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ill A PROGRAN,T FOR CONSTRUCTION OF RADIO TELESCOPES In the ffrst two sections of this report we have discussed the scientiûc prob- Iems presented to radio astronomy and the instrumental specifications de- maod-"d by them. It was shown that existing instrumentation is inadequate to perfoun efiective work on these plogïâms; nevertheless, it was found that the' technical knowledge exists to build instruments tlat can reach beyond the thresholds of information now foreseen. It was for¡nil that instruments of extremely high resolution were required, anil also versatile instruments of considerabíy grãater capacity than those now in existence' Along with these, for student train- a group of lãsser instruments useful in special problems and report pres€nts the-speciffc recommenda- in! is required. This section of the tions of the Panel as to the radio telescopes that it believes lepresent a rea- sonable anil prudent goal for the next decade. A MAJOR HIGH-RESOLUTION INSTRUMENT In the discussion in Sections I and II, it has been shown that the primary need in radio astronomy is a very powerful high-resolutíon instrument' This is needed particularly for the study of the physics of the bright extragalactic radio sources, and for cosmological studies, anil also for other programs in galactic structure and solar-system phenomena. Fo-r reasons already given, ãspecially the need for detailed stucly of the bright extragalactic sources, this instrument must, as its prime goal, achieve a resolution of less than 10 seconds of arc at centimeter wavelengths. Its collecting area must be ade- quate to allow the detection of sources so faint that about 25 sources can be detectecl in each square degree of sþ. The size and distribution of its energy- 50 Copyright © National Academy of Sciences. All rights reserved.

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collecting area must be such tlat sidelobe responses can be suppressed to a satisfactory level As a secondary goal, tle instrument should, if feasible, achieve high quality, extremeþ low sidelobes, and pencil-beam performance with a resolution of about one minute of arc at 21-cm wavelength, for use in studies of galactic structure' The Èanel recommends, as the largest single untlertaking in radio astronomy, the construction of a large array that would achieve t-hese goals' Such an änay might consist, for example, of about l@ separate parabolic antennas, each perhaps 85 feet in diameter. Each should have a surface quality giving gõocl operation down to wavelengths as short as 3 cm' These -"! U" pt"""d in a single line, utilizing the rotation of the earth ""t"o"uãtheá, in efiect, along a second coordinate on the sþ' Or they might to move be arrangeil in the pattern of a cross. Other possibilities may prove efiective, ancl the ãngineering of the array shoultl incluile a search for a format that will give rn'¿ximum returns' \Mhen the antennas are spaced so as to give ooe-rii.nrrte-of-arc resolution at 2L cm, about I0 per cent of the aperture woulcl be fflled, leading to extremely low sidelobe levels. When the antennas are s-ûlllng so as to give resolutions of less than 10 seconds of arc, the aper- paced will be cónsiderably less than 1 per cent, and special proceilures, ture of the kinã outlined in Section II, will be required to provide adequate side- lobe suppression. Every efiort should be made to include as high a degree of ue.r"ìility as can be achieved without increasing costs unreasonably' It ís exþected that a project of this magnítude will cost $40 million' The cost is fairli¡ predictable, since 85-foot paraboloids of the speciffed su-rface háuð ,row been built many times and several engineering and con- """oru"y ffrms have proved their competence (see Table A, Appendix, p' 90) ' strucuon Likewise there is coìsiderable experience in interferometry between single pairs of such dishes over a long baseline. The eartì s atmosphere anil iono- iphere do not introduce appreciable deparhrres from phase coherence, as slown by tests at Joclrell Bank in Englancl at a wavelength of 1'9 meters, over a baseline of 70 miles. Control of line-Iengths and phase stability has been tested at the Owens Valley Railio Observatory at wavelengths of 31 cm antl 12 cm, and in a more limitetl way at the National Ratlio Astronomy Observa- tory ai Green Bank at a wavelength of 10 cm. A precise lineJength monitor- ing system, based on signals sent out from tÏe central receiving point, has beãn'used on tle Stanfoid S2-element anây. In all these tests, stability ade- quate to give an angular resolution of two seconds of arc was demonstrated' - Æthãugh therã is little doubt about the basic feasibility of the large high-resolutìon array, antl although the performance of the entire system Copyright © National Academy of Sciences. All rights reserved.

