1464 PROCEEDINGS O F THE IEEE November

Radio Astronomy with the Australian 210-Foot Telescope*

J. G. BOLTONt

Summary-Major research was begun in March, 1962, with the 2l&foot steerable radio telescope at the Australian National Radio Observatory. Current and planned investigations are numerous and diverse, andinclude observations at wavelengths of 11,21,30,75 and 220 cm. This paper sketches the telescope and equipment details and describes the research program, including results to date.

INTRODUCTION HE 210-FOOT telescope’ (Fig. 1) is the principal instrument of the Australian National Radio Astronomy Observatory, Parkes, New South

Wales. The observatory is owned and operated b’y the Division of Radiophysics of the Commonwealth Sci- entific and Industrial Research Organization. The site is about two hundred miles west of S>-dney in the cen- tral western plains district. I t has an equitable climate, extremes of temperature are 20°F and 100°F and the annual rainfall is 24 inches evenly distributed through- out the year. It is 14 miles from Parkes, a moderate sized country town of 8000 inhabitants with good transport connections to Sydney. The site comprises 400 acres of flat land and facilities, other than the 210-foot tele- scope, include a powerhouse, a workshop, two houses for resident staff and an observers’ lodge to accommodate Sydney-based staff during their observing periods. Presently under construction is a network of rail tracks and a mobile 60-foot telescope for use with the 210-foot telescope as a variable axis-variable spacing interfer- ometer.

The telescope and the necessary funds were secured primarily through the efforts of Dr. E. G. Bowen, Chief of the Radiophysics Division. The funds were partly provided by private donations, the largest being those from the Carnegie Corporation and the Rockefeller Foundation with the balance from the Commonwealth Government. The telescope was designed by Freeman Fox and Partners of London to specifications set by the Radiophysics Division. H. C. Minnett of the division acted as liaison officer in London and, in addition, played a prominent part in the design of the drive and control system. A contract for the construction of the instrument was let in September, 1959. M.A.N. of West Germany were the prime contractors and the main sub- contractors were A.E.I. of England, responsible for the

* Received July 24, 1963. t Radiophysics Laboratory, CSIRO, Sydney, Australia. 1 E. G. Bowen and H. C. Minnett, “The Australian 210-foot radio

telescope,” Proc. IRE (Australia), vol. 24, pp. 98-105; February, 1963.

Fig. 1-The 210-foot steerable telescope.

servo drive and other electrical work, and Askania of West Berlin, responsible for the master equatorial and control desk. Construction was essentially complete a t the dedication of the telescope on October 31, 1961, and the first radio signals had been received a few days prior to the dedication. Final testing and alignment by the contractors, designers and CSIRO personnel continued until March, 1962, parallel with preliminary astro- nomical measurements.

Performance of the telescope has exceeded the design specifications in all respects. Direct survey and radio measurements indicate that the rms deviation of the surface from a true paraboloid is less than 5 mm in the horizontal position. When tilted, the gross deflections are such that no deterioration can be detected in the telescope performance a t 10 cm, the shortest wavelength used so far. The servo control system is excellent, short period following errors are only a few seconds of arc. Over-all pointing errors, including the effects of atmos- pheric refraction, do not exceed 1!5 and, when cor- rected for known effects, positions have an accuracy of better than 0!3 a t declinations north of -40”. The ac- curacy falls off near the south celestial pole, mainly due to the lack of calibration sources of known position. Operational reliability has been very high; about 90

1963 Boltoft: Radio Astronomy with '4 ustralian Telescope 1465

Fig. 2-Part of the interior of the aerial cabin housing the front ends of the radio receivers.

per cent of all scheduled observing time has been se- cured, the balance being due to weather and receiver and telescope malfunction.

For continuum observations, the telescope has mainly been used a t wavelengths of 11, 21, 30, 75 and 220 cm. -All receivers are of the switched type with reference loads of liquid nitrogen cooled resistors or small ref- erence horns. The 220-cm receiver has a tube mixer, the 75- and 30-cm receivers have crl-stal mixers and the two short-wavelength receivers are of the degenerate para- metric type. .All feeds can be centered very precisely in a feed holder which can be rotated for polarization work. .As many as three receivers can be used simultaneously with confocal feeds. Two hydrogen line receivers are available. One is a 48-channel equipment used for work on our own galaxy and the Magellan Clouds and the other is a single channel double parametric sl-stem used for work on distant galaxies. The front ends of all the receivers are situated in the antenna cabin (Fig. 2) be- hind the focal plane, and changes from one equipment to another can be accomplished in about an hour.

