John E. Baldwin
John joined the fledgling Cambridge Radio Astronomy Group in 1952 under the supervision of Martin Ryle. He was one of the most distinguished of the "second generation" of radio astronomers at Cambridge and was to be at the heart of everything in the Radio Astronomy Group during the early years of the development of radio astronomy. The "first generation" was spear-headed by Martin Ryle whose inspiration and strong personality led to the opening up of radio astronomy as an astronomical discipline. But Ryle could not have achieved this without the efforts of an extraordinary team of brilliant colleagues and graduate students such as John Baldwin.
From the very beginning the Cambridge radio astronomy efforts were dedicated to the use of interferometry at radio wavelengths to survey the sky, create catalogues of radio sources and then to make high resolution radio images of the sources. While Ryle drove the program of deep surveys for cosmological purposes, John concentrated on more local aspects of radio astronomy. This involved making low-frequency maps of the sky and disentangling from these the radio structure of our Galaxy and the high energy sources, such as supernova remnants, within it. His pioneering efforts were recognised in this message from Hugo van Woerden: "I first met John at the IAU Assembly in Dublin in 1955. He gave an excellent talk about the Galactic Halo, very impressive for a 24-year old student. Already then, he was a great scientist."
John was involved in the major radio telescope projects as a graduate student (the 2C telescope 1952-55) and research fellow (4C radio telescope 1957-59). In 1957, he was appointed a University Demonstrator, at that time the Cambridge name for an Assistant Lectureship. Following his Demonstratorship, John was appointed an Assistant Director of Research, a post he held until 1981 when he was promoted to a Readership in Radio Astronomy. In 1968 he resigned his Official Fellowship and Director of Studies role at Queens' College in order to concentrate on his research and teaching duties, but was offered and accepted a Bye-Fellowship, which he held from 1968-74. He then retired from the College Fellowship altogether to concentrate on research, but in 1989, on his election to a chair of Radio Astronomy, he returned as a Professorial Fellow.
While Ryle continued the development of the techniques of aperture synthesis to higher frequencies and higher angular resolution with outstanding success, John continued the development of radio astronomy at low radio frequencies, over the years building a succession of world-leading survey instruments. The first of these low frequency telescope systems was the 38 MHz T Synthesis Telescope which operated from 1959-66. These observations were a very major challenge because of the fluctuations in the electron content of the ionosphere. This work culminated in the 6C, 7C and 8C surveys which were the defining low-frequency radio surveys of the sky. As George Miley has written, "The 38MHz 8C survey is still the best survey below 50MHz and was an important stimulus for the next-generation low-frequency arrays, such as LOFAR."
John also pioneered spectral interferometry. He was the project leader for the Half-Mile Radio Telescope which operated from 1967-74 and was the first interferometer to make images in the 21-cm line of neutral hydrogen, a very considerable technical achievement. Hugo van Woerden remarks, "In the sixties his work on neutral hydrogen in galaxies with the Half-Mile Telescope set the scene for our later work at Westerbork."
John's deep understanding of the fundamentals of interferometry and the ways of eliminating the effects of turbulence in the atmosphere were to prove central to his taking up the challenges of applying the techniques of interferometry at optical and infrared wavelengths in the 1980s. This involved a variety of different approaches. From aperture masks on large optical telescopes to the development of the COAST optical interferometer at Lord's Bridge, he demonstrated that optical interferometry is a powerful tool for future optical imaging. With COAST, John and his colleagues produced the first aperture synthesis images of stars with an angular resolution of about one milliarcsecond, about 30 times better than the sharpest images obtained by the Hubble Space Telescope. Among the achievements was the imaging of structures on the surface of nearby giant stars. Harry van der Laan has written, "When in ESO we pushed VLT Interferometry in the late '80s/early '90s, the work of John and his Cavendish team was admired and served to challenge our team." The legacy of his achievements is the involvement of the Cavendish Astrophysics Group in the optical-infrared interferometer at the Magdalena Ridge Observatory. Right up to the months before he died, he was uncovering new features of the fluctuations in the refractive index of the atmosphere at optical wavelengths which are not only surprising, but which also offer new opportunities for optical imaging.
John became the Head of the Radio Astronomy Group in 1987 and Deputy Head of the Cavendish Laboratory in 1988. He was promoted to a Professorship of Radio Astronomy in 1989 and then in 1991 elected to Fellowship of the Royal Society of London. He received numerous awards for his research, including the Guthrie Medal of the Institute of Physics (1997), the Hopkins Prize of the Cambridge Philosophical Society (1997) and the Jackson-Gwilt medal of the Royal Astronomical Society (2001).
In addition to his distinction as a scientist, John was a brilliant teacher and supervisor of graduate students. Richard Hills, now Project Scientist for the ALMA project, wrote, "John is of course one of the main reasons I am in Radio Astronomy - he was my Director of Studies when I first came to Cambridge and I soon learned that trying to think about things and do them in the way that he did was a pretty good way to go."
John's contributions to national and international astronomy were very extensive. He served on many Panels, Committees and Boards of the SRC and SERC and was widely sought after internationally as an advisor on radio astronomical observatories. These included radio observatories in France, the Netherlands, Australia, Brazil and the USA and the European Southern Observatory. He was vice-President (1982-5) and then President (1985-1988) of Commission 40 (Radio Astronomy) of the International Astronomical Union.
John Baldwin died on 7 December 2010.