so that we are forewarned of its next powerful outburst. We also need to understand it so that we can predict future activity and its likely consequences. The second reason for studying the Sun is that the Sun is astar, much like the three thousand or so other stars that we can see in the night sky. All the stars in the night sky, however, are exceedingly remote from uso Even the dosest known star, Proxima in the southern constellation of Centaurus, is some 60 million million kilometres (40 million million miles) from Earth - so far away that its light takes over four years to reach uso Most stars - even the majority of those visible to the naked eye - are much further away still. Even the world's most powerful telescopes show the stars as mere points of light and give us just basic information as to what they are and how they work. But at only 150 million kilometres (93 million miles) from Earth, the Sun is easily dose enough for us to have a detailed view and learn much about it. Armed with a detailed