The flowering of Fleurs: an interesting interlude in Australian radio astronomy

Introduction

Fleurs is situated about 40 km west-south-west of central Sydney near Badgery's Creek, and occupies an expanse of flat land between South Creek and Kemps Creek adjacent to a disused WWII air strip. Between 1954 and 1963, Fleurs was the leading field station of the CSIRO's Division of Radiophysics, and was home to three innovative cross-type radio telescopes, the Mills Cross, Shain Cross and the Chris Cross (Figure 1), all of which played important roles in furthering international radio astronomy (Robertson, 1992). This article discusses these radio telescopes, and the research that was carried out at the Fleurs field station.

Figure 1: The Fleurs field station, showing the old air strip between Kemps Creek (on the east) and South Creek. The Mills Cross is indicated by dashes, the much larger Shain Cross by solid lines, and the Chris Cross by dots.

The Mills Cross

The field station was founded in 1954 in order to erect an innovative new type of radio telescope first tested at the Potts Hill field station in 1953. The Mills Cross was the brain-child of Bernie Mills, and boasted narrow N-S and E-W arrays of dipoles about 450 m long arranged in a cross configuration (Figure 2). Mills et al. (1958: 67) describe how each array "… consists of two rows of 250 half-wave dipole elements backed by a plane wire mesh reflector; the individual dipoles are aligned in an E-W direction." The cross operated at a frequency of 85.5 MHz, giving a 49 arcminute beam _ which in those days was regarded as remarkable. Although the cross was effectively a transit instrument in that neither arm was steerable, by altering the phasing of the dipoles in the N-S arm it was possible to observe different declination strips across the celestial sphere.

Between 1954 and 1957, Bernie Mills, Eric Hill and Bruce Slee used the Mills Cross to carry out a detailed survey of the sky and recorded more than 2,000 sources of discrete radio emission, publishing their results in a series of research papers in the Australian Journal of Physics (the so-called MSH Catalogue). Although a number of these sources were associated with galactic objects, the majority were extragalactic, producing one of the most notable outcomes of this survey: when the numbers of sources plotted against their flux densities (the co-called log N - log S curve) were compared with those derived during the Cambridge C2 survey they were found to differ markedly. Because of the profound cosmological implications of these statistics a heated controversy erupted (see Mills, 1984 for a summary), and it took some years before problems associated with the Cambridge survey were recognised and the Sydney results were fully accepted.

The late 1950s were a time of considerable upheaval at Radiophysics as different senior staff members vied for the right to construct large new radio telescopes, and Mills was one of those who suffered when the Parkes Radio Telescope and Culgoora Radioheliograph were given priority. As a result, in 1960 he moved to a new Chair at the University of Sydney, which provided him with the opportunity to build a new and very much larger cross-type telescope, and in just a few short years the Molonglo Synthesis Telescope, near Canberra, became a reality.

But this staffing change at Fleurs and the fact that the all-sky survey had been completed did not quite spell the end of the Mills Cross, for in 1961 - 1962 Bruce Slee and visiting Cambridge radio astronomer, Peter Sheuer, used the E-W arm of this radio telescope in conjunction with barley-sugar arrays erected temporarily at a number of sites around Sydney to carry out an interferometric sky survey of the MSH sources. Their aim was to determine the sizes of these sources, but unfortunately the observational data returned to England with Sheuer at the end of his term in Sydney and few results of this investigation were ever published.

Figure 2: View looking south showing the N-S arm and most of the E-W arm of the Mills Cross, with the receiver hut at the centre of the array (ATNF Historic Photographic Archive: 3476-3).

The Shain Cross

Two years after the Mills Cross was operational, the larger low frequency Shain Cross was completed (Shain, 1958). This new cross was built alongside the Mills Cross, operated at a frequency of 19.7 MHz, and had a beam width of 1.4 degrees. Although it had N-S and E-W arms of 1105 m and 1036 m respectively, the construction of this particular cross was much simpler than its Mills predecessor in that it comprised a series of dipoles strung between what look like telegraph poles (Figure 3), with the ground itself serving as a reflector.

Yet this apparent simplicity was deceptive for the Shain Cross worked perfectly, and Alex Shain (after whom it was named) was able to carry out a survey of the galactic plane (where some HII regions were seen in absorption) and monitor decametric burst emission from Jupiter.

Several years after Shain's untimely death in 1960 at the age of just 38, Bruce Slee and Charlie Higgins used a small 19.7 MHz array at Fleurs in conjunction with temporary 19.7 MHz antennas erected at a number of distant sites (with spacings up to 200 km) in order to investigate Jovian bursts. Through this interferometry, they were able to place an upper limit of 10 arcseconds on the size of the emission region, and investigate the radio scattering in the outer solar corona.

Figure 3: The `power poles' mark part of the N-S arm of the Shain Cross, and behind it are the Mills Cross and then the Chris Cross (ATNF Historic Photographic Archive: 5192-9).

The Chris Cross

Fleurs gained its third large radio telescope in 1957 when a major new solar array, the Chris Cross, was completed. The brain-child of Dr W N Christiansen, this comprised 378-m N-S and E-W arms each containing 32 parabolic dishes 5.8 m in diameter (Figure 4). These dishes were constructed of wire mesh suitable for operation at 1420 MHz, and were equatorially-mounted in order to be able to track the Sun. The Chris Cross was the first radio telescope to generate daily two-dimensional high-resolution radio pictures of the Sun. In addition, the array has been used to investigate major solar bursts and solar microwave transients (see Christiansen and Mullaly, 1963).