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may be predicted with some con.ffilence, the replication of so many com- ponents, even though each is of a proven design, and the tying together of the sígnal Iines from all the wiileþ spaced receiving points make the whole undertaking a very complex one. The proiect thus appears to be beyond the capabilities of a single university, and, ín fact, falls naturally into the category of instruments that shoulil be constructed by the National Radio Astronomy Obsewatory. Since this will be a major national enileavor, the National Radio Astronomy Observatory should make every efiort to avail itself of the knowledge anil experience in the required tecbniques possessed by the scientiffc communþ. Means should be provided for extensive par- ticipation by scientists who are not members of the NR {O stafi in the plan- níng and development of the instrument. It may take little less than a decatle to build, and so should be started as soon as possible. LIMITED CAPABILITY A HICH-RESOLATrcN ARRAY OF Going beyond the major instrument proposeil above, the Panel feels that the neeil for high resolution is great enough to warrant proposâl of an alter- native, simpler, less expensive, and quicker approach to tÏe high-resolution It problem pet se. is highly desirable to have an instrument ttrat can quite high resolution on the brighter radio sources. This would achieve provide many advantages: 1 ) Some prime data would be fortlcoming at the earliest possible time; 2) these data would be valuable guides to the design of the giant high-¡esolution antenna; 3) this antenna would provide a teslíng ground for techniques possibly useful to the high-resolution antenna, such as methoils of sidelobe suppression and the interconnection of various en- ergy-collecting elements. To implement this neeil, the Panel recommends the funding of the already-proposed extension of the obsewing facilities at the Owens Valley Observatory of the Califomia Institute of Technology. This proposeil exten- sion can be achieved relatively quickly, and will produce high-resolution measurements on a limited number of radio sources. The extension includes the construction of four new steerable parabolic reflectors of about 130-foot apertu-re, bringing to six the number of antennas at tÏe site, and an increase in the length of the inte¡ferometer track on which the antennas are car¡ied. Based on existing alata, it appears virtuaþ certain that the presently approved, but unfunded, additions will prove highly successful. In tÏis case, a further increase in the available equipment by a factor of about two will 52 ¡ll- Copyright © National Academy of Sciences. All rights reserved.

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allow useful resolutions of less tha¡ I0 seconds of arc, tìe speciûcation set previously in thís report. Thus, the Panel feels that one should anticipate now the need for this further adilition to tTe Owens Valley faciJities. The cost of the approved extension to the Owens Valley system has been care- fully estimated to be about $5 million; the second extension recommended here would cost about the same amount. Therefore, the total funding re- quired for this development is $10 million. It should perhaps be emphasized that this inskument is not an adequâte substitute for the giant telescope previously recommended; but it will serve as ân efiective interim instrument and guide to optimum design of the very large array, and continue to be V L fl useful in its own right. Sínce t}re very large array will take almost a decade to build, construction should be commenced immediately. The results from the extension at tìe Owens Valley Observatory can be e¡pected to come in good time to contribute to the ffnal success of the very large array. VL A LARGE PARABOLOIDS It was shown in Section II that tìere is a great need for additional powerful, multi-purpose, easily used instruments to complement the high-resolution instruments proposed above. These are required for 21-cm galactic studies, polarization studies, measurernents of source spectra, monitoring of variable cosmic radio sources and planets, and radar experiments, among other proi- ects. They should be quickly convertible from one type of observation to another, and adaptable to radar operation by aildition of a transmitter. The growth in manpower, and problems to which such instruments are appli- cable, indicate a national need for more than one development of this natu¡e, It would appear that the appropriate instrument for these tasks is the fully steerable paraboloid of about 300-foot diameter, with a surface accuracy adequate for 10-cm or shorter wavelengths, Any smaller inshument ís not suftciently powerfu.l; a larger instrument encounters severe technical and financial obstacles. After due.consideration, the Panel feels that the appro- priate number of such instruments to be built within the next ten years is two. Progress with any less would be too slow; any more might well exceed the foreseeable observational requirements and manpower availability. The cost of each of these instruments will be $8 million, calling then for a total investment in such instruments of $16 million. These instruments could appropriateþ be built by a single university able to provide the exten- sive personnel support required, by a regional group of universities, or by 53 Copyright © National Academy of Sciences. All rights reserved.

