Photo: Bill Erickson (in Australia) in 2005 around the time he received the inaugural Grote Reber medal. In the background is one of his last antennas, adapted for use in both the Bruny Island Solar Radio Spectrometer and the Long Wavelength Array.
“It is often said that one learns by one’s mistakes. Based upon this criterion, I can claim to have extensive knowledge of low frequency radio astronomy.” These words capture the wit and wisdom of a pioneering radio astronomer of the twentieth century, William Clarence Erickson. “Bill” Erickson died on September 5, 2015 in Hobart, Australia after several years of deteriorating health.
Bill Erickson was born in Chicago, Illinois on November 21, 1930. With a life spanning the entire history of radio astronomy, he later joked “Since I was two years old at the time, I was unimpressed by Jansky’s discovery of Cosmic Static.” As a second generation descendent of Scandinavian immigrants, he spent his youth in Duluth, Minnesota where his father worked in the steel industry. As a teenager he often raced dangerous, wind-powered iceboats across the frozen lakes of the upper Midwest, foreshadowing his love of adventure and classic sea sailing that endured throughout his adult life. He was educated in the “relative tropics” of Minneapolis, as he fondly compared it to Duluth, at the University of Minnesota, receiving undergraduate degrees in both math and physics. He also received his PhD in physics at UMN in 1956 under the theoretical physicist Charles L. Critchfield, future director of the Los Alamos National Laboratory. His thesis topic considered whether spinning dust could be responsible for the recently discovered cosmic “radio noise.” Nonthermal synchrotron emission inevitably became a primary target for the many instruments he later developed.
Bill was a consummate intellectual, physicist, and technologist. While building his first optical telescope at age 14 and being an amateur radio (aka “HAM”) operator, his motivation to pursue low frequency radio astronomy was partly happenstance and came later in life. He was visiting the Department of Terrestrial Magnetism (DTM) shortly after B. F. Burke and K. L. Franklin discovered Jovian decametric emission in 1955, one of the most exciting early discoveries in radio astronomy. As Bill remarked whimsically much later “It gave me the mistaken impression that low frequency work was easy. All that you needed to do was to erect some poles, string some dipoles between them, and great discoveries would result.”
After a Carnegie fellowship at DTM from 1956-1957 pursuing HI observations developed under Howard Tatel, Bill was hired by Critchfield, who had become the director of the Convair Corporation’s Scientific Research Laboratory near San Diego. It recalled a different era when the US private sector invested in basic scientific research, and was less consumed with the financial bottom line. As Bill remarked, “We were given complete freedom to choose our research projects within the confines of a very limited budget.” At around this time Bill developed his passion for flying, second only to sailing in his love for adventure. He spent a period chasing and retrieving balloon-born scientific payloads across the American landscape from California to Oklahoma. It was likely during one of these excursions that he was among the first to recognize the Plains of San Agustin (in western New Mexico) as a prime site for what would eventually be the premier radio interferometer in the world, the Very Large Array (VLA).
Bill’s first task at Convair was to search for a flat site in the San Diego area to conduct low-frequency radio astronomy research. He located the Clark Dry Lake in the beautiful Anza-Borrego desert, a setting that would dominate his career over the next several decades. Commencing in the late 1950s and lasting through the late 1980s, Bill and a hardy band of colleagues and their students developed a succession of innovative radio telescopes at Clark Lake, catalyzed by the early momentum in low frequency work generated by Carl Jansky’s discovery of Galactic radio emission and Reber’s pioneering follow-on work characterizing it as nonthermal radiation.
While he was at Convair, Bill was offered a faculty position by John S. Toll, president of the University of Maryland. Bill also convinced Maryland to take over the Clark Lake facility from Convair/General Dynamics, which it could no longer afford to operate after being nearly driven into bankruptcy by competition with Boeing in the burgeoning jetliner industry. Before starting his position in College Park, Bill was invited by Jan Oort, the great Dutch astronomer in Leiden, to help develop the proposed, mechanically complex (>100 large parabolic dishes) Benelux Cross Antenna Project designed with Jan Högbom and Chris Christiansen. Professor Oort hired Bill (1962-1963) to head the endeavor, reflecting respect for his skills in both radio interferometry and project management. Shortly thereafter the team recognized that aperture synthesis, recently pioneered at Cambridge and in Australia, informed a much simpler design. They convinced Oort that it would not be practical to build the much larger telescope until a much simpler, though slower instrument was developed, one utilizing Earth-rotation synthesis and requiring multiple 12-hour tracks to produce a map. This was the basic design of what became the Westerbork Synthesis Radio Telescope (WSRT), which remains a premier research instrument today.
Bill subsequently spent the heart of his career developing instruments and training students at Clark Lake while on the faculty at Maryland. His finest telescope, the Clark Lake TPT (so named because it was a T-shaped array of conical, log-spiral antennas), was far ahead of its time and foreshadowed a technical revolution still underway. It was an electronically steerable, broad-band array that could be tuned in fractions of a second. It could even form images in near real time, a level of innovation only now being achieved by emerging dipole-based, digital arrays. Modified versions of the antennas, originally commissioned together with Bill’s former student Rick Fisher, are still in use at the Nançay radio observatory today. Sadly, Clarke Lake was dismantled in the early 1990s. This was partly due to a reduction in funding from the NSF for small, university-operated observatories, but more importantly it highlighted a fundamental technical limitation. Below ~100 MHz, ionospheric waves introduce rapid phase variations in the visibility phase measurements of interferometer elements separated by more than a few kilometers. As a result, low frequency radio astronomy was limited to poor angular resolution and, in turn, poor sensitivity due to source confusion, compounded by a naturally high sky background. Thus, effectively blinded at long wavelengths, the mainstream in radio astronomy migrated to higher frequencies, and to instruments like the Very Large Array. Operating with improved receivers, and in a regime where background noise levels were lower and ionospheric effects were much less severe, arc-second resolution and high sensitivity revolutionized radio astronomy. In the early 1980s the technique of self-calibration was developed, and Bill recognized it could lift the short-baseline limit on low frequency interferometry. Unfortunately it came too late to save the Clark Lake TPT, that, in principle, could have been extended beyond its maximum baseline of 3 km to achieve significantly higher resolution and sensitivity.