A major development occurred at Fleurs in 1959 when an 18-m prefabricated American parabola was installed at the eastern end of the Chris Cross. By using this new antenna in combination with the dishes of the E-W arm of the Chris Cross the Division's scientists were able to access a compound interferometer that could be used at night for non-solar radio astronomy (see Labrum et al., 1963). Operating at 1420 MHz, and with a 1.5 arc-minute fan beam, this new array was used for a high-resolution survey of selected southern sources.

Figure 4: The centre of the Chris Cross array showing the southern part of the N-S arm, and one of the E-W arm dishes (ATNF Historic Photographic Archive: 9097-12).

Emergence of the Fleurs Synthesis Telescope

In 1963 the 18-m antenna was transferred to Parkes to be used in conjunction with the 64-m radio telescope, and by this time the research programs carried out with Mills and Shain Crosses had come to an end and Fleurs was no longer required as a field station. It had served its purpose, and the Division's focus shifted to Parkes and later to Culgoora.

Yet this did not spell the end for Fleurs. Instead, the field station and the three cross-type radio telescopes were handed over to School of Electrical Engineering at the University of Sydney, led by Professor Christiansen (who in 1960 had also left Radiophysics). Christiansen and his team (which included other ex-Radiophysics staff) then developed the Chris Cross by installing six 13.7-m stand-alone antennas at and beyond the ends of the N-S and E-W solar arrays. The resulting Fleurs Synthesis Telescope (Figure 5) had a resolving power of 20 arc seconds, and during the 1970s and 80s was used to study individual radio sources, but particularly large radio galaxies, supernova remnants and emission nebulae (e.g. see Bunton et al., 1985).

In addition to this major development, another staff member from the School of Electrical Engineering refurbished the E-W arm of the Shain Cross, and he and a graduate student used it in conjunction with two small arrays spaced in the N-S direction to carry out a sky survey at 29.9 MHz (see Finlay and Jones, 1972). This provided the first decametric flux densities for many southern sources.

Figure 5: View of part of the Fleurs Synthesis Telescope, showing one of the 13.7-m antennas and a section of the E-W arm of the Chris Cross (ATNF Historic Photographic Archive: 9087-11).

Concluding remarks

During the ten-year interval, 1954 - 1963, Fleurs was one of the world's leading radio astronomy field stations, and it played an important role in furthering solar and non-solar radio astronomy. Taffy Bowen (1984: 97) would go so far as to claim that the three Fleurs cross-type radio telescopes "… were among the great successes of the 1950's and were responsible for a large part of the Division's research output over that period." They consolidated the international standing of Christiansen and Mills, helped to build the emerging reputations of people like Shain, Sheridan and Slee, and served as stepping stones to the Division's next major advances in instrumentation: the 64-m Parkes Radio Telescope and the Culgoora Radioheliograph. Through these radio telescopes Australia was able to maintain its place at the forefront of international radio astronomy.

After the Division of Radiophysics relinquished Fleurs, the field station was taken over and developed further by the School of Engineering at the University of Sydney, and for more than a decade this site continued to make a valuable contribution to international astronomy.

After the Fleurs Synthesis Telescope was closed in 1988, the site passed to the Engineering Faculty at the University of Western Sydney and for some years was used as a teaching facility. During this period the Mills and Shain Crosses rapidly deteriorated and the Chris Cross dishes and larger antennas of the Fleurs Synthesis Telescope continued to rust. Today, all that remains of the three historic radio telescopes of the 1950s are the twelve centrally-located Chris Cross aerials, which in 1991 were cleaned and painted. The site also features a small reconstructed section of the Mills Cross N-S arm, plus the rusting remains of the six 13.7-m Fleurs Synthesis Telescope dishes.

References

Bunton, J.D., Jones, I.G., and Brown, D.R., 1985. Proc.A.S.A., 6, 93-100.

Christiansen, W.N., and Mullaly, R.F., 1963. Proc.I.R.E.Austr., 24, 165-173.

Christiansen, W.N., in Sullivan, 112-131.

Finlay, E.A., and Jones, B.B., 1972. Proc.A.S.A., 2, 115-117.

Labrum, N.R., Hartung, E., Krishnan, T., and Payten, W.J., 1963. Proc.I.R.E.Austr., 24, 148-155.

Mills, B.Y., 1984, in Sullivan, 147-165.

Mills, B.Y., Little, A.G., Sheridan, K.V., and Slee, O.B., 1958. Proc.I.R.E., 46, 67-84.

Robertson, P., 1992, Beyond Southern Skies. Radio Astronomy and the Parkes Telescope, CUP, Cambridge.

Shain, C.A., 1958. Proc.I.R.E., 46, 85-88.

Slee, O.B., and Higgins, C.S., 1966. Austr.J.Phys., 19, 167-180.

Sullivan, W.T., 1984, The Early Years of Radio Astronomy, CUP, Cambridge.

Wayne Orchiston and Bruce Slee
Wayne.Orchiston@csiro.au
bslee@atnf.csiro.au