Ronald N. Bracewell - Contributions and Honours

Contributions and Honours

Professor Bracewell was a Fellow of the Royal Astronomical Society (1950), Fellow and life member of the Institute of Electrical and Electronic Engineers (1961), Fellow of the American Association for the Advancement of Science (1989), and was a Fellow with other significant societies and organisations.

For experimental contributions to the study of the ionosphere by means of very low frequency waves, Dr. Bracewell received the Duddell Premium of the Institution of Electrical Engineers, London in 1952. In 1992 he was elected to foreign associate membership of the Institute of Medicine of the U.S. National Academy of Sciences (1992), the first Australian to achieve that distinction, for fundamental contributions to medical imaging. He was one of Sydney University's three honourees when alumni awards were instituted in 1992, with a citation for brain scanning, and was the 1994 recipient of the Institute of Electrical and Electronic Engineers' Heinrich Hertz medal for pioneering work in antenna aperture synthesis and image reconstruction as applied to radio astronomy and to computer-assisted tomography. In 1998 Dr. Bracewell was named Officer of the Order of Australia (AO) for service to science in the fields of radio astronomy and image reconstruction.

At CSIRO Radiophysics Laboratory, work that in 1942-1945 was classified appeared in a dozen reports. Activities included design, construction, and demonstration of voice-modulation equipment for a 10 cm magnetron (July 1943), a microwave triode oscillator at 25 cm using cylindrical cavity resonators, equipment designed for microwave radar in field use (wavemeter, echo box, thermistor power meter, etc.) and microwave measurement technique. Experience with numerical computation of fields in cavities led, after the war, to a Master of Engineering degree (1948) and the definitive publication on step discontinuities in radial transmission lines (1954).

While at the Cavendish Laboratory, Cambridge (1946–1950) Bracewell worked on observation and theory of upper atmospheric ionisation, contributing to experimental technique (1948), explaining solar effects (1949), and distinguishing two layers below the E-layer (1952), work recognised by the Duddell Premium.

At Stanford Professor Bracewell constructed a microwave spectroheliograph (1961), a large and complex radio telescope which produced daily temperature maps of the sun reliably for eleven years, the duration of a solar cycle. The first radio telescope to give output automatically in printed form, and therefore capable of worldwide dissemination by teleprinter, its daily solar weather maps received acknowledgement from NASA for support of the first manned landing on the moon.

Many fundamental papers on restoration (1954–1962), interferometry (1958–1974) and reconstruction (1956–1961) appeared along with instrumental and observational papers. By 1961 the radio-interferometer calibration techniques developed for the spectroheliograph first allowed an antenna system, with 52" fan beam, to equal the angular resolution of the human eye in one observation. With this beam the components of Cygnus A, spaced 100", were put directly in evidence without the need for repeated observations with variable spacing aperture synthesis interferometry.

The nucleus of the extragalactic source Centaurus A was resolved into two separate components whose right ascensions were accurately determined with a 2.3-minute fan beam at 9.1 cm. Knowing that Centaurus A was composite, Bracewell used the 6.7-minute beam of the Parkes Observatory 64 m radiotelescope at 10 cm to determine the separate declinations of the components and in so doing was the first to observe strong polarisation in an extragalactic source (1962), a discovery of fundamental significance for the structure and role of astrophysical magnetic fields. Subsequent observations made at Parkes by other observers with a 14-minute and wider beams at 21 cm and longer wavelengths, though not resolving the components, were compatible with the dependence expected from Faraday rotation if magnetic fields were the polarising agent.

A second major radiotelescope (1971) employing advanced concepts to achieve an angular resolution of 18 seconds of arc was designed and built at Stanford and applied to both solar and galactic studies. The calibration techniques for this leading-edge resolution passed into general use in radio interferometry via the medium of alumni.

Upon the discovery of the cosmic background radiation:

• a remarkable observational limit of 1.7 millikelvins, with considerable theoretical significance for cosmology, was set on the anisotropy in collaboration with Ph. D. student E.K. Conklin (1967), and was not improved on for many years
• the correct theory of a relativistic observer in a blackbody enclosure (1968) was given in the first of several papers by various authors obtaining the same result
• the absolute motion of the Sun at 308 km/s through the cosmic background radiation was measured by Conklin in 1969, some years before independent confirmation.

With the advent of the space age, Bracewell became interested in celestial mechanics, made observations of the radio emission from Sputnik 1, and supplied the press with accurate charts predicting the path of Soviet satellites, which were perfectly visible, if you knew when and where to look. Following the puzzling performance of Explorer I in orbit, he published the first explanation (1958-9) of the observed spin instability of satellites, in terms of the Poinsot motion of a non-rigid body with internal friction. He recorded the signals from Sputniks I, II and III and discussed them in terms of the satellite spin, antenna polarisation, and propagation effects of the ionised medium, especially Faraday effect.

Later (1978, 1979) he invented a spinning, nulling, two-element infrared interferometer suitable for space-shuttle launching into an orbit near Jupiter, with milliarcsecond resolution, that could lead to the discovery of planets around stars other than the sun. This concept was elaborated in 1995 by Angel and Woolf, whose space-station version with four-element double nulling became the Terrestrial Planet Finder (TPF), NASA's candidate for imaging planetary configurations of other stars.

Imaging in astronomy led to participation in development of computer assisted x-ray tomography, where commercial scanners reconstruct tomographic images using the algorithm developed by Bracewell for radioastronomical reconstruction from fan-beam scans. This corpus of work has been recognized by the Institute of Medicine, an award by the University of Sydney, and the Heinrich Hertz medal. Service on the founding editorial board of the Journal for Computer-Assisted Tomography, to which he also contributed publications, and on the scientific advisory boards of medical instrumentation companies maintained Bracewell's interest in medical imaging, which became an important part of his regular graduate lectures on imaging, and forms an important part of his 1995 text on imaging.

Experience with the optics, mechanics and control of radiotelescopes led to involvement with solar thermophotovoltaic energy at the time of the energy crisis, including the fabrication of low-cost solid and perforated paraboloidal reflectors by hydraulic inflation.

Bracewell is also known for being the first to propose the use of autonomous interstellar space probes for communication between alien civilisations as an alternative to radio transmission dialogs. This hypothetical concept has been dubbed the Bracewell probe after its inventor.