Tools of Radio Astronomy

476 Pages·1996·40.398 MB·English

by  Kristen Rohlfs, T. L. Wilson

S 1{0 OMY A D
\STROPHY ICS LIB ARY
LIBRARY
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Physics and Astronomy
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K. Rohlfs T. L. Wilson
Tools of
Radio Astronomy
Fourth Revised and Enlarged Edition
With 133 Figures and 21 Tables
~
Springer
Professor Dr. K. Rohlfs
Universitat Bochum
lnstitut fiir Astrophysik
44721 Bochum, Germany
Dr. T. L. Wilson
MPI fiir Radioastronomy
Auf dem Hiigel 69
53121 Bonn, Germany
Cover picture: The new 110-m Green Bank Telescope (GBT). The design as an off-axis paraboloid was chosen in
order to minimize the sidelobes and baseline ripple.
Library of Congress Cataloging-in-Publication Data applied for.
ISSN 0941-7834
ISBN 978-3-662-03268-8 ISBN 978-3-662-03266-4 (eBook)
DOI 10.1007/978-3-662-03266-4
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Preface to the Fourth Edition
Progress in the fields of science and technology is again the reason for updat
ing this text most changes occur in Chaps. 4, 7, 8 and 14; in the others we
have made a number of minor improvements.
In Chap. 4, Signal Precessing and Receivers, we have shortened the presen
tation of maser and parametric amplifier front ends, which are no longer com
monly used as microwave receivers in radio astronomy. Instead, we have ex
panded the presentation of cooled transistor and superconducting front ends,
and we have added a short section on coherent multi-beam receiver systems.
The chapter titled Observational Methods now appears as Chap. 7, and
mainly addresses single dish observations. Chapter 8 now concerns Interfer
ometers and Aperture Synthesis. Aperture synthesis has become the single
most important imaging technique in observational astronomy, and its use
has spread from the radio frequency range into other wavelength bands pro
viding the only general method available for obtaining images of extremely
high resolution and quality. The discussion of the foundations for aperture
synthesis therefore have been extended and hopefully improved. Both the
hardware and the software instrumentation is now collected in this chapter.
Chapter 14, Molecules in Interstellar Space, has been extensively revised
and updated. The increased availability of astronomical satellites starting
with the Infrared Astronomy Satellites, IRAS and ISO, the mid IR satellite
MSX and the Submillimeter Water Satellite SWAS have resulted in a great
number of new molecular line data for the interstellar medium. More recently,
the Very Long Baseline Satellite VSOP /HALCA has provided high resolution
data, and the spectral line mission ODIN is providing sub-millimeter data for
transitions that cannot be measured from the ground. These missions have
dramatically changed molecular line astronomy and some of these new results
have been included.
Shortly after the third edition of this book was published, a book on
problems and solutions by T.L. Wilson and S. Hiittemeister appeared as an
extension of the "Tools." The basic understanding of observational radio as
tronomy can be learned only by practice, and this is provided for in this
problems supplement. We hope it will serve a useful purpose.
Bochum and Bonn Kristen Rohlfs
July 2003 T. L. Wilson
From the Preface of Previous Editions
This is a book describing the tools a radio astronomer needs to pursue his/her
trade. These tools consist, on one hand, of the radio telescopes and the various
kinds ofreceivers needed to analyze cosmic radio signals, and on the other, of
the physics of the radiation mechanisms responsible for the continuous and
line radiation. This book grew out of a one-year graduate course that was
given repeatedly at the Ruhr-Universitat at Bochum. We hope that this text
will also be useful for all scientists who use data and results obtained by radio
astronomical means and will help them to understand both the strong points
and their characteristic drawbacks. Finally, this book may occasionally save
some scientists working in the field of radio astronomy from long searches in
the literature when questions concerning tools occur.
