advances in radio telescopes [scanning the issue]

4
SCANNING THE ISSUE Advances in Radio Telescopes By JACOB W. M. BAARS Guest Editor LARRY R. D’ADDARIO , Member IEEE Guest Editor A. RICHARD THOMPSON , Life Fellow IEEE Guest 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 the drawing 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 an enormous 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 aperture telescopes. 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 antenna telescopes, 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 ANTENNA TELESCOPES 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 offset paraboloid 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-diameter BArecibo 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 new single-aperture and synthesis array radio telescopes and covers advances in antenna metrology, imaging techniques, and the use of phased array technology. Digital Object Identifier: 10.1109/JPROC.2009.2022885 Vol. 97, No. 8, August 2009 | Proceedings of the IEEE 1373 0018-9219/$25.00 Ó2009 IEEE

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Page 1: Advances in Radio Telescopes [Scanning the Issue]

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

Page 2: Advances in Radio Telescopes [Scanning the Issue]

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

Page 3: Advances in Radio Telescopes [Scanning the Issue]

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

Page 4: Advances in Radio Telescopes [Scanning the Issue]

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