The Modern Amateur Astronomer

167 Pages·1995·5.71 MB·English

by  R. W. Forrest (auth.)

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