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OCR for page 57
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-
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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.
OCR for page 61
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
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ái. .
Copyright © National Academy of Sciences. All rights reserved.
OCR for page 62
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
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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.
