Ordinary Meeting, 2009 May 27


The European Extremely Large Telescope – Fabricating the Mirror Segments

Dr Walker opened by explaining that his talk would outline some of the engineering challenges posed by the manufacture and maintenance large telescope optics. It would go on to look in detail at the work which was in progress to develop new techniques for fabricating mirror segments for the European Extremely Large Telescope (E-ELT), which was to be built in the mid-2010s.

Dr Walker first took a survey of the largest telescopes which were currently available to professional astronomers. The largest telescope ever to have been built was the Gran Telescopio Canarias (Gran TeCan), a 10.4-m telescope on the island of La Palma in the Canary Islands, which had taken over seven years to build and which was only just beginning to take its first observations. Behind it in the league table of size, there were around a dozen telescopes worldwide with apertures of between 8 and 10 metres, including the two American 10-m Keck Telescopes and the Japanese 8.3-m Subaru Telescope on the summit of Mauna Kea, 13,796-feet above sea level on the island of Hawaii. These were complemented by around a dozen more telescopes with 4- to 8-metre apertures, including the Anglo-European William Herschel Telescope (WHT) on La Palma.

The speaker remarked that it was interesting to compare this current generation of telescopes with those which had been in use in the 1930s, to see how things had changed in the intervening 70 years. In the 1930s, the available telescopes had included the 100-inch (2.54-m) Hooker Telescope at the Palomar Observatory on Mount Wilson, which been in operation since 1908, and work had been well under way on a new 200-inch (5.08-m) Hale Telescope at the same site. The mirror for this new telescope had been silvered in 1934, though it had not entered service until 1948 owing to delays brought about by the Second World War. So, perhaps rather surprisingly, the change was comparatively slight: telescope apertures had grown by a mere factor of two in the past 70 years.

Over that time, there had, however, been tremendous advances in the sensitivities of the detectors placed at the foci of these telescopes. In the 1930s, photographic plates with a typical light-collecting efficiency of 1% had been state-of-the-art, whereas modern CCD cameras were typically able to record 70-80% of the light which landed on their sensors. As a result, a modern photographic exposure could reveal equivalent detail in less than a seventieth of the time that it would have taken in the early twentieth century, and it was consequently possible to detect much fainter structures on the sky. But, putting these advances to one side, the lack of corresponding progress in the building of telescopes with ever-larger apertures seemed to beg an explanation. Astronomy was, after all, surely unique among the sciences in that the optics of telescopes such as the Hale Telescope remained among the best in the world over 60 years after their construction.

Dr Walker went on to explain that, in essence, large telescope mirrors were very difficult and expensive to manufacture and handle. Taking as an example the two 8.4-metre mirrors of the Large Binocular Telescope (LBT; completed 2002), he explained that in the accounts of such projects, the phenomenal cost of building and equipping a vacuum chamber which was large enough to use for silvering such a mirror was often exceeded only by the more-mundane-still cost of building a road which was good enough to use for the transportation of such a bulky mirror up to a good mountain-top observing site. Aside from these cost considerations, there were naturally hazards associated with handling such large and unwieldy mirrors: a single crack could render it entirely useless. For this reason, the 8.4-m mirrors built for the LBT were the largest monolithic mirrors ever to have been built: all larger telescopes used tessellating hexagonal mirror segments which could be manufactured individually and put together as tiles. For example, the primary mirrors of the two 10-metre Keck telescopes were built up from 36 segments, each measuring 1.8-m across.

The principles involved in the construction of segmented telescope mirrors had first been demonstrated in the 1990s with instruments such as the Keck Telescopes, but the task was not easy. Typically, a sophisticated computer system was needed to continuously monitor the positions of the segments and make corrections for any errors in the shape of the mirror using actuators placed behind each segment. Issues such as the thermal expansion of the telescope structure, and its flexion as the telescope slewed across the sky, introduced errors into the shape of the mirror which needed to be painstakingly corrected for. These challenges grew massively more difficult as telescope apertures got larger, owing to the increased weight of the mirrors which were having to be manipulated and kept in shape.