RESEARCH PROGR~M ,4 considerable number of separate investigations are

being carried out on the 210-foot telescope. These in- clude the following.

1) Investigations of the structure and dynamics of the galaxy from observations with the 48-channel hy-

drogen line receiver. About 5000 line profiles have so far been made a t intervals of 6 minutes of arc along the galactic equator and along lines of galactic longitude 10' apart extending to L- 3" in galactic latitude.

2) Obervations of the large and small Magellan Clouds in the hydrogen line and in the continuum a t wavelengths of 11, 21, 75 and 220 cm.

3) Observations of the atomic hydrogen content and rotation of a number of relatively nearby southern galaxies with the double parametric line receiver.

4) Studies of selected regions of our galaxy and the structure of galactic sources a t wavelengths of 11, 21 and 75 cm.

5 ) A survey for radio sources in the southern hemi- sphere a t 75 cm (beamwidth 48') with subsequent posi- tion and intensity measurements a t 11 and 21 cm. Identification of sources is being attempted on plates taken with the 74-inch telescope a t Mount Stromlo Observatory in a cooperative program.

6) Determination of precise positions and brightness distributions of radio sources by the method of lunar occultation.

7 ) A systematic survey for linear polarization of the radiation from galactic and extragalactic radio sources and measurement of the associated Faraday rotation.

8) Observation of certain flare stars for possible ef- fects in the radio spectrum.

9) Investigations of objects in the solar system, particularly Jupiter where observations of the po- larized emission from the Jovian Van Allen belt have been made over the wavelength range of 10 cm to one meter.

In each of these fields the new telescope is yielding important and often dramatic new results. Often these results can be directly attributed to some unique feature of the instrument-its high gain or resolving power, the wide range of wavelengths over which it is possible to make useful observations or the precision of its motion. There is not space to mention all the new results but only to briefly discuss a selection of them.

IDENTIFICATION OF RADIO SOVRCES Among the cosmic radio sources that we can detect,

there is an extremely large range in the ratio of radio to optical brightness. At the low end of this range are ob- jects such as the planets in our solar system, the emis- sion nebulae and planetary nebulae in our own galaxy and the normal galaxies in our own neighborhood. In- termediate in this range are the supernova remnants, or remains, of exploded stars in the galaxy and at the top end of the scale are the radio galaxies, extremely rare and generally distant objects in which the radio emis- sion can occasionally equal or exceed that in the visible spectrum. Identification at the low end of the range generally involves the recognition of weak radio emis- sion from a bright and well-known optical object, often against a complex background of stronger sources and general galactic emision. Hence the use of an instrument

1466 PROCEEDINGS

Fig. 3-Photograph of the large Magellan Cloud in the light of Ha

Observatory.) taken with an 8-inch Schmidt camera. (Courtesy Mount Stromlo

O F THE IEEE AIrovember

with high resolving power and gain is of great value. These features of the 210-foot telescope have been ex- ploited by E. R. Hill and M. 14. Komesaroff in observa- tions of sources in our own galaxy and by D. S. Ifathew- son, R. X. McGee and J. V. Hindman in studies of the Magellan Clouds.

Hill and Komesaroff have found that a large part of the intense ridge of emission, near the galactic equator, breaks up into discrete sources and that many extended objects, found with small telescopes, exhibit detailed- often ring-likestructure. In some cases, the radio emission can be attributed to visible objects on sky survey plates. In other cases, particularly over a large section of the southern Milky Way, obscuring clouds of interstellar dust prevent any direct identification. How- ever, the nature of the object can frequently be deter- mined by radio measurements alone. Emission nebulae, clouds of ionized hydrogen surrounding hot stars, give rise to a flat radio spectrum; that is, the flux density is independent of wavelength. The supernova remnants have a nonthermal spectrum S = X I where x is of the order of 0.7. Emission nebulae usually have a brightness distribution with a sharp central concentration, whereas the supernova remnants often show part or complete

Fig. 4-The large Magellan Cloud as Seen in the 21-cm hydrogen line. The contours are of integrated brightness, 50 units is equivalent to 1 hydrogen atom per cm* in a line of sight column. The heavy line encloses a region of double-peaked profiles.

1963 Bolton: Radio Astronomy with Australian Telescope 1467

IM"Im

Fig. 5-The large Magellan Cloud as seen in the continuum at 21 cm. The contours are of brightness temperature in units of 0.17"K.

rings presumably due to the projection of a partial or complete thin shell structure.

