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Final Report, Part One VLA FEASIBILITY STUDY (A SUMMARY) Edited by: R. I. presnell Prepared for: NATIONAL SCIENCE FOUNDATION OFFICE OF NATIONAL CENTERS AND FACILITIES OPERATIONS WASHINGTON, D C. 20550 CONTRACT NSF-C706 STANFORD RESEARCH INSTITUTE Menlo Park, California 94025 • U.S.A.
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Page 1: VLA FEASIBILITY STUDY (A SUMMARY) - library.nrao.edu

Final Report, Part One

VLA FEASIBILITY STUDY (A SUMMARY)

Edited by: R. I. p r e s n e l l

Prepared for:

NATIONAL SCIENCE FOUNDATION OFFICE OF NATIONAL CENTERS

AND FACILITIES OPERATIONS WASHINGTON, D C. 20550

CONTRACT NSF-C706

STANFORD RESEARCH INSTITUTE Menlo Park, California 94025 • U.S.A.

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^ STANFORD RESEARCH INSTITUTEMenlo Park, California 94025 ■ U.S.A.

Final Report, Part One February 1972

VLA FEASIBILITY STUDY (A SUMMARY)

Edited by: R. I. PRESNELL

Prepared for:

NATIONAL SCIENCE FOUNDATION OFFICE OF NATIONAL CENTERS

AND FACILITIES OPERATIONS WASHINGTON, D.C. 20550

CONTRACT NSF-C706

SRI Project 1517

Approved by:

DAVID A. JOHNSON. Director Radio Physics Laboratory

RAY L. LEADABRAND, Executive Director Electronics and Radio Sciences Division

Copy No.

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ABSTRACT

This report summarizes the results of a three-month feasibility study of a large radio telescope system called the Very Large Array (VLA). The National Radio Astronomy Observatory (NRAO) is proposing to construct a 27-element radio telescope array in a wye configuration at a site in the southwest part of the United States. The objectives of this study are to verify the technical feasibility of the VLA system design, confirm the cost and time schedules, and evaluate the project- management method proposed by NRAO. The overall study procedure has been to break down the VLA system into subsystems or components and assign a team of specialists to study each of the subsystems and report their findings. The feasibility study results are as follows:

• The VLA technical feasibility is confirmed.• The cost has been accurately estimated by NRAO.• The time for construction could be as short as four years.• NRAO’s technical competence for this task has been confirmed.• The project-management method proposed by NRAO is generally

good, but could be improved.

iii

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CONTENTS

ABSTRACT........................................................ iii

ACKNOWLEDGMENTS ................................................ v

CONTRIBUTORS....................................................... vi

I INTRODUCTION.............................................. 1

II BACKGROUND................................................ 4

III TECHNICAL FEASIBILITY...................................... 7

IV COST AND TIME FEASIBILITY.................................. 9

V NRAO PROJECT MANAGEMENT.................................... 12A. General................................................ 12

B. Proposed VLA Project Management ....................... 121. Development and Construction Phase ............... 122, Operational Phase.................................. 14

C. Conclusions............................................ 15

REFERENCES......................................................... 17

TABLES

Table 1 VLA Cost-Estimate Summary............................. 9

iv

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ACKNOWLEDGMENTS

The project is indebted to Dr. Hein Hvatum and the staff of the National Radio Astronomy Observatory for their complete cooperation during our many visits to their facilities at Charlottesville and Green Bank, which allowed this study to proceed quickly and thoroughly. The project is also indebted to Dr. Ronald Bracewell of the Stanford University Radio Astronomy Institute for his help in explaining correlator array concepts to the project staff and reviewing the VLA specifications.

v

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CONTRIBUTORS

For the purposes of this study the VLA system was divided into sub­systems or components, and specialists were assigned to study them. Each specialist wrote three contributions to the final report, as follows:

• Subsystem specification• Technical feasibility• Cost feasibility.The subsystems and the specialists who studied them are listed

below.

Subsystem SpecialistAntenna N. Stafford and H. KunnesTrack system w. HullTransporter G. DurfeyRF system G. Augus tReceiver w. EdsonLocal-oscillator system w. EdsonIF distribution system w. EdsonDelay-line system A. BahrCorrelator A. BahrComputer system M. BaronSite w. HullFacilities w. Hull

NRAO management was studied by L. Gray and L. Clarke.