Although the students to whom this course was addressed had a rather
thorough background knowledge of physics, a characteristic difficulty often
turned up when the instrumental tools were discussed. Obviously there is a
difference between how a subject is treated in a genuine physics book and the
way it is dealt with in a text intended for engineers - one example is the way
in which four-terminal networks are used. We have tried to explain everything
using concepts that are familiar to both astrophysicists and general physicists.
For each chapter, a list of references is given. Usually this list has two
parts: general references give a list of papers and books that cover the gen
eral aspects and which often give a more thorough treatment of the subjects
covered, and special references document the sources for specific topics. How
ever, these references do not give a complete or even nearly complete review
of the relevant literature. The papers cited are those that we found to present
the subject in a way convenient for the purpose intended here.
Beginning with the second edition of this book T.L. Wilson from the MPI
fiir Radioastronomie in Bonn, Germany, joined as coauthor to insure compe
tent and up-to-date coverage of modern developments. Many of the changes
and additions to the text are due to him. In particular, the readability of the
text should profit from his being a native English speaker.
Another fundamental change from the first edition is that the book is
now produced from a Ib'IEX script. This is more than a mere technical change
since extensive changes in the text and its ordering are thus fairly easily done.
The text of the first edition was scanned, and although the plain text could
VIII From the Preface of Previous Editions
be successfully recognized automatically, the formulae had to be converted
manually to the TEX syntax. All changes to the text were then done on this
reconstructed electronic version of the first edition.
The basic concepts used in the first edition have remained unchanged,
however. This book attempts to give an outline of the methods and tools of
radio astronomy. Results are given only as examples or in order to make the
approach used plausible. The book is intended to be of help in applying radio
astronomy; it is not a description of the many results obtained with its help.
Another problem encountered when writing a textbook is that of consis
tent designations, symbols, and units. One could introduce a consistent set
for a whole area of research, but then comparisons of this text with results in
the literature would often be difficult. Too close an adherence to the customs
in the different fields of research, on the other hand, results in contradictory
terminology, and therefore we searched for a suitable mean between these
extremes. Since the astronomical community stubbornly sticks to their tradi
tional mixed set of units, we have defected from the one and only true creed
and switched from SI units, used in the first edition, back to the CGS system
augmented and mixed with various other units, as used in the research liter
ature. Wherever possible, we have written equations, with numerical values
for constants, as a relation between ratios of physical quantities and their
respective units. Only if these expressions turned out to be too clumsy, have
we dropped the units and explained them in the accompanying text. In par
ticular, this is so in Chap. 14 on Molecules in Interstellar Space. The scientific
arguments are not affected by this in any way, but it brings the presentation
into line with Physical Processes in the Interstellar Medium by L. Spitzer,
which is the best-known text in this field.
Chapters 1 to 8 form the technical part of the book, while Chaps. 9 to 14
discuss the physical background. We have moved the chapter on Receivers
from 7 to 4 renaming it Signal Processing and Receivers. The reason for this
was that we felt some concepts of signal processing should be discussed first
so that they could be freely used in later chapters. The discussion of re
ceiver front ends has been extensively revised and modernized, the section
on receiver sensitivity now contains a discussion of the minimum noise per
formance possible with a coherent system. In the last few years, incoherent
detectors, usually bolometers, have been used in the far infrared. For this
reason, we have included a short discussion of incoherent detectors (bolome
ters) and their noise limits. The long discussion of parametric amplifiers in
the first edition has been replaced by that of modern semiconductor devices
such as HEMT amplifiers and SIS mixers.
Chapter 5 on Fundamentals of Antenna Theory and Chap. 6 on Filled
Aperture Antennas remain basically unchanged, although the wording of
many aspects has been changed and, we hope, improved. In the discussion of
practical filled aperture antennas, modern systems have been selected.
