Forty years ago, Broten et al. (1967a) heralded in the new field of VLBI: “The measurement of the diameter of radio sources in the range 0.01 sec of arc or less has recently been made possible by a new technique on radio astronomy. Independent stable local oscillators are used to convert the signals at the two stations to frequencies which are recorded on magnetic tapes. In principle, these long baseline interferometers can operate at any separation.” As an undergraduate summer student employed at NRC, I remember the excitement when the results were presented at the URSI meeting in May, but as a novice was unaware of the drama that had played out over the previous months, indeed right up to days before the meeting, when the first fringes on a transcontinental baseline (3100 km, Algonquin Park to Penticton) were finally detected. Insight into this momentous accomplishment can be gained from the review by Kellermann and Moran (2001) and the more anecdotal account by Broten (1988).
Broten recalls the making of documentary film “To the edge of the universe” (“Aux confins de l'univers”) by the National Film Board of Canada (released in 1969), and the added chaos that the filming caused for the researchers. The film contains interesting footage of the construction of the 150-ft radio telescope at the Algonquin Radio Observatory, but was made more “on spec” that the VLBI experiment would indeed work and so provide a dramatic story line for the film! A re-enactment has Allen Yen, the project leader, and colleagues gathered round a chart recorder saying excitedly “look, fringes”. To mark this 40th anniversary, we showed the film at the recent CASCA meeting in Kingston; those interested in celebrating and preserving this historic time in Canadian astronomy can acquire a DVD from the NFB (www.nfb.ca/trouverunfilm/fichefilm.php?id=11351&v=h&lg=en&exp).
Broten summarizes by noting that “A team effort by individuals from Canadian universities and government laboratories prevailed to bring success.” He also notes that a pre-requisite was “that the political attitude be favorable to basic or “curiosity-oriented” research which may not have immediate results,” something that does not always obtain in synchronism with other pre-requisites, like having the skilled team and engaged leadership. In the late 1970s, as CFHT was coming on line, the Canadian Astronomical Society began developing an ambitious plan for the Canadian Long Baseline Array, building on Canadian leadership and expertise, and costing $70 M (Legg 1984). In the end it was never built, a bitter disappointment to many. In addition the 150-ft telescope was soon closed for radio astronomy (its non-identical twin, the Parkes telescope in Australia, is still being productive).
That did not spell the end of VLBI activities and innovations by Canadians, however. As is clear from the written historical accounts and the film, the tape systems and the post-observation correlation were technically very challenging. One approach was to eliminate the tapes altogether, using satellite communications, at the same time boosting the sensitivity by employing a broader bandwidth. This was demonstrated by Yen et al. (1977) using the Hermes satellite. Another approach was to improve the reliability, capability, and cost-effectiveness of the tape-based systems. Kellermann and Moran (2001) write: “A major breakthrough occurred in the late 1970s with the introduction of the remarkably inexpensive home Video Cassette Recorder (VCR). Allen Yen, who had engineered much of the Canadian VLBI system, was intrigued by the potential opportunities of using consumer electronics for VLBI. During a series of visits to Caltech, the Max Planck Institut fur Radioastronomie (MPIfR) in Bonn, and the NRAO, Yen succeeded in recording MK II-compatible data on a standard VCR. As the sensitivity of the MKII VLBI system was restricted by the limited bandwidth of 2 MHz (4 Mbps), Yen began a program to develop a VCR-based system that would allow reliable digital recordings over a 6-MHz bandwidth. [Such a system was used in the new Canadian geophysical long baseline interferometer (Yen 1991).] Following Yen’s untimely death in 1993, Canadian radio astronomers continued this work, which led to the development of the inexpensive S2 record system based on professional-model VCRs.” S2 recording systems have been very productive, for example in the Japanese-led VSOP, in which Canadians participated with support from the CSA. A powerful correlator for the S2 system was developed at DRAO.
Even without new radio telescopes on Canadian soil, Canadian radio astronomers continue to innovate. The challenge is to have these innovations incorporated in the future facilities being developed by large international collaborations, like the SKA.
Bibliography
Broten, N. W. et al. 1967, Nature, 216, 44-45. Diameters of Some Quasars at a Wavelength of 66.9 cm (See also Broten et al. 1967, Nature, 215, 38, and Broten, N. W. et al. 1967, Science, 156, 1592-1593.)
Broten, N. W. 1988, JRASC, 82, 233-241.Early Days of Canadian Long-Baseline Interferometry - Reflections and Reminiscences
Kellermann, K. I. and Moran, J. M. 2001, ARAA, 39, 457–509.
The Development of High-resolution Imaging in Radio Astronomy
Legg, T. H. 1984, IAU Symposium 110, 383-389. The Canadian Long Baseline Array
Yen, J. L. et al. 1977, Science, 198, 289-291.Real-Time, Very-Long-Baseline Interferometry Based on the Use of a Communications Satellite
Yen, J. L. et al. 1991, Radio Science, 26, 89-99 The Canadian geophysical long baseline interferometer