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It would be desirable if at least one of these were constructed the NRAO. adjacent to one of tle large arrays previously proposed, since electrical inter- connection of the ar:ray and the large paraboloiil may ofier unique observa- tional capabilities of great value in some problems. Since tlree years or more will be rãquired to complete these instruments, once started, their construc- tion should be authorizetl at the earliest possible time. OSE IN ST RU MEN T S SM ALLER SPECI AL-PU RP In addition to the major costþ instruments recommended above, the Panel feels shongly that both an adequate program in radio astronomy and the proper suppãrt of student training call for extensive investment in lesser i"str"me"is over the next ten years. These will be, in general, instruments aimed at special problems, often of an exploratory nature, and will normally be located at universities active in graduate education' The Panel does not consider it proper to specify all the instruments required; in many specific cases, the fãrm of a rãquirecl instrument will develop naturaþ from the special interests and areas of competence of the proposers of the instrument' In fact, it should be emphasizecl that the speciffcations of many of the instru- ments to be built cannot b. predicted today' The past history of radio astronomy has shown that we are still in a stage of development in which many of the more important celestial phenomena and observing techniques are yet to be discoverecl. Examples from the recent past of such new devel- opments include the iletection of the quasi-stellar sources, radio emission fàm flare stars, and the use of lunar occultations to achieve resolution' Nevertleless, the Panel sees some areas in which signiûcant instruments are clearly justiûed, and presents these here as examples of suitable projects for support. These include large telescopes suitable for millimeter wave- lengths; thes" may be steerable paraboloids up to 60 feet in diameter, antl ,rp to $2 million. Another example is ímproved arrays for the stutly "or-tirrg decameter railiation of Jupiter antl perhaps Satu¡n' Another is ex- of the tended arrays for high-resolution studies o{ solar racliation. Telescopes par- ticularþ designed tð monitor known flare stars would be most valuable' One of the laiger university-operated paraboloids coulcl be used as a radar system for stuãies of lunar anil planetary surfaces and atrnospheres, the interplanetary medium, anil the motions of bodies in the solar system' In ,o*"-""."r, uníversity departments will develop new and original electronic ilevices to be used in existing arrays or paraboloids. These may be perfected 54 ái. . Copyright © National Academy of Sciences. All rights reserved.

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on a local antenna system during the development perioil, and later trans- ferred to the major instuments at NR-AO ancl ottrer places; examples of the success of this procedure have alreaily established a precedent. These de- vices may include special low-noise receivers, multi-channel receivers for hydrogen-line studies, receivers for the search lor atd/or study of other spectral lines, and radar astuonomy instnmentation' After due consideration, the Panel feels tlat approximately 15 universi- ties will be capable of such projects over tÏe next ten years, Based on esti- mates of the cost of the examples given, we estimate that tle average cost to implement one of ttrese projects will be about $2 million. Thus tle Panel recommends that approximately $30 million be provided over tlie next ten years for proiects of this nature. The limited power of such instruments shou-ld not be misconstuueil as an indication that ttrey are unimportant' In many cases, they will be capable of producing certain speciffc data beyond the capabilities of the giant instruments to produce; an example is the study of flare stars, to which the valuable time of the maior instuuments cannot be assigned, But more important, these lesser instruments will probably provide the principal practical experience to graduate students, and thus snongly influence the quality of personnel available in tle latte¡ portion of the dec- ade. Thus they are of crucial importance, ín that they will proviile a training ground for the personnel who will be needecl iI the maior instruments are to produce optimum results. Indeed, the Panel feels that particular emphasis must be given to the continuing support of radio astronomy groups in the r:niversities. Älthough annual operating support for existing programs is outside the scope of this . report (see Section VI, p. 76), the Panel reports its opinion that'the present level of support of university departments with on-going radio astronomy programs is less than adequate. This situation has arisen perhaps because universities have had to assume the operation of large and rather complex new research instruments over a short interval of time, antl have had to rely heavily on extramural support in a new ffeld of research that has not been integrated into the traalitional academic structure in the same way as has optical astronomy. In the opinion of the Panel, there is already danger of an i-b"l"tt"" between the strong federal support given to the national center for radio astronomy, on the one hand, and, on the other hand, the support given to the varied activities in the same ûeld in the universities. The efiectiveness of the support to university departrnents would be markedly increased if it were in the form of long-term block-funding rather than on ân annual basis, as is now the common plactice. Long-term funding 5b Copyright © National Academy of Sciences. All rights reserved.

