Darragh O'Donoghue died on 25 June 2015 following a short illness. He worked in the Department of Astronomy at the University of Cape Town (UCT) for the South African Astronomical Observatory (SAAO) and, most recently, for the Southern African Large Telescope (SALT). He was elected to the Fellowship of the Royal Society of South Africa in 2005. He is survived by the family he adored – his wife, Liz, and daughters, Andrea and Alexandra. He left a hole in many of our lives and a chasm in the small community of South African astronomy.
Darragh was born on 29 January 1957 and attended Moore Park School, Ludlow, Shropshire, UK and Durban Boy's High school before studying for his BSc in Physics at the University of Natal in Durban. In 1977 he arrived at UCT to study for his PhD with Brian Warner, working on the atmospheres and pulsations of white dwarf stars – the cooling end-products of stellar evolution. After completion of the thesis in 1982, he continued to work on white dwarf stars, especially pulsations, and on cataclysmic variables – binary stars in which a small cool star loses matter to a white dwarf, resulting in light variations due to matter transfer and, in extreme cases, runaway thermonuclear reactions. His interest in these objects remained throughout his life.
He was one of the original members of the “Whole Earth Telescope” (WET) consortium: a multi-site collaboration set up in the mid-1980s to carry out continuous monitoring of variable stars – especially white dwarfs – for typically 2 or 3 weeks. WET eventually comprised over 20 observatories spread around the globe and often had more than 50 astronomers working on a given collaboration. In the first campaign, Darragh was the principal investigator for the interacting binary white dwarf V803 Cen and was consequently lead author for one of the earliest WET papers. He participated in many of the WET campaigns, including early attempts to determine the rates of stellar evolution in white dwarfs by measuring slow changes in their pulsation frequencies. In the late 1980s, Darragh joined the Edinburgh-Cape (EC) Blue Object survey, the purpose of which was to find new, interesting celestial objects for detailed study. During the next 10 to 15 years, Darragh was part of a small team at the SAAO which put considerable amounts of observing effort into the survey. Undoubtedly, the high point of the EC survey was the discovery and investigation of a completely new class of variable star – the rapidly pulsating hot subdwarf B-type stars. Darragh was substantially involved in the discovery and analysis of several of the first known examples of this class.
In the 2000s, Darragh's direct astronomical research was partly constrained as he became more and more involved with problems surrounding SALT. In the last few years, however, having got SALT back on track, he was starting to realise his goal – that the large telescope would provide him with substantial research material for years to come. His recently published papers, whilst continuing his work on evolved stars, show a remarkable diversity of astronomical interest – from asteroids to active galactic nuclei and supernovae.
In a career cut tragically short, Darragh was author, or co-author, of over 300 papers and conference proceedings which attracted more than 6500 citations. In parallel with his stellar interests over the last 35 years, Darragh maintained a keen interest in instrumentation, software and methods of analysis. By the mid-1990s he had designed and overseen the building of a stellar photometer using one of the first rapid read-out charge-coupled devices (CCDs). It used frame-transfer mode to get around the problem of the CCD read-out time which seriously limits continuous photometry. The instrument was run by Darragh's software, which enabled data-reduction and differential photometry in (almost) real time. Even frequency analysis could effectively be done at the telescope. The system ran for around 20 years and produced vast quantities of research material, leading to many scientific papers.
Darragh joined the SAAO in 1997 and spent some months reading optics textbooks and experimenting with ray-tracing software. Astonishingly, he was soon designing complex optical systems. He took it upon himself to work on the optical design of SALT which was to be a copy of the Hobby-Ebberly Telescope (HET) at the McDonald Observatory in Texas. A critical point, which enabled both telescopes to be funded, was that a 10-m class telescope can be obtained at a fraction of the cost of a conventional telescope – and this cost reduction is achieved by utilising, amongst other things, a spherical primary mirror. This requires a spherical aberration corrector at prime focus. In the original design, this resulted in an effective primary mirror size of about 9.2 m at best and good images over a 4 arcminute field-of-view; Darragh's new design gave an effective 10 m primary and good images over a field twice as large. The HET recently installed a new spherical aberration corrector which is based on Darragh's design.
He also designed the optical system for SALTICAM – the SALT Imaging Camera and the “first light” instrument for SALT. Because of his longstanding interest in rapid phenomena and the importance of their measurement in compact objects, Darragh ensured that both SALTICAM and the SALT prime focus spectrograph had high speed capabilities – unusual, and perhaps unique, for a 10 m telescope. One of the first SALT science papers, lead authored by Darragh, was a study of the fine detail, hitherto unseen, of the accreting polar caps of an eclipsing cataclysmic system.
In the mid-2000s, it became clear that SALT was experiencing image quality problems – effects which seriously affected the telescope performance. Darragh threw himself totally into trying to fix this problem and, working with a dedicated SAAO/SALT team, eventually succeeded in finding and fixing it – restoring SALT images to the quality that his optical design allowed. Following years of dedicated work on the SALT, Darragh was awarded the 2011 Gill medal of the Astronomical Society of Southern Africa for exceptional services to astronomy.
In 2012, Darragh spent a sabbatical working with an old friend and colleague, Chris Clemens, at the University of North Carolina. They developed a compact and revolutionary spectrograph design using curved volume-phase holographic (VPH) diffraction gratings. Typical astronomical spectrographs use complex optical systems to produce the best image quality, often involving significant light loss – highly undesirable, given the low light levels received from celestial objects. Their new design uses a very simple optical system, and the robust VPH gratings are very efficient and relatively cheap. By manufacturing them to a spherical shape they can also be used to correct optical aberrations. Such spectrographs are very compact and might well have many applications outside astronomy. In a tribute to Darragh, Chris Clemens said, “Those who know Darragh know that he defies any description I can write here. He was affable, outrageous, brilliant, multi-talented, and capable of teaching himself any subject.”
Darragh O'Donoghue was an excellent teacher, with endless patience and a great love of analogy. He was kind, generous, helpful, devoted to his family, an impish wit, a great collaborator and a wonderful friend. South Africa has lost a scientist who has contributed significantly to his field and who was perhaps only beginning to show his real capabilities.