However, the speaker added that the scientific rewards for having a telescope with an aperture which was very much larger than anything which was currently available were potentially great. For example, in order to image Earth-like extrasolar planets around other stars it would be necessary to have a telescope with a tremendously high resolving power, but which was simultaneously able to detect an intrinsically very faint planet. Likewise, in order to see the formation of the first generation of galaxies out of the primordial gas which had been produced by the Big Bang, a telescope was needed which could detect objects which were not only at distances of billions of lightyears away from us, but also intrinsically rather small and faint.

In response to these challenges, designs for several very large optical telescopes had been proposed. In the US, plans were currently under way to build a Thirty Meter Telescope (TMT), whose primary mirror would be made up from 492 hexagonal segments, and which it was hoped would be operational by around 2017-8. In Europe, plans for a similar telescope had stemmed from two competing proposals, which had recently been merged into a plan for a single telescope, taking the most realistically achievable aspects from each of the earlier designs.

The first of these two earlier designs had been the 50-m Euro50 Telescope, which had been designed around an aspherical primary mirror made from 619 segments. The speaker explained that the aspherical shape of the segments required to build such a mirror was not insignificant. Spherical segmented mirrors were relatively easy to manufacture, because every segment was of the same shape and mass-production was possible. The segments of an aspherical segmented mirror, on the other hand, all needed to have different curvatures. Not only was mass-production impossible, but the fabrication of high-precision aspherical surfaces was costly and difficult. Many amateurs would be familiar with the difference in cost between Schmitt-Cassegrain telescopes with spherical mirrors and corrector plates, and more traditional Cassegrain telescopes with parabolic mirrors. The idea of building a 50-m aspherical mirror was thus highly ambitious.

The competing design, the OverWhelmingly Large telescope (OWL), was in one sense simpler: its 100-m primary mirror was spherical, made from 3,048 identical segments. However, to correct for the spherical shape of this primary mirror, a 25.6-m aspherical secondary mirror was required, which would itself be made from 216 segments.

Dr Walker explained that the merged proposal was to build a more realistic 42-metre telescope, called the European Extremely Large Telescope (E-ELT), which would have an aspherical primary mirror built from 984 hexagonal segments, each measuring 1.42-m across. In practice, a total of 1,148 mirror segments would need to be manufactured in order to ensure an adequate supply of spare parts. The resulting telescope would be configured with a field-of-view of around ten arcminutes and a focal ratio of f/1. Despite the move to a less ambitious aperture, the task of manufacturing the segments for the E-ELT remained a serious challenge, however. With the best technology currently available, each segment would take around six months to polish, at a cost of several million pounds, and so it was essentially that new techniques be developed.

The speaker explained that his own involvement with the project came through his company, Zeeko Ltd, which was pioneering a new way of polishing aspherical mirror surfaces. He explained that standard polishing tools worked rather poorly on aspherical mirrors because the target curvature of the mirror was not constant across its surface. Consequently, a polishing tip which had the right curvature to hug the surface of the mirror at one point would have the wrong shape to hug the surface of other parts of the mirror. The speaker's idea had been to use an inflatable membrane with abrasive surface in place of a traditional polishing tip. One simply needed to press down on the membrane by different amounts to change the pressure inside it and change its curvature to match the desired shape at any particular point on the mirror's surface. This inflatable membrane could be mounted on a computerised (CNC) polishing machine and controlled automatically.

The speaker explained that having patented his idea, he had formed a collaboration with OpTIC Glyndŵr Ltd in North Wales, and the partnership had accepted a contract a build seven full-size prototype segments for the E-ELT. To provide a fair test of the most challenging engineering which was needed for the E-ELT mirror, these prototype segments corresponded to the most extremely aspherical parts of its surface, around the edges. There was also a competitive element to the contract: a rival company in France had been contracted to independently manufacture a similar set of prototype segments.

The speaker closed by reporting that he was making good progress on his own contract: in a recent test he had polished a single mirror segment to the desired surface accuracy within 31 hours. Looking ahead, he hoped to be able to deliver his first two prototype segments in April 2010, and the remaining five in late 2010. If he completed the contract successfully, he hoped to form an industry consortium and be in a position to bid for the manufacture of all 1,148 of the E-ELT's segments when the time came.

Following the applause, the President invited Dr Richard Miles, Director of the Association's Asteroids and Remote Planets Section, to present Sky Notes.




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