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13"-* will enable tÏe universities to retain efiective stafis and to carry ou.t time- consuming development programs extending over a nu:nber of years. DESIGN STUDY FOR THE LARGEST POSSTBLE STEERABLE PARABOLOID The Panel, after consiilerable discussion with a broad segment of the radio- askonomy community, feels tlat, by the end of the decade, a need may arise for fully steerable paraboloids even larger than the 300-foot paraboloids previously proposed in this report. The basis for this opinion is the present youthful state of railio astronomy, a situation in which it has been found, as can be supported by many events in recent history, tìat many important phenomena are yet to be discove¡ed. Throughout the short history of radio astronomy, every increase in telescope size has resulted in the discovery of new and very important phenomena in the universe. It is realistic to assume that this sequence of events will continue as the maior instruments described here are brought into selvice. The extension of the large high-resolution arrays may be made simply by adding to them more collecting elements of ttre same type as those originally used in the array. Thus, no great difrcuÌty is foreseen in extending these instruments, should tle need become apparent. However, an increase in the size of large paraboloids requires extensive design and engineering studies, since each increase in size confronts the builder with new technological obstacles. When paraboloids larger than 300 feet'in diameter are studieil, severe tech¡ical problems are encountered and the possible solutions are quite complex, incluiling, for example, the use of servo-operated controls on the shape of t}re reflector surface. The design anil evaluation of these solutions are costþ and very time-consuming, as has been shown in the ulsuccessful attempt at Sugar Grove to build a 600-foot paraboloid. Clearly, construction of a successful giant telescope for astronomical purposes requires a thorough- going engineeríng study. If the reasonable assumption is made tlrat toward ttre enil of the decade a need for paraboloids larger than 300 feet in diameter will have appeared, studies of possible antenna designs should be com- menced in the not-too-distant futwe. Thus the Panel recommends that design studies for the largest feasible steerable paraboloids be commenced at an earþ date. The control of the reflector surface through a sewo system should be investigated in the course 56 Copyright © National Academy of Sciences. All rights reserved.

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of this study. The funding probably required to make an adequate study, as here proposeil, ís $1 million. A SOLAR RADAR SYSTEM The possibility of using radar techfiques to produce unique data on the solar corona has now been demonstrated. Such studies could contribute important data on solar phenomena, especially when used in coniunction with passive radio observations of the active sun. However, such an installa- tion would cost perhaps $15 millíon, and the Panel considers that the data per dollar tlrat might accrue ftom such an installation a¡e not commensurate with tle data per dollar to be produced by the instruments proposed above. Furthermore, since such installations have had practical utility for space and military operations, they have hitherto found sources of funds through agencies that do not ordinarily contribute to conventional asbonomical instrumentation. Ther.efore, the Panel does not wish to recommend that funding of such a radar system should be conside¡ed within the framework of ttre present report. SUMMARY OF RECOMMENDATIONS FOR B,ADIO TELESCOPES I Ä very-high-resolution array with great collecting area and low side- lobe levels. Construction time, approximateþ one decade. Cost: $40 million. 2 Two additions to the interferometer at the Owens Valley Observatory of the California Institute of Technology. Six years to complete. Cost: $10 million. 3 Two fully steerable 300-foot paraboloids. Five years to complete. Cost: míllion. $16 4 Smaller special-purpose instruments, approximately 15, costing an average of $2 million each. $30 million. 5 Design study of largest feasible steerable paraboloid. Cost: $I million. The cost of this ffve-part program for radio astronomy thus totals $97 million, not including operating expense. õt Copyright © National Academy of Sciences. All rights reserved.