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SALT’s timing The suggestion to build SALT came in 1998, as South Africa emerged from isolation. It was an opportunity to catch up in the field of astronomy, and to stimulate broader interest in science and technology. The new ANC government’s White Paper on Science and Technology: Preparing for the 21st Century (1996) explicitly supported astronomy, and with government backing, the SAAO had a real chance of launching the project and raising further international funding. The motivation was compelling. ■ South Africa’s astronomers and astrophysicists were internationally well regarded. ■ The Sutherland field station was a good observing site. ■ SALT would be an affordable 10-m class telescope in which relatively small-scale partners, such as individual university departments, could participate, with generous amounts of observing time. Synergies were also possible with other existing facilities. ■ SALT was not competing directly with other large telescopes (which emphasize the red to infrared region of the electromagnetic spectrum) nor would it attempt high spatial resolution imaging. It would concentrate instead on the relatively neglected shorter wavelengths, to the ultraviolet cut-off determined by the Earth’s ozone layer, together with time resolved studies, for which South African astronomers were already known. These shorter wavelengths need appropriately designed instruments. The telescope itself collects all the light that passes through the Earth’s atmosphere, but the instrumentation determines what is detected and measured or imaged. (High-performance short-wavelength instruments are more difficult and expensive to construct.) ■ SALT’s complex queue-scheduling allows for time domain studies of objects (that is, looking at how things change over timescales of days, months, or years), and builds on South African pioneering work in high time resolution astronomy: looking at objects that vary on very short timescales of seconds or less. Queue-scheduling differentiates SALT from almost all other major telescope facilities and turns the viewing angle constraint into a scientific advantage. ■ The proposal for the 10-m SALT – renamed the Southern African Large Telescope to reflect its regional importance – immediately won support from Khotso Mokhele (President of what is now the National Research Foundation), Rob Adam (of the Department of Arts, Culture, Science & Technology and later its Director-General), Minister Ben Ngubane, and the full cabinet. In the 1998 budget vote, R50 million was earmarked for SALT, as long as a similar total contribution was secured from partner institutions. (The necessary funding was provided by the USA, Poland, Germany, New Zealand, and the UK.) Many people helped to design SALT, including the SALT team, SAAO technical staff and astronomers, specialist instrument designers, and engineers and scientists from other telescopes, in particular Keck (in Hawaii) and HET. The project took five and a half years to complete, nearly three years less than any comparable project to date. The ground-breaking ceremony took place on 1 September 2000, and, exactly five years later, on 1 September 2005, ‘first light’ pictures were officially released (see www.saao.ac.za/news/salt_light.html).
Far right: Primary mirror segment number 5 before being aluminized. Right: Unlike conventional telescopes, SALT’s primary mirror (composed of hexagonal segments, below) remains stationary as it collects starlight during observation. The only movement is from the tracker (the dark structure, above), which carries instruments on the prime focus payload and migrates across the primary mirror.
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astronomers a very good deal. In addition, instead of the massive telescope tube structure moving to track an object as the Earth rotates, the tracking in both HET and SALT is done by moving a much smaller ‘payload’ of instruments on a ‘tracker’ across the focal plane of the telescope (called the Prime Focus)5. The fact that the tracker, although a very complex and expensive item, was the only part of the telescope that needed to move to follow an object also kept costs down. The South Africans were delighted at the suggestion of using the HET as a model for the new, improved SALT. Suddenly the plan had changed. No longer would they have a 4-m class telescope but a far more powerful 10-m class one for the same price6, as the cost of manufacturing 91 identical spherical mirrors and aligning them is far less than manufacturing a single surface of the same area. SALT would now be the largest single optical telescope in the southern hemisphere and equal to the largest in the world.
Trailblazing design The SALT Project Team used the HET concept as a template. They were able to improve the design by benefiting from the HET experience and from the technological advances since its completion in 1997. The primary mirror of each of these telescopes is stationary, with an array of 91 identical hexagonal mirror segments, each with a spherical surface. Although the single large mirror in conventional telescopes is easier to use, the cost of building one this size would be prohibitive and technically difficult to produce. (In fact, the largest casting furnaces currently available can produce mirrors no greater than 8 m in diameter.) A mirror composed of segments is no less efficient than a single one, provided the segments are properly aligned. A major area of improvement in the SALT design was in the alignment system of its primary mirror segments. Some 480 capacitative edge sensors were added to measure the mirror segments’ relative movements and then to correct for this motion by moving 273 ‘actuators’ (that is, small motors at the back of the mirrors that can 5. The SALT tracker is used as follows. When an astronomer wishes to observe an object (for example, a star, galaxy, cluster, or nebula) as it moves through the field of view of SALT, the tracker’s job is to move with the image of the object produced by the telescope and make sure that its light is constantly focused onto the instrumentation that produces an image or spectrum. During the ‘track’, the main telescope structure stays fixed in position and does not need to move (which contributes to SALT’s stability and resistance to wind-shake), because only the tracker moves when a given object is being observed. 6. SALT is known as a 10-m class telescope. It can be awkward to define telescope size precisely when some telescope mirrors (such as SALT’s) are no longer the traditionally circular kind. SALT’s mirror is hexagonal (because of the hexagonal shape of the segments) so its dimensions are technically close to 10 m 11 m. In addition, because of moving the tracker, the telescope’s effective mirror size changes during an observation.