Table Of ContentR Hanbury Brown FRS
THE INTENSITY
INTERFEROMETER
its Application to Astronomy
R. HANBURY BROWN, F.R.S.
Professor of Physics (Astronomy)
Unfrersity of Sydney
TAYLOR & FRANCIS LTD
10-14 Macklin Street London \\\W H
1974
First published 1974 by Taylor & Francis Ltd
10-14 Macklin Street, London WC2B 5NF
© 1974 R. Hanbury Brown
All rights reserved. No part of this publication
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ISBN 0 85066 072 6
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10-14 Macklin Street, London WC2B 5NF
Distributed in the United States of America
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List of Symbols
.-l area of light detector or mirror
a( i·) amplitude of Fourier component of light wave
a fraction of photomultiplier anode current due to stray light
and dark current
B apparent blue magnitude of star
Bo bandwidth of light (eqn. (5.2))
b,. electrical bandwidth (eqn. (5.10))
c wlocity of light
c(d) correlation observed in 100 s at a baseline d
c (d) obsen·ed correlation corrected for zero-drift ( eqn. ( 10.1))
0
c,(d) normalized correlation ( eqn. ( 10.f))
Cs zero-baseline correlation normalized to nlue expected for
a single unresoh-ed star
D zero-drift of correlator
d baseline length
E amplitude of electric wctor of light waw
e charge on the electron
voltages
voltage gain at frequency f
.\bsolute monochromatic flux emitted per unit area of
stellar surface at wawlength ,\
f frequency of electric current or voltage
f1 absolute monochromatic flux recei\·ed from a star at
wawkngth ,\
Gr electrical gain of correlator
G(1·) mutual spectral density of two light beams (eqn. (3.12))
g(1·) transmittance of optical system at frequency v
lz the Planck constant
I liaht intensitv
l(S) li~ht intensit;· per unit area of source
i (§ f) angle of inclination of orbital plane of a binary star
11· 12 electric currents
IX
Preface·
In this book I have tried to write a history
of the intensity interferometer together with
a brief account 0f the theory, practice and
application of this new instrument to a
classical problem in astronomy-the
measurement of the apparent angular
diameters of stars. Much of the scientific
material has already appeared in print but
it is scattered through more than twenty
years of scientific literature. My aim has
been to collect it into one book and to tell
how and why the work came to be done.
There are at least three good reasons for
doing this. Firstly, the intensity
interferometer has proved to be of lively
interest to physicists and incomprehensible
to astronomers; there should be a single
written account appropriate to both groups.
Secondly, the measurements of the angular
diameters of stars made at Narrabri
Observatory have been widely used by
astrophysicists and there is no other existing
instrument capable of checking them. It is
therefore doubly important that there should
be a detailed account of exactly how the
observations were made and analysed so
that their credibility can be assessed.
Finally, I want to preserve our experience
so that other groups, perhaps in the Northern
Hemisphere, will one day follow our lead.
Those of us who have tried to build on the
experience gained with the unsuccessful
50 ft. stellar interferometer built at Mount
Wilson by Hale and Pease have found that
although there are descriptions of the
v
instrument, like so many scientific papers,
they don't tell us all we need to know-what
checks were made and what went wrong.
With this in mind I hope the reader will
excuse some of the details in this book
which, at first sight, may seem unnecessary.
During my life I have been fortunate to take
part in two other scientific adventures: the
early work on radar and the beginnings of
radio-astronomy. Books about those
pioneering days have always seemed to me
to leave out the essential ingredient of such
adventures-the sense of purpose and
excitement which transfigures the hardest
work and dullest routine. Telling my own
story has made me well aware that a lifetime
of writing scientific papers is not a good
training for conveying that same sense of
ad,·enture which was felt by all who helped
t0 build the stellar interferometer at Narrabri.
If only for one moment I could take you
into the great plains of northern New South
Wales and show you this elegant instrument
looking at the stars, I believe you would
understand what is missing from my book.
Not only was the work we did at Narrahri
worth while in strictly scientific terms, but
in personal terms it was exciting, exhausting
and profoundly rewarding.
The success of a project lasting more than
two decades has involved the help and
support of many people over the years.
Much of the work described in this book
has been done by my colleagues, Dr R. Q.
Twiss, Dr J. Davis and Dr L. R. Allen.
VI
Others who have "done time" at Narrabri
are Dr D. Herbison-Evans and Dr C. Hazard
and a small galaxy of postgraduate students:
Dr M. J. Yerbury, Mr J.M. Rome,
Mr R. J. W. Lake, Mr D. W. Keenan,
Dr R. J. Webb and Mr R. K. Outhred.
Mr Gifford has taken care of the maintenance
of the Observatory for ten years, Mr S.
Owens has looked after the electronics and
Miss V. Raymond has been throughout,
secretary to the Department of Astronomy
in the University of Sydney.
The original design of the instrument owes
much to the staff of Dunford and Elliott
(Sheffield), Mullard Ltd. (Salford) and
MacDonald, Wagner and Priddle (North
Sydney). The financial support was provided
by the Department of Scientific and
Industrial Research (now the Science
Research Council), the Nuclear Research
Foundation (now the Foundation for
Research in Physics) within the University
of Sydney, the Office of Scientific Research
of the United States Air Force, the
Australian Research Grants Committee and
the Research Grants Committee of the
University of Sydney.
I am grateful to Sir Bernard Lovell for
giving the original work at Jodrell Bank his
support and to Professor Harry Messel who,
as Head of the School of Physics in the
University of Sydney, has always supported
us with characteristic enthusiasm. Among
our many friends overseas, I must mention
vii
Dr D. M. Popper of U.C.L.A., Dr E. P. Ney
of the University of Michigan and Dr D. C.
Morton of Princeton, all of whom have
worked at Narrabri.
Finally I thank my wife Heather for making
our life in the bush so happy and for her
help with everything described in this book
except the mathematics.
Narrabri Observatory R. HANBURY BROWN
New South Wales
Australia
June 1973
Ylll
Jc wave noise (eqn. (4.18))
Jn shot noise (eqn. (4.19))
k the Boltzmann constant
lo coherence length of light (eqn. (3.33))
N,n number of counts of pulses
N(T) r.m.s. noise or uncertainty after a time T
n(v) number of photons per unit time, per unit area, per unit
light bandwidth at frequency v
R radius of star
Ri> R distances from observer to points on a light source
2
(S/N) signal to noise ratio
T,t time
Te effective temperature of a star
u limb-darkening coefficient (eqn. (10.12))
v
apparent visual magnitude of star
V(d) fringe visibility with baseline d
V(t) electric vector of light wave
W(f) spectral density of electrical fluctuations
z (§ 5.6) zenith angle, (§ 5.8) coordinate parallel to line from
observer to source
quantum efficiency
ratio of brightness of components of binary star
polarization factor (eqn. (5.7))
normalized correlation factor ( eqn. (5.8))
mutual coherence function (eqn. (3.16))
complex degree of coherence (eqn. (3.18))
partial coherence factor ( eqn. (5.11))
difference in brightness of components of a binary star
light bandwidth
electrical bandwidth
intensity fluctuation
x
