advances in radio telescopes [scanning the issue]
TRANSCRIPT
SCANNING THE ISSUE
Advances in Radio TelescopesBy JACOB W. M . BAARSGuest Editor
LARRY R . D ’ADDARIO , Member IEEEGuest Editor
A . R ICHARD THOMPSON , L i f e Fe l l ow IEEEGuest Editor
I . INTRODUCTION
The editors of the third Special Issue on Radio Telescopes, which appeared in
the Proceedings of the IEEE in May 1994, surmised in their introduction that
Bperhaps yet a future issue is merited, one devoted to those new telescopes that
are still on the drawing boards.[Now, 15 years later, such an issue lies in front of
you, featuring 16 papers describing both the realization of new instruments and
the status of several giant radio telescopes, most of which are moving from thedrawing board to different stages of
construction. The development of
astronomy over this period has led
radio astronomers to concentrate on
both the highest and the lowest ranges
of the radio spectrum. The tech-
nological advance in the millimeter
wavelength domain has enabled anenormous improvement in observing
capabilities. In the low frequency
range, roughly 10Y2000 MHz, new
telescopes are being planned that
combine a large instantaneous field
of view with a large number of high-
resolution antenna beams. In addition
to these developments, this issue features papers on several new single aperturetelescopes. We also have three papers covering advances in technologies that are
applicable to multiple projects, namely, antenna metrology, imaging techniques,
and the use of phased array techniques.
The issue begins with a short paper by the guest editors on BRadio
Astronomy in the Early Twenty-First Century.[ There we attempt to put the
topics of the following papers in historical perspective and to provide
background information for readers whose expertise lies outside astronomy.
The remaining papers are organized into three broad categories: single antennatelescopes, synthesis array telescopes, and the Square Kilometre Array (SKA).
Although the last is also a synthesis array, the intensity of SKA-related work
now under way around the world justifies a separate set of papers devoted to it.
II . SINGLE ANTENNATELESCOPES
The largest fully steerable instrument
in this category is the Green Bank
Telescope of the National Radio
Astronomy Observatory (NRAO), a
100-m-diameter clear-aperture offsetparaboloid with an active reflector
surface. It can be operated either as
prime focus instrument for frequen-
cies below 1.2 GHz or in a Gregorian
mode for higher frequencies. Under
benign conditions, it is can be used at
a shortest wavelength near 3 mm.
In China, a 500-m-diameterBArecibo type[ fixed-reflector tele-
scope, called FAST, is being built in
a natural depression of the Karst
region of Guizhou province. The re-
flector surface shape can be adjusted
by a control mechanism to provide a
parabolic surface that is a function of
viewing angle. This improves efficien-cy and enables the use of simple point
source feeds. Construction has started
and completion is expected in 2015.
The paper BPreparatory Study for
Constructing FAST[ outlines the pro-
ject and describes the studies and
prototyping in the development of
this instrument.Radio technology can now be
applied to frequencies above 1 THz,
but telescopes for these frequencies
must be deployed above the terrestrial
atmosphere. The European Space
This issue features newsingle-aperture andsynthesis array radiotelescopes and coversadvances in antennametrology, imagingtechniques, and the use ofphased array technology.
Digital Object Identifier: 10.1109/JPROC.2009.2022885
Vol. 97, No. 8, August 2009 | Proceedings of the IEEE 13730018-9219/$25.00 �2009 IEEE
Agency missions Herschel and Planck,to be launched in May 2009, are de-
scribed. Located at the Lagrangian
point L2, these are passively cooled
telescopes of 3.5 and 1.5 m diameter,
respectively, equipped with cryogeni-
cally cooled receiver systems. Herschel
will be operated as an observatory in the
frequency range of 450 GHz to 5.3 THz.To study the Cosmic Background radia-
tion, Planck will map the entire sky cov-
ering frequencies from 30Y857 GHz.
