The Telescope Program for the National Radio Astronomy Observatory at Green Bank, West Virginia

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<ul><li><p>Emberson and Ashton: Telescope for the National Radio Astronomy Observatory</p><p>Using fluctuation voltage it is possible14 to deduce thesize of the elementary charges causing this voltage.When the result was published, a value correspondingto three electrons was given. This was an error causedby an incorrect and much too low value of capacity.Afterwards, the mistake was found and the proper valueof electron charge secured in agreement with othermethods.</p><p>These old experiments at Wheaton were quite thrill-ing at the time. My present experiments3 at the otherend of the spectrum in Tasmania using cosmic static atkilometer wavelengths fully equal the old in the realmof the unexpected.Much remains to be done.23 Grote Reber, 'Between the atmospherics," J. Geophys. Res. (in</p><p>press).</p><p>The Telescope Program for the National RadioAstronomy Observatory at Green Bank,</p><p>West Virginia*R. M. EMBERSONt, SENIOR MEMBER, IRE, AND N. L. ASHTONt</p><p>Summary-A brief account is given of the initiation of the feasi-bility study on the establishment and operation of the NationalRadio Astronomy Observatory at Green Bank, West Virginia. Theprincipal research facilities will be the radio telescopes, and a seriesof such telescopes have been proposed. The desired performancecharacteristics are reviewed. A 140-foot steerable paraboloid on anequatorial mount has been designed. The steps leading to this de-sign are described, as well as the general features of the designedand the expected operating performance. The National Radio Astron-omy Observatory is being sponsored by the National Science Foun-dation.</p><p>INTRODUCTIONN January, 1954, a conference' on radio astronomywas held in Washington, D. C., jointly sponsoredby the National Science Foundation, the California</p><p>Institute of Technology, and the Department of Ter-restrial Magnetism of the Carnegie Institution of Wash-ington. Participants included both United States scien-tists and engineers, as well as numerous distinguishedforeign visitors. The conference made clear to the USparticipants that the lead in radio astronomy had beentaken by other countries, this despite such noteworthycontributions as Karl Jansky's initial discovery,2 G. Reb-er's pioneering work,3 the tremendous advances in our</p><p>* Original manuscript received by the IRE, November 8, 1957.This work was supported by the National Science Foundation.</p><p>t Associated Universities, Inc., New York, N. Y.t University of Iowa, Iowa City, Iowa.1 "Proceedings of conference on radio astronomy, January 4-6,</p><p>1954," J. Geophys. Res, vol. 59, p. 140; March, 1954. See also "RadioAstronomy Conference," Science, vol. 119, p. 588; April 30, 1954.</p><p>2 K. Jansky, "Directional studies of atmospherics at high fre-quencies," PROC. IRE, vol. 20, pp. 1920-1932; December, 1932.</p><p>-, "Electrical disturbances apparently of extraterrestrial origin,"PROC. IRE, vol. 21, pp. 1387-1398; October, 1933.</p><p>3 G. Reber, "Cosmic static," PROC. IRE, Vol. 28, pp. 68-70;February, 1940.</p><p>-, Astrophys. J., vol. 91, pp. 621-632; June, 1940.</p><p>electronics techniques, particularly those related to ra-dar and radio communications, and the first observationof the 1420-mc radiation of interstellar hydrogen byEwen and Purcell.4 The conference also indicated thatUS radio astronomy would fall behind further andfurther if steps were not taken to provide observing fa-cilities comparable to those planned or being built inother countries.The equipment requirements of radio astronomers</p><p>are of three general classes: the antenna system; the re-ceiver and data processing equipment; and the ancillarypower supplies, time and frequency standards andsimilar items. In most instances, the antenna is, by far,the most costly part of the observing equipment. Be-cause of the cost and other factors most US radioastronomers have been working with antennas that aresmaller and less effective than those of foreign scientists,who, soon after World War II, gained support of theirrespective governments for the construction of largeradio astronomy equipment. The smaller size, or gain ofan antenna, can be compensated for in part throughbetter electronics-a receiver with a better noise figureand extreme stability to permit integration of the veryweak celestial signals over longer periods of time; butfor angular resolution and the avoidance of confusion,which is important to astronomers, there is no substi-tute for aperture size. Hence, the conclusion from theJanuary, 1954, conference was that larger and more ef-fective antennas were needed in the United States. Howto obtain them was another question. The suggestionwas made that several institutions might join together</p><p>4 H. I. Ewen and E. M. Purcell, Nature, vol. 168, p. 356; 1951.