Bill was, if nothing else, persistent, and fortunately the closure of Clark Lake marked not the end of his career but the start of what was arguably one of its most influential chapters. After self-calibration was developed, he and a former student, Rick Perley, proposed developing a large, low-frequency array based on the infrastructure of the ~35 km VLA. The only part that would be new would be large, dipole-based phased-array stations in place of the 25-m parabolic dishes. While funding for this scale of project proved unrealistic at the time, they regrouped, recruited another former student, Namir Kassim, and embarked on a much less expensive system based on the VLA dishes fed by 4-meter (74 MHz) dipole feeds. The system was less intended as a scientific workhorse than as a demonstration that sub-arcminute angular resolution, low-frequency radio astronomy was possible through the ionosphere. The system successfully overcame the “ionospheric barrier” and proved so successful that many of its pioneering observations have not yet been surpassed. Much of this work emerged in collaboration with the radio astronomy group at the Naval Research Laboratory led by Bill’s last graduate student, Kassim. Bill had tremendous creative influence with the NRL group and others throughout the remainder of his life. The success of his VLA “demonstrator system” helped fuel a resurgence in the field resulting in the emergence of a suite of ambitious new international instruments. His last antenna design, for the Long Wavelength Array, is still actively in use at both the VLA site in New Mexico and at the Owens Valley Observatory in California.
Bill’s technical savvy was often sought for projects outside of his main stream in low frequency radio astronomy. He served as informal advisor to the technical group at NRAO that designed the VLA starting in the 1960s, and later in his career he played an important role in the construction of the Berkeley-Illinois-Maryland Array antennas at the Hat Creek site in California. A noteworthy anecdote involved Bill’s response to the plea of a crew on a San Francisco to Sydney flight. He was able to extend the life of a newborn in an incubator by improvising a step-down transformer from the only available 110 V power supply on the plane.
While Bill’s forte was instrumentation, his scientific achievements were significant, marking him as one of the last of an early generation of classical radio astronomers with balanced scientific and technical backgrounds. Over the course of his career he wrote prescient papers spanning solar and planetary radio science, Galactic and extragalactic radio astronomy, and interplanetary and interstellar propagation effects. Many of the sky surveys he motivated are still used today, most notably the VLA Low Frequency Sky Survey that is still serving as a fundamental calibration grid for a suite of new instruments. He helped lead the first successful experiments in very long baseline interferometry, and also conducted one of the earliest searches for emission from magnetized extra-solar planets. His Clark Lake observation that the curious radio source 4C21.53 had a steep spectrum akin to that of the fast spinning Crab Nebula pulsar augured the discovery of the first millisecond pulsar. He also discovered a new class of travelling ionospheric disturbances using radio interferometry, presaging a new field of ionospheric remote sensing only now catching up to his original work. As a consummate radio scientist for all seasons, Bill was honored in 2005 as the inaugural recipient of the Grote Reber Medal “for lifetime innovative contributions to radio astronomy.” Bill was also honored by a workshop organized by his students and held in 2004. “From Clark Lake to the Long Wavelength Array: Bill Erickson’s Radio Science.”
Though he would remain active for many more years, the closure of Clark Lake led Erickson to take an early formal retirement from Maryland. He thereafter moved to Australia with his wife Hilary Cane who had begun her career in low frequency radio astronomy at the University of Tasmania. They flourished together for many years on Bruny Island, off the coast of Tasmania, where Bill continued to be scientifically and technically active. They spent twenty northern-hemisphere summers in their motorhome, parked in the driveway of former student Robert Hanisch in Maryland, allowing Bill to collaborate with NRL and Hilary to work at the NASA Goddard Space Flight Center. They shared a love for the outdoors, including gardening and fishing for much of their own food at home. He continued to be an avid sailor, circumnavigating Tasmania twice as a fitting southern complement to his earlier transatlantic voyage in the 1970s in the northern hemisphere.
On Bruny Island Bill built and operated his last instrument, a broad-band, solar radio spectrometer that provided valuable data to the solar community up until about a year before his passing. The system was copied and fielded at several other sites around the world, as were many of his earlier technical innovations. He also spent many years on Bruny Island developing low frequency antennas, and he and Hilary continued to publish papers together in solar physics. Finally, and as he wrote in one of his last papers, he was proud that “The most enduring legacy of Clark Lake is the students that were trained there,” many of whom went on to play actives roles in modern astronomy. Professor Emeritus William C. Erickson is survived by his wife Hilary in Australia, his sons Bill, Steve, and Tim in the US, eight grandchildren, and his always devoted former students.
“From Clark Lake to the Long Wavelength Array: Bill Erickson’s Radio Science”, Astronomical Society of he Pacific Conference Series, Volume 345, 2005 (eds. N. Kassim, M. Perez, W. Junor, P. Henning).
“Lessons Learned from the Clark Lake Experience”, W.C. Erickson, ASP Conference Proceedings v. 345, p. 89, 2005 (a selected paper from the above referenced conference proceedings, and available on-line at: http://www.aspbooks.org/a/volumes/article_details/?paper_id=2809).