The requirement for active con-
trol, as in the GBT and FAST, and for
extremely high surface and pointing
accuracies, as in submillimeter tele-
scopes, has increased the need foraccurate metrology methods and the
associated sensing and measuring
devices. This subject is sometimes
called mechatronics and is reviewed
in BPerformance Improvement of a
Flexible Telescope[ with interesting
examples of existing radio telescopes,
which illustrate control and correc-tion of structural deformations caused
by gravity and temperature gradients.
III . SYNTHESIS ARRAYS
Because of the longer wavelength,
obtaining an angular resolution com-
mensurate with optical telescopes(better than one arcsecond) has been
a challenge to radio astronomers. This
has been met by the application of
interferometry and synthesis map-
ping. In synthesis mapping, the bright-
ness distribution of a radio source is
reconstructed from measurements of
its Fourier components in the spatialfrequency domain by a set of two-
element interferometers. The prime
example of this is the Very Large Array
(VLA) of the NRAO in New Mexico.
Its ongoing expansion and a number of
new, large synthesis arrays for fre-
quencies from 10 MHz to 1 THz, along
with a review of recent developmentsin imaging techniques, are described
in the following group of papers.
The Long Wavelength Array
(LWA) operates over the range 10Y88 MHz using 53 arrays of broadband
dipoles that will be spread over an area
about 400 km wide, roughly centered
on the VLA site in New Mexico. Fourbeams are formed simultaneously, one
of which is primarily devoted to cali-
bration of the ionosphere. The LWA
can address a wide range of nonthermal
radio sources. The frequency range is
similar to that of the Low Frequency
Array (LOFAR)’s Low Band, but the
LWA is located at a latitude approxi-mately 21� further south, which facil-
itates galactic-plane studies.
In The Netherlands, the deploy-
ment of the LOFAR Telescope is
under way for the frequency range of
15Y240 MHz. Each antenna is a large
set of barely visible dipoles rising out
of the flat Dutch terrain. The heart ofthe telescope is an enormous elec-
tronic and computer system to accom-
modate multiple beams within the
large field of view, and fast data
collection and handling. The core
station of about 2 km in diameter is
under construction in the northeast of
The Netherlands. LOFAR will growwith stations in The Netherlands and
neighboring countries providing base-
lines up to about 800 km.
The next paper, the Allen Tele-
scope Array (ATA), describes a syn-
thesis array with many innovative
features. It is actually the first instru-
ment to exploit the idea that largecollecting area can be achieved inex-
pensively by using many small reflec-
tor antennas, an approach sometimes
referred to as Blarge N, small D.[ The
ATA uses 6-m-diameter antennas, and
eventually the intention is to have
350 of them, although it is currently
operational with 34 antennas. To min-imize cost even more, the electronics
at each antenna is substantially sim-
pler than in other telescopes. Its entire
22 : 1 frequency range (0.5Y11 GHz) is
covered by a single cryogenically
cooled low-noise amplifier per polar-
ization and a single dual-polarization
log-periodic feed. The instrument isespecially powerful because its signal
processing is arranged to allow several
independent scientific programs to
proceed simultaneously. It is being
built by the SETI Institute and Uni-
versity of California using mostly
private funding, which is unusual in
radio astronomy. One of its principalpurposes is to support the search for
extraterrestrial intelligence, but since
most fields of view of the 6-m anten-
nas contain SETI target stars, other
astronomical observations can be car-
ried out at the same time.
Over the past 20 years, the VLA
has proved to be one of the mostsuccessful and widely used radio
telescopes. The Expanded Very Large
Array (EVLA) is a technical upgrade
using the same antennas with a new
receiving system that covers the
entire frequency range 1Y50 GHz in
ten bands. The maximum observing
bandwidth is increased by almost twoorders of magnitude over that of the
original VLA, resulting in an approx-
imate factor of ten increase in sensi-
tivity. The upgrade has required a new
receiving system including broadband
feeds, replacement of the waveguide
signal-transmission system with opti-
cal fiber, and new back-end andcorrelator.