From the Preface of Previous Editions IX
The second half of the book, Chaps. 9 to 14, is concerned with what
we consider to be the most important conceptual tools needed by the radio
astronomer if he/she wants to relate the message from interstellar space to
physical processes. We realize that in a book of about 450 pages we cannot
adequately cover all of the details, so we ask everyone for charity if his/her
favorite subject is treated only cursorily or even not mentioned at all. We
have aimed the presentation at the newcomer to the field, and we hope that
the overall presentation is reasonably well balanced, even if many interesting
and important topics had to be skipped. Chapters 9 and 10 discuss emission
mechanisms of continuum radiation and present some examples. Here the
discussion of the synchrotron radiation and the corresponding sources has
been enlarged compared to the first edition.
As several reviewers had commented, a discussion of pulsar physics was
missing. So we have included a section on this in Chap. 10. Again, as in the
other parts of this book, this is not intended to be a review of the entire field
of pulsar radio astronomy in the literature but describes only the basic and
undisputed concepts and results.
An introduction to spectral line studies can be found in Chaps.ll-14. We
treat the study of hyperfine and fine structure lines in Chap. 12, in Chap. 13
we present an introduction to the study of recombination lines. Chapter 14
describes the study of molecular lines. This chapter has grown to more than 60
pages, and in it we try to collect the many aspects of physical theory needed
in this field, which are largely unknown to the other fields of astrophysics.
Although we tried to eliminate unnecessary details from the text as far
as possible, the book has gained about 1/3 in length compared to the first
edition. This material is obviously more than can be covered in a two semester
course.
References to books have remained, mostly, as in the first edition. The
references to the current literature have been updated. We do not attempt
to give a complete review and we chose those references that are the most
recent or cover the subject most comprehensively.
Several colleagues sent us notes drawing our attention to errors, both
typographical and erroneous arguments. Particularly extensive comments on
the first edition were from Anders Winnberg, Onsala, Sweden. We thank all
and the remarks have been taken into account and the new text is hopefully
improved. Many people have helped us in preparing the book, we would like
to thank in particular H. Kampmann, Bochum, who was largely responsible
for the ]}.'lEX work.
Contents
1. Radio Astronomical Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 On the Role of Radio Astronomy in Astrophysics . . . . . . . . . . 1
1.2 The Radio Window . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Some Basic Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4 Radiative Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.5 Black Body Radiation and the Brightness Temperature..... 10
1.6 The Nyquist Theorem and the Noise Temperature . . . . . . . . . 14
2. Electromagnetic Wave Propagation Fundamentals . . . . . . . . 16
2.1 Maxwell's Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.2 Energy Conservation and the Poynting Vector . . . . . . . . . . . . . 17
2.3 Complex Field Vectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.4 The Wave Equation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.5 Plane Waves in Nonconducting Media . . . . . . . . . . . . . . . . . . . . 22
2.6 Wave Packets and the Group Velocity . . . . . . . . . . . . . . . . . . . . 25
2. 7 Plane Waves in Dissipative Media . . . . . . . . . . . . . . . . . . . . . . . 27
2.8 The Dispersion Measure of a Tenuous Plasma . . . . . . . . . . . . . 28
3. Wave Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.1 Vector Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.2 The Poincare Sphere and the Stokes Parameters . . . . . . . . . . . 37
3.3 Quasi-Monochromatic Plane Waves . . . . . . . . . . . . . . . . . . . . . . 40
3.4 The Stokes Parameters for Quasi-Monochromatic Waves.... 42
3.5 Faraday Rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4. Signal Processing and Receivers . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.1 Signal Processing and Stationary Stochastic Processes . . . . . . 48
4.1.1 Probability Density, Expectation Values
and Ergodicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.1.2 Autocorrelation and Power Spectrum . . . . . . . . . . . . . 50
4.1.3 Linear Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.1.4 Gaussian Random Variables . . . . . . . . . . . . . . . . . . . . . 55
4.1.5 Square-Law Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4.2 Limiting Receiver Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.2.1 The Minimum Noise Possible with a Coherent System 61