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I INTRODUCTION

On October 4, 1971, the National Science Foundation awarded Stanford Research Institute (SRI) a contract to study the feasibility of a synthetic-aperture radio telescope designated the Very Large Array (VLA). The National Radio Astronomy Observatory (NRAO) operated by Associated Universities, Inc. has submitted to the Foundation a proposal for the VLA Telescope system. The results of the feasibility study are summa­rized in this report. The study included the following tasks:

• Determination of the technical feasibility of the proposed system and the ability of the design to meet the performance specifications, in the light of existing technology.

• Confirmation of the cost and time schedules included in the financial and management plans for the construction, develop­ment, and operation of the VLA.

• Evaluation of the project-management method proposed by NRAO.

The scientific objectives of the VLA system have already been established, and it is not within the scope of this study to reexamine either the scientific objectives or the system performance requirements based on these objectives. Furthermore, the evaluation will be based on the array size, shape, antenna number, and gain as currently proposed, and a redesign or update of the proposed system will not be considered in this study.

The proposed VLA consists of twenty-seven 25-m-diameter antennas, together with their associated electronics and other equipment. The system is operated as a correlator array, in which the output of each antenna is correlated with that of every other antenna in the array.

1

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The multiple simultaneous correlator outputs, each representing one component of the complex Fourier transform of the brightness distribution of the region of sky under observation, are processed and accumulated in a computer for a period of up to 14 hours, during which time the array tracks the region of sky as the earth rotates thereby utilizing the

4principle of aperture supersynthesis. Some 10 independent Fourier components are obtained in 8 to 14 hours, which are then transformed into a "picture" of the source or region observed.

The individual antennas are paraboloids of revolution with solid surface, permitting operation to wavelengths as short as 3 cm. They are elevation-over-azimuth mounted, giving full-sky coverage to within 5° of the horizon. A central computer controls the positioning and tracking of the antennas.

The antennas are placed along the arms of an equiangular wye configuration. Each arm of the wye is approximately 21 km in length.A pair of standard-guage railroad tracks runs the full length of each arm of the wye. The characteristics of the array (resolution and side- lobe levels) may be modified as needed by moving the antennas along the arms of the wye on the track system. Three self-propelled transport systems are used to move the antennas, which are normally bolted to concrete foundations when in use. A complete change of array configura­tion can be accomplished in two days.

The electronic system is basically that of a conventional double- sideband-correlation interferometer, with some complexity introduced by the long distances and large number of elements involved. Low-noise, uncooled, degenerate parametric amplifiers are employed in the front ends, giving high stability and total system noise of less than 100°K.A phase-stable local-oscillator (LO) signal is provided at each antenna through use of a coaxial—cable round-trip phase—correcting system.

2

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A central computer is used to perform data processing and all monitor and control functions for the antennas and electronics.

Section II of the report gives a short historical account of the VLA concept from 1960 to the present time. Section III is a discussion of the technical feasibility question. Section IV reviews the cost and time schedule of the VLA, and Section V covers NRAO’s Project management plan.

3

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II BACKGROUND

Higher resolution has always been a major goal of radio astronomy instrumental development. The need for resolution led, in the 1950s and early 1960s, to the extensive development and use of radio inter­ferometers, particularly at the radio observatories in Cambridge and Manchester, England; Sydney, Australia; and Cal Tech in the United States. In 1960, and before, ideas began to develop at various places for an instrument that would map, in a reasonable observing time, a region of sky with a high-resolution pencil beam. Since then, various technical developments have occurred that make feasible the achievement of such an instrument with a 1-second resolution.

The general concept of the VLA as an instrument to obtain radio "pictures" with high resolution, sensitivity, and speed began developing at the NRAO in 1961, and some preliminary studies were done in 1961 and 1962. In the spring of 1963, development of a variable-baseline interferometer giving 8-second resolution was begun in order to investi­gate and develop the techniques that would be required for the VLA. In particular, the technique of supersynthesis appeared to offer the most promising means of realizing the VLA concept, and the interferometer was built to be operated using that technique.

The Panel on Astronomical Facilities of the National Academy ofoSciences’ Committee on Science and Public Policy published its report

*The majority of this section was abstracted from the NRAO Proposal Vol. I1' and contributed by Dr. Hein Hvatum of NRAO in private communi­cation.

t References are listed at the end of the report.