The Magellan Clouds, irregular galaxies that are our nearest neighbors in space, are not affected by external or internal absorption so that close radio and optical correlation is possible. Jloreover, all objects within each Cloud can be considered as a t essentially the same dis- tance. The correlation is particularly good for the large Magellan Cloud as is shown in Figs. 3-5. Fig. 3 is a photograph of the large Magellan Cloud taken with the 8-inch Schmidt camera at llount Stromlo Observatory in the light of Ha. Figs. 4 and 5, which are on the same scale, are brightness contours a t 21 cm in the con- tinuum observed by D. S. llathewson and J. R. Healey and in the 21-cm hydrogen line by R. X. XlcGee. hfathewson has identified over 40 of the radio sources

with the Ha emission regions and a number of McGee's pronounced concentrations of atomic hydrogen appear to surround the major radio continuum and H a emis- sion regions. Recently, Mathewson and Dr. Bengt Westerlund of Moun t Stromlo Observatory have iden- tified one of the radio sources with a supernova rem- nant, the first to be discovered in an external galaxy.

Mathewson and J. M. Rome? have measured the radio emission from twenty of the other relatively nearby normal galaxies. An interesting feature of their observa- tions is that the radio emission in the bright Sc galaxies is strongly concentrated towards the nuclei of these systems as opposed to a more general disk, and perhaps coronal distribution, in the later type spirals such as our

sion from normal galaxies, Aust. -7. Phys. vol. 16, pp. 360-369; 1693. * D. S. Mathewson andwJ. M. Rome, "Observations of radio emis-

1468 PROCEEDINGS OF THE IEEE November

own galaxy and the Andromeda nebulae M31. B. J. Robinson and C. van Damme have detected the hydro- gen line radiation from some of the nearby galaxies and in several cases determined the rotation curves. For NGC 55, a magellanic-type irregular galaxy, they find the center of rotation is quite some distance from the optical centroid, a result which is similar to that found for the large Magellan Cloud.

Identification of radio galaxies which are, in general, very distant objects requires high positional precision. The absolute accuracy of present position measurements with the 210-foot telescope varies from about 01 2, in northern areas of the sky where there exists sufficient calibrators or sources of accurately known position, to 1' or worse near the south celestial pole. Accuracy of 1' is sufficient to identify galaxies as faint as perhaps 17th magnitude; the author and Westerlund have secured about ten such identifications between - 35" and - 65' declination. Radio and optical centers do not always co- incide however; a t 17th magnitude the difference may be as much as O! 2. Such identifications will, however, be used to form a temporary calibration grid. On this basis, it is hoped to secure more distant identifications where the assumption of coincidence will involve smaller errors, and thus, continuously improve the position calibration of the telescope.

3c 273 For those sources suitably situated, lunar occulta-

tion offers the most precise method known for position determination. The method is free from errors of other methods such as pointing accuracy, wavelength and baseline accuracy in interferometry and even atmos- pheric refraction. The factors involved in the position calculation are the times of immersion and emersion, the latitude and longitude of the observatory and a knowledge of the moon's motion. An error of a second in the timing results in a position error of only one sec- ond of arc. With a good signal-to-noise ratio, times of immersion and emersion can be determined to a small fraction of a second and the limiting factor is then the lack of knowledge of the detailed topography of the moon's limb which may introduce errors of up to 0!3.

About ten complete occultations have now been ob- served with the 210-foot telescope. Where the sources are only a few seconds of arc in diameter, a Fresnel dif- fraction pattern is seen as in Fig. 6, facsimiles of the records obtained a t 75 cm of the occultation of the source 3c 273 on August 5, 1962. On this occasion, simultaneous measurements were made a t 220 cm. On other occasions, simultaneous observations were made a t 75 and 20 cm, and 75 and 11 cm. From these observa- tions, C. Hazard, M. B. Mackey and A. J. Shimmins have constructed a very precise radio picture of this ~ o u r c e . ~ 3c 273 is not a simple source but a double; each

C . Hazard, M. B. Mackey and A. J. Shimmins, "A st$y of the radio source 3c 273'by the method of lunar occultations. Nature vol. 197. p. 1037; 1963.

component is about 6" by 2" with a separation of about 19" between the two centers. The location of the centers of the two components is shown in Fig. 7. The spectra of the two components differ greatly; that of A has a flux density which varies approximately as the wave- length whereas that of B is independent of wavelength. The two are almost equal a t 75 cm. On a plate taken with the Palomar 200-inch telescope, component B agrees closely with a 13th magnitude stellar object and A with a jet pointing away from the star along the line of centers of the radio sources.