</p><p>1958 23</p></li><li><p>PROCEEDINGS OF THE IRE</p><p>in the construction of a large antenna, larger than anysingle institution could afford. In the spring of 1954,there were conversations between representatives ofHarvard, Massachusetts Institute of Technology, andthe Naval Research Laboratory on a proposal to con-struct a large steerable paraboloid. This would be a gen-eral purpose research tool, applicable to astronomicalproblems as well as to more immediately utilitarianstudies, such as scatter propagation. The enthusiasm ofthe scientists and engineers was dampened by the highestimated cost of a large paraboloid. But the conversa-tions continued, and a parallel was found in the researchreactor and large accelerators at the Brookhaven Na-tional Laboratory. This laboratory was established andis operated by Associated Universities, Inc.5 under acontract with the Atomic Energy Commission. Theresearch facilities there are enjoyed by visiting scientistsas well as by those on the permanent staff. Why notestablish a radio astronomy observatory along the samegeneral pattern?</p><p>After further conferences, the National Science Foun-dation made a grant to Associated Universities, Inc. fora study on the feasibility of establishing and operating aradio astronomy observatory. The study included asearch for a suitable site, plans for the site development,several investigations contributing to the design andconstruction of large antennas, and a plan for the or-ganization and operation of the proposed observatory.'More recently, the National Science Foundation hascontracted with Associated Universities, Inc. to estab-lish and operate a National Radio Astronomy Observa-tory at Green Bank, West Virginia.</p><p>In the remainder of this article, the portion of thefeasibility study and of subsequent contract activitiesrelated to large antennas will be discussed. To help inunderstanding decisions that have been made, it is de-sirable to recognize that what an engineer regards as anantenna system is a telescope to the astronomer, andthe astronomer routinely deals with signals that thecommunication engineer would regard as a smallamount of noise. The term "telescope" will be employedin deference to the nomenclature of the astronomers,and we will regard it as an instrument for research, eventhough the tonnage of materials involved are about thesame as for a destroyer.The nature of the signals studied by radio astrono-</p><p>mers is described in other articles in this issue of thePROCEEDINGS.7 Let us note briefly here that the signalsare weak as judged by radio communications or radar</p><p>* Associated Universities, Inc. is a nonprofit corporation charteredby the Board of Regents of the State of New York; the corporationis sponsored by nine universities: Columbia, Cornell, Harvard, JohnsHopkins, Massachusetts Institute of Technology, Pennsylvania,Princeton, Rochester, and Yale.</p><p>6 B. J. Bok, "A national radio observatory," Sci. Amer., vol. 195,pp. 56-64; October, 1956.</p><p>L. V. Berkner, 'National radio observatory site," Sky and Tele-scope, vol. 15, p. 398; July, 1956.</p><p>7 J. W. Findlay, 'Noise levels at a radio astronomy observatory,"this issue, p. 35.</p><p>standards. All wavelengths transmitted by the earth'satmosphere are represented (e.g., from less than 1-cmwavelength to more than 10 meters). There are somefrequencies of particular astronomical importance be-cause of relatively strong bright-line radiations associ-ated with natural atomic and molecular processes, (e.g.,the 21-cm radiation of the hydrogen atom). The typicalcelestial radiation is unmodulated white noise, andin some cases may be polarized. The signals come frommany sources from the sun, from the center of ourMilky Way system or galaxy, from un-ique objects suchas the Crab Nebula, from external galaxies, and fromsources of such low visual luminosity that no opticalidentification has been possible. All that precedes is conI-cerned with radio astronomy in the passive or receiving-only phase, wherein the transmitters are natural phe-nomena and not under the control of the experimenter.The active or radar phase of radio astronomy is limitedto the solar system, and there is no sharp line of de-marcation between these studies and those of the upperatmosphere that stem from geophysical and radio com-munications interests. Throughout this paper the dis-cussion will generally be directed to the passive phase ofradio astronomy, but most of what is said is equally ap-plicable to the active phase.</p><p>GENERAL DISCUSSIONAt the outset, the Advisory Committee8 for the</p><p>feasibility study pointed out that the observing require-ments in the microwave region required a telescope withboth high gain and high resolution. At meter wave-lengths and longer, high resolution was the dominantrequirement because of the more satisfactory status ofthe receiver art for such radiations. It was also notedthat resolution at the longer wavelengths was a linearaffair, e.g., a simple array or Mills Cross,9 whereas thehigh-gain, high-resolution telescope for microwaveswould involve a paraboloid, or something equivalent,and the cost would vary as the square or some higherpower of the aperture. Thus a concept for the telescopesat Green Bank was formed early, namely, that the rela-tively expensive paraboloids would be part of the perma-nent equipment, whereas