For observations in the millimeter
wavelength range (30Y300 GHz), as
well as that portion of the submilli-
meter wavelengths accessible from
the Earth (300Y1000 GHz), an inter-
national collaboration of institutes in
Europe, North America, and Japan isin the process of constructing the
Atacama Large Millimeter Array
(ALMA) on the high desert in north-
ern Chile. ALMA consists of more
than 60 high accuracy reflectors with
receivers covering all observable parts
of the above frequency ranges and is
reconfigurable over baseline distancesof up to 15 km. ALMA will provide
observers with more than two orders
of magnitude improvement in sensi-
tivity and angular resolution com-
pared with existing telescopes in the
same frequency range.
Experience of more than four
decades in synthesis imaging, usingobservations of the complex visibility
measured between pairs of spaced
antennas, has resulted in increasingly
sophisticated imaging algorithms.
These are based on the Fourier trans-
form relationship between visibility
and the brightness distribution on the
Scanning the Issue
1374 Proceedings of the IEEE | Vol. 97, No. 8, August 2009
sky but include deconvolution, theuse of closure relationships to in-
crease dynamic range, methods of
imaging with wide angular fields,
wide signal bandwidths, use of multi-
frequency data, etc. A review is given
in the paper on BAdvances in Calibra-
tion and Imaging Techniques.[
IV. SKA AND PATHFINDERS
The largest new telescope now on the
drawing board and in prototype devel-
opment is the Square Kilometre Array.
It will provide unsurpassed sensitivity
and clean, multiple beams of very high
angular resolution in the frequencyrange from about 100 MHz to 10 GHz
(and possibly to higher frequencies
eventually). Some 19 institutes world-
wide are now collaborating in its
design. Realization of the full SKA
will be a massive undertaking, envi-
sioned as requiring unprecedented
international cooperation and thesharing of funding among multiple
governments. At present, national and
regional funding is supporting tech-
nology development efforts in Europe
and the United States, as well as the
construction of Bpathfinder[ instru-
ments in South Africa and Australia.
Although the latter are small com-pared with the full SKA, each has
significant capability by itself. The
SKA, its pathfinders, and related
research are described in the last
group of papers.
An overview paper, BThe Square
Kilometre Array,[ is presented by
longtime leaders of the project. Thepaper describes how the scientific
priorities of twenty-first century as-
tronomy are pushing not only for a far
more sensitive (and hence larger)
instrument than those available now
but also for one that can rapidly
achieve high sensitivity over large
areas of the sky. For this reason, sur-vey speed, which combines sensitivity
with instantaneous field of view, has
become the parameter of greatest in-
terest. Technologies that can achieve
high survey speed over the SKA’s two-
decade frequency range are described.
More than one antenna type is
needed. Advanced digital signal pro-cessing will also play a critical role.
The Murchison Widefield Array
(MWA) will consist of 512 arrays of
16 dipoles operating over 80Y300 MHz
and spread over an area about 3 km
wide in the Murchison Radio Observa-
tory, Western Australia. The design is
based on the use of a large number ofsmall antennas, which, for a given total
area, increases both the field of view
and the number of baselines over which
the visibility is measured. Major scien-
tific goals include the detection of the
hydrogen line (1420 MHz rest fre-
quency) in the red-shift range 6Y10,
studies of the sun and inner helio-sphere, and time-varying astronomical
phenomena. The MWA will also act as
a demonstrator for a method of imag-
ing in which the phase reference
position is fixed at the zenith, which
is a departure from the usual practice.
The Australian SKA Pathfinder
(ASKAP) will be an array of 36 para-bolic antennas of 12 m diameter oper-
ating in the range 700Y1800 MHz.
The antenna mounts are alt-azimuth
with a third axis that allows the
reflector and quadrupod structure to
rotate about the paraboloid’s axis so as
to maintain a constant angle with
respect to the sky. To provide high-speed sky coverage, the instantaneous
field of view is 30 square degrees. This
is achieved by the use of a phased
array feed system that produces mul-
tiple simultaneous beams, using a
pattern of conducting elements that
resembles a checkerboard. The array
will be located at the MurchisonRadio Observatory, in a radio-quiet
region of Western Australia.