4

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in August 1964. The report of the Panel stated ’’...that the primary need in radio astronomy is a very powerful high-resolution instrument." The Panel recommended construction, by the NRAO, of a large array that would achieve a resolution of less than 10 seconds at centimeter wave­lengths, with low sidelobe levels, and with sufficient sensitivity to de­tect 25 sources per square degree of sky. The Panel further recommended, as a secondary goal, that if possible the array should achieve 1-minute resolution at 21-cm wavelength, with extremely low sidelobe levels.

In response to these recommendations, the NRAO began detailed systems studies in the summer of 1964. Funds for design work were provided by the National Science Foundation in NRAO budgets for Fiscal Years 1965, 1966, and 1967. The design was carried out by the NRAO staff with the assistance of a number of radio astronomers from other institu­tions. Several general meetings were held during the course of the work to review progress and problems. Approximately twenty-five radio astronomers from throughout the country participated in each of these review meetings. The discussions at these meetings have had a strong influence on the final design; in particular, the resolution of the instrument has been progressively improved from 10 seconds to 1 second.A preliminary progress report3 of the design work was issued and widely distributed in January 1966.

1 4In January 1967 a detailed two-volume proposal * for the VLA was published by NRAO, following one more year of effort in firming up the design. During the period from 1966 to 1969 many subcontracts were let to study and/or prototype all of the various electronic and mechanical subsystems of the VLA as well as complete site surveys and facility plans.

5In January 1969 a third volume of the VLA proposal was published by NRAO, following two more years of study. Several technical subsystems

5

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and components that were considered unique or critical to the successfuloperation of the planned array were designed, prototyped, and tested.Many radio astronomers, both from the U.S. and abroad, discussed,criticized, and commented on the original proposal, and the direction of

5the work reported on in Vol. Ill was strongly influenced by these discussions. In particular, the recommendations by the ad hoc Advisory Panel for Large Radio Astronomy Facilities were taken into account.

Although the basic concept of the Very Large Array remains unchanged, some of its performance specifications were slightly modified when it was found that the original performance goals were either unwarranted or could be only achieved by undue costs and/or unreasonable technical efforts. For example, the maximum acceptable peak sidelobe level within the field of view was relaxed from -20 dB to -15 dB, and this change was a main contributing factor in the reduction from 36 to 27 antennas.

6In August 1969 the report from the second Dicke Committee meeting was published. In it, the committee recommended an immediate start on the construction of the VLA. The report also noted that most of the technical uncertainties of the VLA system had been resolved since the previous Dicke Committee meeting in 1967.

During the past two years, NRAO has not done any new system research or development work on the VLA. The proposal has been in the hands of the government, awaiting approval.

6

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Ill TECHNICAL FEASIBILITY

The study project approach to the technical-feasibility question has been to break the VLA system down into subsystems and/or components and to study specifications for each item. Each of eight project staff members was assigned an area for study. The question of technical feasibility was then answered by establishing that the subsystem is within the state of the engineering art or that it has been adequately prototyped by NRAO. In addition, the following questions were addressed:

• Are the specifications given by the NRAO proposal sufficient?• Are the specifications correct?

The general answer to the feasibility question is that the VLAsystem is entirely feasible. Although some minor imperfections doexist in the NRAO proposal specifications, they are easily corrected.Detailed feasibility discussions of each subsystem are given in Part

7Two of this final report.

The concept of using two-element-antenna-array correlations with aperture synthesis in a radio telescope has been in existence at least since 1961. Since that time many researchers have made good use of the technique, including those at Cambridge, England; Cal Tech; NRAO; and Stanford. Consequently, there can be little doubt about the feasibility of the concept. The two characteristics that make the VLA different from other correlator array systems are the total number of elements (27) in simultaneous use, and the maximum element spacing of 35 km. The first characteristic is feasible because a few antennas could malfunction without making the output data completely unusable, even though the antennas are operating simultaneously. In other words, the array tends

7

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to degrade gradually rather than suddenly. Long-baseline interferometers (a 35-km-baseline system at Green Bank, West Virginia, and cross-continent very-long-baseline interferometers) have been adequately demonstrated.

8

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