In the preceding three years, four other such "stars" had been identified with radio sources from combined Jodrell Bank-Caltech radio measurements and optical investigations at the Mount Wilson and Palomar Ob- servatories. Failure to identify the lines in their optical spectra or resolve the images had convinced the aston-

Fig. &Records of the occultation of 3c 273 at 75 cm. The source is

right angles to the moon's limb, a t emersion almost parallel. double; at immersion the two components were aligned almost a t

7 45"

Posltlon angle

Fig. 7-Positions of the two components of 3c 273 from three lunar occultation measurements. The small triangles enclosing the positions are formed by the moon's limb at the instant of emersion or immerslon.

1963 Bolton: Radio Astronomy with Australian Telescope 1469

onlers that these objects were stars and not galaxies. The 210-foot observations were responsible for a sudden reverse of this decision, for 3Iaarten Schmidt4 of Mount Wilson and Palomar Observatories was able to fairly easily identify the lines in its stellar counterpart. He found the lines were shifted to the red by an amount im- plying a velocity of recession of 0.15 times the velocity of light. Such an object can only be a galaxy with a distance of a about 5 X lo9 light years. With this clue, J . L. Greenstein and T. -A. llatthews5 successfully identified the lines in another of the stars-3c 48-and were able to show that it was even more distant with a red-shift of 0 .37~. These objects represent an entirely new class. Phi-sically they are of great interest, their energy output in the visible spectrum is a hundred times greater than that of the brightest galaxy pre- viously known, )ret the emission is concentrated into a volume or less of that of a normal galaxy. Cosmo- logically, they may be of great significance for they could be detected out to distances well in excess of normal galaxies or radio galaxies and their study may provide important clues to the nature and evolution of the universe. One thing is quite clear-it will be the task of the radio astronomer to hunt them down and sort them out from among the myriads of “stars” that are not superluminous galaxies.

POLXRIZATIOB OF RADIO SOVRCES For about a decade, it has been believed that syn-

chrotron radiation, or the emission from relativistic elec- trons in a magnetic field, is responsible for the high level radio emission from sources such as supernova remnants and radio galaxies. Such radiation is linearly polarized with the electric vector in the plane a t right angles to the direction of the magnetic field. Due to possible random variations in field direction, only a small net polarization might be expected from an un- resolved object. There have been many searches for such polarization but up to a year ago it had onl!. been detected in two objects-the Crab nebula a t optical and centimeter wavelengths and from the Van Allen belt of Jupiter. Then two almost simultaneous observa- tions provided a sudden breakthrough. C. H. Mayer, T. P. McCullough and R. lf. Sloanaker6 a t the U. S. Naval Research Laboratory found that the emission from the radio galaxy C\-gnus-A was 8 per cent linearly polarized a t 3 cm, although no polarization was found a t 10 cm. R. N. Bracewell, B. F. C. Copper and T. E. Cousins’ using the 210-foot telescope detected 17 per

Mature, vol. 197, p. 1040; 1963. M. Schmidt, “3c 273: a star-like object with a large red-shift,”

.adio source: 3c 48,“ vol. 197, p. 1041; 1963. J. L. Greenstein and T. A. Matthew, “Red-shift of the unusual

6 C. H. Mayer, T. P. McCullough and R. M. Sloanaker, “Evi- lence for wlarized 3.15 cm radiation from the radiwalaxv Cvmus-A.” 4stro@hyi. J., vol. 135, p. 656; 1962.

7 R. N. Bracewell, B. F. C. Cooper and T. E. Cousins, “Polariza- :ion in the central component of Centaurus A,” Nature, vol. 195, p. 1289; 1962.

- , ,-

cent linear polarization in one of the two small central sources of Centaurus-A. Then, F. F. Gardner and J. B. Whiteoak* again working with the 210-foot telescope found that the extended components of Centaurus-A were polarized a t 21 cm and 38 per cent linear polariza- tion was recorded in one area. The!. also detected polarization in one out of three supernova remnants and in seven out of nine radio galaxies examined.