In South Africa, the Bmany small
dishes with wide-band feeds[ ap-
proach to the SKA will be demon-
strated by BMeerKATVThe South
African Array.[ The goal is to build
an array of 80 dishes of 12 m diameterreceiving the 1Y10 GHz band. The
paper describes technological devel-
opments towards this in using reflec-
tors built from composite materials,
wide-band feed design, and packet-
based signal processing. The telescope
will be located in the Karoo desert of
the northern Cape Province, a largeprotected area proposed for the SKA.
To achieve the large instantaneous
field of view desired for the SKA, two
techniques involving phased arrays are
expected to be exploited. One involves
filling the focal plane of a reflector
antenna with multiple feeds so as to
generate multiple simultaneous beamson the sky, expanding the field of view
far beyond that of a traditional single-
feed radio telescope. This can be done
most efficiently if the feeds are syn-
thesized from an array of many small
antennas that tile the focal plane. The
second technique, applicable mainly
at the lowest frequencies of the SKA, isto construct each station of the large
synthesis array as a phased array of
subwavelength-sized elements, each
of which has a beam covering most of
the sky and which therefore need not
be mechanically steered. With suffi-
cient signal processing, the phased
array can provide enough simulta-neous beams to cover a large part of
the sky. The final paper of the issue,
BExtending the Field of View with
Phased Array Techniques: Results of
European Research,[ describes these
concepts in some detail and reports
the results of several projects in-
volving each technique. Some ofthese are already being exploited to
expand the capabilities of existing
telescopes. h
Acknowledgment
The editors are happy to thank theauthors of the papers for this Special
Issue and the numerous reviewers, all
of whom they had to keep to a rather
tight schedule, for providing a set of
interesting contributions. They also
thank the editorial staff of this
Proceedings, in particular J. Sun,
for support and help with masteringthe intricacies of the Manuscript
Central system. It has been a reward-
ing experience for them to act as
Guest Editors and they hope this issue
will ignite renewed interest among
the IEEE community for the science
and technology of radio astronomy.
Scanning the Issue
Vol. 97, No. 8, August 2009 | Proceedings of the IEEE 1375
ABOU T THE GUE ST EDITORS
Jacob W. M. Baars was born in The Netherlands
in 1937. He received the master’s and doctor
degrees in physics from Technical University Delft,
The Netherlands, in 1963 and 1970, respectively.
From 1963 to 1966, he was a Research Assis-
tant with the National Radio Astronomy Observa-
tory, Green Bank, WV. In 1966, he joined the
Netherlands Foundation for Radio Astronomy
(NFRA), Dwingeloo, where he participated in the
design and construction of the Westerbork Syn-
thesis Radio Telescope. From 1972 to 1975, he was Head of the Telescope
Division of NFRA. In 1975, he joined the Max-Planck-Institut fur Radio-
astronomie (MPIfR), Bonn, Germany, to become Head of a new Division
for Millimeter Technology. He also was Project Manager of the 30-m
Millimeter Radio Telescope in Granada, Spain. Since 1985, the operation
of this telescope has been in the hands of IRAM. Following the 30-m
telescope, he managed the design and construction of the 10-m diameter
Heinrich Hertz Submillimeter Telescope, a joint project with the
University of Arizona. From 1992 to 1994, he was Director of the
Submillimeter Telescope Observatory, Tucson, AZ. In 1997, he obtained a
leave of absence from MPIfR to become Chief Scientist of the Large
Millimeter Telescope Project, a joint project of the University of
Massachusetts and Instituto Nacional de Astrofisica, Optica y Electronica,
Mexico. He left this project after two years to join the ALMA Project at the
European Southern Observatory, where he held several functions in
project management and system engineering and, lastly, in the perfor-
mance evaluation of the two prototype antennas at the NRAO site in New
Mexico. He has been a Consultant to several radio telescope projects.