arrays would be regarded asexpendable equipment and tailored to fit the particularneeds of each observing program. This concept recog-nizes that a paraboloid is inherently a broad-band de-</p><p>8 The feasibility study started early in 1955, with an AdvisoryCommittee under the chairmanship of J. P. Hagen, Naval ResearchLaboratory, and composed of B. J. Bok, Harvard College Observa-tory; A. J. Deutsch, Mt. Wilson and Palomar Observatories; H. I.Ewen, Harvard College Observatory; L. Goldberg, University ofMichigan; W. E. Gordon, Cornell University; F. T. Haddock, Uni-versity of Michigan; J. D. Kraus, Ohio State University; A. B.Meinel, University of Chicago; M. A. Tuve, Carnegie Institution ofWashington; H. E. Wells, Carnegie Institution of Washington; andJ. B. Weisner, Massachusetts Institute of Technology. When Dr.Hagen was made the head of Vanguard, he resigned his position onthe Advisory Committee, and Dr. Bok served as chairman until heleft to become the director of the Australian National Observatory.</p><p>9 B. Y. Mills, A. G. Little, K. V. Sheridan, and 0. B. Slee, 'A highresolution radio telescope for use at 3.5 cm," this issue, p. 67.</p><p>24 January</p></li><li><p>Emberson and Ashton: Telescope for the National Radio Astronomy Observatory</p><p>vice, limited at one extreme when the aperture is thesame order of magnitude as the longest wavelength tobe observed, and at the other extreme when the irregu-larities of the paraboloid surface (openings, if a mesh;deviation from the best-fitting true paraboloid) are asignificant fraction of the shortest wavelength.</p><p>There have been numerous theoretical studies on theshort-wavelength limitations of paraboloid surfaces.10Experimental evidence and the theoretical predictionsare in general agreement, Friis" and his colleagues hav-ing shown with the Homdel 60-foot paraboloid that sur-face deviations of 1/16th the observed wavelength havesignif cance if utmost gain is the objective. From theexperimental point of view, the radio astronomer doesnot have a free choice of what wavelength will be ob-served. There are practical factors of interference fromman-made sources. If these devices are intentionaltransmitters, they generally radiate in fixed bandswhich the radio astronomer will try to avoid. The as-tronomers are also limited by the transparency of theearth's atmosphere. And, finally, the celestial radiationshave a continuum plus certain bright lines (sometimesthese may be observed as absorption lines,"2 similar tothe Fraunhofer lines in the solar spectrum). As previ-ously mentioned, one of the most important of these"bright lines" is the 21-cm radiation from atomic hydro-gen, because hydrogen is a fundamental building blockof the universe and the most abundant element foundnot only throughout our Milky Way system, but inother galaxies as well. Because of the great importanceof this hydrogen radiation, any general purpose radiotelescope must be capable of working effectively at 21cm.'3 Thus it is seen that, independent of the diameter,a paraboloid for a radio astronomy telescope must meeta surface tolerance of 2 inch to be ideal for 21-cmwork; of course, the paraboloid would have value if thesurface irregularities were greater than 1/16th the wave-length, but the over-all antenna gain would suffer. Fur-thermore, if the irregularities were systematic in char-acter because of the manufacturing process, curious</p><p>10 S. Silver, Ed., "Microwave Antenna Theory and Design,"Mass. Inst. Tech. Rad. Lab. Ser., vol. 12, McGraw-Hill Book Co.,Inc., New York, N. Y.; 1949.</p><p>R. C. Spencer, "Multiple Feed High-Gain Antennas for RadioAstronomy," paper presented at IAU conference; May 27, 1955.</p><p>D. J. Cheng, "Study of Phase Error and Tolerance Effects inMicrowave Reflectors," Syracuse Univ. Res. Inst., Syracuse, N. Y.,Contract No. AF 30(602)-924; December 31, 1955.</p><p>11 J. Robieux, "Influence of the Manufacturing Precision of anAntenna on its Performance," Compagnie Generale de T.S.F., Paris,France, Contract No. AF 61(514)-737C; May 27, 1955.</p><p>A. B. Crawford, H. T. Friis, and W. C. Jahes, Jr., "A 60-foot di-ameter parabolic antenna for propagation studies," Bell Sys. Tech.J., vol. 35, pp. 1199-1205; September, 1956.</p><p>J. Ruze, "Physical Limitations on Antennas," Mass. Inst. Tech.,Res. Lab. Electronics, Cambridge, Mass., Tech. Rep. No. 248;1952.</p><p>12 A. E. Lilley, "The absorption of radio waves in space," Sci.Amer., vol. 197, pp. 48-55; July, 1957.</p><p>13 The giant 250-foot telescope of the University of Manchester,nearing completion at Jodrell Bank, was conceived by Prof. Lovellbefore the hydrogen radiation had been observed: a redesign of thereflector has been accomplished while the telescope was being con-structed in order that 21-cm observations might be undertaken.</p><p>effects might be introduced into some experiments, suchas polarization measurements.With the minimum acceptable specification for the</p><p>surface tolerance essentially predetermined, there re-mains for selection only the diameter and focal length(or f/d ratio). Of these, the...</p></li></ul>

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