Emeritus Professor John Davis (left) at the PhD graduation of Andrew Jacob in 2009. Dr Gordon Robertson also of Sydney University is on the right. Picture courtesy Andrew Jacob
One of Australia’s foremost astronomers, Professor John Davis died a few weeks ago on the weekend of 16/17 January 2010. He is best known for his work on two unique instruments, the old Narrabri Stellar Intensity Interferometer and the current Sydney University Stellar Interferometer (SUSI). He had a long association with the School of Physics at Sydney University where both of us, Andrew Jacob and Nick Lomb, had the privilege of knowing him while we were postgraduate students. Andrew was his last research student, receiving his PhD in 2009, while Nick knew him from 1969 when he became a research student in the small Department of Astronomy within the School of Physics. Though John Davis was not his supervisor, he was in charge of research students in the department.
The Narrabri Stellar Intensity Interferometer. Picture courtesy NASA
The Narrabri Intensity Interferometer originated in Manchester where Robert Hanbury Brown with help from the mathematician Richard Twiss realised that a technique he had developed and used for radio astronomy could be adopted to optically measure the tiny angular diameters of stars (1). In 1955 and 1956 Hanbury spent 60 nights trying to make observations at Jodrell Bank near Manchester with a prototype instrument to prove that the concept worked. During some of these nights John Davis, who was making optical observations of meteors, would call in to commiserate about the poor observing weather (2).
One of the small mirrors that made up the Narrabri Stellar Intensity Interferometer in the collection of the Powerhouse Museum (97/278/1). It was donated in 1997 by John Davis on behalf of the then Chatterton Astronomy Department in the School of Physics, University of Sydney. Picture courtesy Powerhouse Museum
A few years later when work began on the full instrument at Narrabri in NSW John Davis was an important member of the small team involved. This instrument consisted of two giant optical mirrors each 6.7 metre in diameter mounted on a circular railway track. It was John who solved one of the early problems with instrument, the decrease in angular resolution (sharpness of vision) as the mirrors were pointed closer to the zenith (3).
Working as a senior academic at Sydney University as well as observing with an instrument almost 600 km away was extremely difficult for Hanbury, John and their two colleagues. With much of the road between Sydney and Narrabri unsealed their regular journeys out to the site of the instrument were not only tedious, but hazardous as well.
By the time the observing program was completed at Narrabri the Intensity Interferometer had measured the diameters of 32 stars. According to Hanbury the list of stars, ‘included the first measurements ever made of main sequence stars and also of any star hotter than type M.’ (4) After combining these measurements with observations of the distribution of radiation reaching Earth made by John Davis at Mt Stromlo Observatory, a temperature scale for hot stars could be established. This scale is still in use today although John stated: ‘It is one of the regrets of my career that, in spite of having done the bulk of the work and writing for the paper…I had to agree to [others who contributed satellite photometry] having first authorship since we were not allowed to pay the page charges’ (5) levied by the journal. This important research at Narrabri was supplemented with a few experimental observations especially that of the spectroscopic double star Spica, which yielded an exceptionally accurate distance measurement of 25.7 +- 1.2 light years for the star.
The central portion of the Sydney University Stellar Interferometer (SUSI) at Narrabri. Picture courtesy School of Physics, University of Sydney
Following the success of the Narrabri Stellar Intensity Interferometer thoughts turned to a successor instrument. John initially suggested building a larger intensity interferometer, but developments in optics and understanding of atmospheric turbulence (which causes stars to twinkle) convinced him a modern Michelson-type interferometer was the way to go. Soon the Sydney University Stellar Interferometer (SUSI – one of the few uncontrived acronyms in astronomy) was born.
SUSI is a long-baseline optical interferometer, in other words a very long telescope that looks at very small things. Beams of light from two small mirrors separated by hundreds of metres are carefully brought together at a block of glass with a precision of millionths of a metre. A digital camera records the resulting ‘interference’ pattern. It’s a bit like photographing the patterns waves make on the ocean surface as they pass by each other.
SUSI is designed to do fundamental astrophysics: measure star diameters, binary star separations and pulsations of variable stars. Although this may seem dull compared to searches for black holes and the beautiful images returned by the Hubble Space Telescope this work provides the foundation for our understanding of how long stars live, what their mass is and how big and old the universe is.
SUSI would dominate the remainder of John’s career. In a remarkable feat a world-class instrument was designed, constructed and commissioned by just a handful of people and all within budget. The basic infrastructure was contracted out, but the electronics, servo systems, mechanical components and software were developed ‘in-house’ and much of that between teaching duties.
Cutting edge science is always prone to unforseen difficulties and SUSI was no exception. Difficulties in measuring the position of the two light beams, problems with the mirror coatings and a low sensitivity (your eye could see fainter stars than the young SUSI) were gradually overcome.
Nevertheless, a steady stream of science has been produced. The SUSI program has produced over 40 refereed papers, 12 completed PhDs and 6 completed MScs. SUSI has influenced the design of several subsequent interferometers – the enormous VLTI in Chile and CHARA at Mt Wilson owe parts of their design to the pioneering SUSI. In 2010 SUSI continues to evolve with a program underway to further improve the sensitivity, expand the observing program and allow remote operation from Sydney, Europe or the USA. John was thrilled to know that his pride and joy was still growing.
John himself was dedicated to the instrument and he spent a substantial proportion of his life on site under the clear skies west of Narrabri. He has been described as ‘meticulous’, ‘thorough’ and a ‘perfectionist’ – all great qualities for a research scientist. As John’s last PhD student I discovered his thoroughness early on – for meetings I learnt to be extremely well prepared. John always liked to check my analysis and test my conclusions himself.
On the long drives back and forth from Sydney John would regale Andrew Jacob with his stories of the Intensity Interferometer and SUSI: how his three-year visit to Australia morphed into a lifetime; his car packed to the rafters with family and supplies slewing through the mud of the Breeza plain; his disappointment at having to relinquish first-authorship of the 32-stars paper; his incomprehension at how the Australian Research Council was willing to fund construction but not the staff to make the most of the instrument; his passion for optics; his appreciation for the strong support of international colleagues when local interest flagged and his joy that the ‘astronomy gene’, which had bypassed his children, may have settled with his granddaughter.
John continued working on research papers throughout 2009, being regularly at his desk at the University of Sydney forensically delving into some data analysis issue. At Andrew’s last meeting in mid-December he was still carefully investigating our pulsating star data for an upcoming publication.
An authoritative obituary, mainly written by long-time colleague Bill Tango, has appeared in the Sydney Morning Herald.
Emeritus Professor John Davis will be sadly missed.
1. Boffin, a personal story of the early days of radar, radio astronomy and quantum optics, R Hanbury Brown, Adam Hilger imprint of IOP Publishing Ltd, Bristol, 1991, p117.
2. Ibid p125
3. Ibid p142
4. Ibid p153
5. Davis J. (2006) Forty Years of Progress in Long-Baseline Optical Interferometry: 2005 Robert Ellery Lecture, Publications of the Astronomical Society of Australia 23, pp94–104.
Andrew Jacob and Nick Lomb