Since his retirement in 2004, he is a Guest Scientist with MPIfR, Bonn. His
interest lies in all aspects of radio telescope design and operation, in
observational techniques and calibration, and in the influences of the
atmosphere on millimeter wavelength observations. He has published
about 100 papers in radio astronomy and its technical aspects.
Dr. Baars is a member of the International Astronomical Union, URSI,
the American and German Astronomical Societies, and the Dutch Physical
Society.
Larry R. D’Addario (Member, IEEE) received the
S.B. degree from the Massachusetts Institute of
Technology, Cambridge, in 1968 and the M.S. and
Ph.D. degrees from Stanford University, Stanford,
CA, in 1969 and 1974, respectively, all in electrical
engineering.
At Stanford, his thesis work was connected
with the development of a five-element synthesis
radio telescope and its use in astronomical
observations of ionized hydrogen regions and
Jupiter. He subsequently joined the National Radio Astronomy Observa-
tory, where he worked from 1974 through 2004 on a wide variety of
projects, including assisting in the development of the VLA, VLBA, GBT,
and ALMA radio telescopes. He also contributed to the early develop-
ment of SIS mixer technology for millimeter wavelength receivers. From
1989 through 1995, he led the design and construction of an Earth station
to support orbiting radio telescopes for VLBI; the station was successfully
used with the Japanese VSOP mission. Since 2004, he has been with the
Jet Propulsion Laboratory, California Institute of Technology, Pasadena,
where he has been working to improve communication with spacecraft in
deep space, including development of transmitting arrays for possible
use in the Deep Space Network and, most recently, the modernization of
transponders for use on spacecraft.
A. Richard Thompson (Life Fellow, IEEE) was
born in Hull, Yorkshire, U.K. on April 7, 1931. He
received the B.Sc. degree (with honors) in physics
from the University of Manchester, U.K., in 1952
and the Ph.D. degree from the University of
Manchester, U.K., in 1956.
From 1952 to 1956, he was graduate student at
the Jodrell Bank Experimental Station. From 1956
to 1957, he was with E.M.I. Electronics, Middlesex,
U.K., working on missile guidance and telemetry.
In 1957, he joined Harvard College Observatory as a Research Associate
and later Research Fellow. During this period, he was engaged in solar
studies at the Harvard Radio Astronomy Station, Fort Davis, TX, initiated
for the International Geophysical Year (1957Y1958). In 1962, he joined the
Electrical Engineering Department, Stanford University, Stanford, CA,
becoming a Senior Research Associate in radio astronomy. During
1966Y1972, he also held a visiting appointment at the Owens Valley Radio
Observatory of the California Institute of Technology, Pasadena. In 1973,
he joined the National Radio Astronomy Observatory (NRAO) and, with
the VLA project, served as Systems Engineer, Head of Electronics, and
Deputy Project Manager. From 1984 to 1992, he worked on the VLBA
project as Systems Engineer and Deputy Manager. From 1992, he was
Assistant Head of the NRAO Central Development Laboratory and retired
in 1999. While with NRAO, he was actively engaged in frequency
coordination for radio astronomy, and from 1978 to 1998 was a member
of U.S. Study Group 7 of the International Telecommunication Union
(earlier Study Group 2 of CCIR) and Chairman of the U.S. Study Group 7D
on radio astronomy. He was a member of the Committee on Radio
Frequencies (CORF) of the National Academy of Sciences in 1980Y1991,
and subsequently a Consultant to CORF. From 1982 to 1988, he was
Secretary of the Interunion Commission for Allocation of Frequencies for
Radio Astronomy and Space Sciences. He is currently an Emeritus
Scientist at NRAO.
Dr. Thompson is a member of the International Union of Radio
Science, Commission 40 of the IAU, and the American Astronomical
Society.
Scanning the Issue
1376 Proceedings of the IEEE | Vol. 97, No. 8, August 2009