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Racine’s work with rapid guiding had shown, as had others (such as the Roddiers at the University of Hawai‘i), that the introduction of adaptive optics (AO) on large telescopes was inevitable. Adaptive

Figure 1.

The Nuclear Astrophysics Laboratory, Oxford U.K.

optics correction (as then envisaged) was really only possible over the few seconds of arc covered by the isoplanatic patch (i.e., where atmospheric aberrations are instantaneously the same), so the large focalratio infrared field would be ideal. The much later successful introduction of laser guide stars allowed not only greater sky coverage, but also much larger fields. The final optical specification was 0.07 arcsecond fullThat first morning, Fred Gillett and René Racine presented two basic sets of science requirements which, to be met, would demand some ingenuity. Gillett made the strong case that optimum thermal infrared sensitivity required ultra-low emissivity from the telescope mirrors, a large Cassegrain focal-ratio (f/15), a marginally undersized secondary, a small central hole (1.6 meters) in the primary, and images close to the telescope diffraction limit. Only then could we approach the huge potential D-4 advantage in exposure times (D is the primary mirror’s diameter) relative to other large telescopes. The low emissivity level of the Mauna Kea sky dictated an overall reflectivity of > 96% - equivalent to a 4% emissivity. While there was wide appreciation for the remarkable results coming from infrared observations and the development of increasingly large, low-noise detector arrays, at that time there were few infrared astronomers in Canada— where interest was overwhelmingly in the optical. René Racine then laid out his requirements for optical imaging. His qualifications were unmatched, having been director from 1980 to 1984 of the CanadaFrance-Hawai‘i 3.6-meter telescope, which provided a benchmark of optical performance. He had assessed the contributions to image quality from telescope aberrations, dome and boundary layer seeing, and presented convincing evidence that natural seeing < 0.35 arcsecond occurred some 25% of the time. He proposed a requirement that the image quality delivered by the large telescope must not degrade the best natural seeing by more than 10%. This demanded an image size of < 0.1 arcsecond, similar to Hubble! Racine had been a vocal skeptic of the large telescope and this was his challenge. He made his case and the SAC had no trouble adopting it.

width half-maximum (FWHM) images with AO over a narrow field, and 0.1 arcsecond images over a narrow field. These also satisfied the needs of the thermal infrared. The need for extra-high reflectivity led to the development of special silver coatings and the process for regular cleaning of the mirrors. To preserve as much of the ultraviolet part of the spectrum as possible, the silver needed to be specially overcoated. As AO was only expected to be effective at red/yellow and longer wavelengths, the loss of the ultraviolet was not considered a major sacrifice. It would take another six years for the science requirements document to be completed (see: http:// www.gemini.edu/science/scireq3.html),

by

which

time it ran to fifty pages. However, those first few hours in Oxford set the most critical requirements for image quality and reflectivity and were essentially those eventually achieved now at the Gemini Observatory. For the thermal infrared, an emissivity of ~3% is regularly achieved from the primary/secondary combination, f/16 was the final focal ratio adopted and the hole in the primary is just 1.18 meters in diameter. All of this is even better than Fred Gillett had hoped for. Pat Roche took us to an excellent pub that evening. I might have been less relaxed if I had known that Canada would withdraw from the project, albeit temporarily, only a few months later. But, that’s another story! Gordon A. H. Walker is Professor Emeritus at UBC. He can be reached at: gordonwa@uvic.ca

June2009

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Issue 38 - June 2009  

GeminiFocus