-4 further important discovery came when Cooper and R. AI. Priceg found that the plane of polarization of the radiation from Centaurus-+\ changed with wave- length. From observations a t a number of wavelengths between 10 and 7 3 cm, they were able to show that the rotation of the plane of polarization varied as the square of the wavelength as in Faraday rotation. From ob- servations of one object alone, the position of the Far- aday medium could not be deduced although our own galaxy appeared the most likel), place. That this is so has now been amply demonstrated by Gardner and Whiteoak‘” in observations of rotation for sixteen po- larized sources. They find that sources near the galactic poles show little or no rotation and the rotation in- creases rapidly for decreasing galactic latitude. Gard- ner’s most recent results are shown in Fig. 8. I t was also observed that sources near the galactic plane showed rapid depolarization with increasing wavelength. This is presumably due to irregularities in the line-of-sight elec- tron density across the source resulting in differential Faraday rotation across the source. I t accounts for earlier unsuccessful attempts to observe linear polariza- tion a t longer wavelengths.

There are three important results from the polariza- tion work.

’ lo[

17.0

Fig. 8-Faraday rotation measures for 20 extragalactic and 2 galactic sources plotted against galactic latitude.

wavelength radiation from radio sources, Phys. Rev. Lett., vol. 9, p. 8 F. F. Gardner and J. B. Whiteoak, “Polarization of 20 crn

197-1997 1962.

associated with the radio source Centaurus A,” Nature, vol. 195, PD. 9 B. F. C. Cooper and R. M. Price, “Faraday rotation effects

1084-1085; 1962. _ _

and their Faraday rotation by magnetic fields,” Nature, vol. 197, 10 F. F. Gardner and J. B. Whiteoak, ’Radio source polarizations

p. 1162; 1963.

1470 PROCEEDINGS O F THE IEEE zVoYember

1) There is now ample proof of the synchrotron process for the generation of high level radio emission in many objects.

2) There is evidence for preferentially aligned mag- netic fields over very large distances in space. In some of the larger radio galaxies this is of the order of a million light years.

3) There is a magnetic field system in our own galaxy, the form of which may well be elucidated by observa- tions of Faraday rotation on galactic and extragalactic sources.

Polarized sources within our own galaxy have also been studied. J. A. Roberts and Gardner have confirmed the earlier Dutch results that certain regions are highly polarized at 75 cm. From the results on rapid depolariza- tion of extragalactic sources, it is clear that these re- gions must be close and this has been confirmed by the absence of appreciable Faraday rotation at 75 cm. Extensive and complex polarization effects have been found by D. K. Milne in the Vela-X source, a supernova remnant. As this source is about 5" in diameter and the telescope beamwidth a t 11 cm is only 7', the resolution he obtains is comparable to that used in photoelectric observations of the Crab Nebula.

AN OUTBURST FROM A FLARE STAR

For several years, workers a t Jodrell Rank and the Radiophysics Laboratory have been engaged in a search for radio outbursts similar to those accompany- ing solar flares from certain types of stars. These stars are known as flare stars and it would appear that the whole of the surface undergoes a change similar to that which takes place over a few thousandths of the sun's surface during a solar flare. Such searches require considerable patience. Intervention of cloud during

visual observations and terrestrial interference during radio observations are two of the major frustations. Results up to a few months ago had suggested a statis- tically significant correlation between radio and optical events for certain flare stars. However, 0. B. Slee has recently observed an almost certain outburst from the star V 371 Orionis. A strong increase a t 75 cm was seen on the 210-foot telescope approximately coincident with an optical flare observed photographically with the Baker Nunn Schmidt camera a t Woomera, South Aus- tralia, and visually on two 10-inch telescopes manned by amateur astronomers in New South Wales. The 75- cm record is shown in Fig. 9. At its peak, the flux density reached 12 X watts m-2 (c/s)-l. Rather smaller ef- fects were observed at 21 cm on the same telescope. At Fleurs, New South Wales, signals were also detected a t 19.7 Mc with part of the north-south arm of the 19.7 Mc cross antenna directed at the star. No signals were de- tected with a second section of this aerial directed 3" away from V371 Orionis. The optical flare was estimated as having lo3 times the energy of a strong solar flare. At radio frequencies the corresponding factor is prob- ably lo6. From radio spectroscopy of the sun, much has been learned about the outer atmosphere of the sun under disturbed conditions. The use of large steerable teles- copes like the 210-foot may make possible similar inves- tigations of certain types of stars.

I . . , . l o r n r & P r l r a m

uolv-1 nnm 71 *

Fig. %Tracing of the record of the out- burst from V 371 Orionis a t 75 cm.