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Roadmap endorses extremely large telescope

18 Feb 2009

The 42 m Extremely Large Telescope is one of the key priorities identified in Europe's newly unveiled 20 year astronomy roadmap. Jacqueline Hewett finds out how this decision was reached and the monumental challenges facing the scientists developing the massive mirror.

Making unanimous decisions by committee is always an arduous task. However, next time you find yourself in this situation, spare a thought for Michael Bode and his colleagues on the ASTRONET project who have spent the last three years compiling a roadmap that prioritizes both the ground- and space-based facilities that Europe should invest in over the next 20 years to ensure its leading position in astronomy.

This massive task in diplomacy involved intensive interaction with the astronomy community. The end result is a comprehensive document that is backed by astronomers in 28 member and associated states across Europe.

"This project was very ambitious and the entire European astronomy community has had a chance to influence the decisions," Michael Bode, task leader for the ASTRONET roadmap, told OLE. "We now need to sustain the momentum and make sure that our recommendations get implemented."

One of two clear top priorities was the European Extremely Large Telescope (E-ELT), a 42 m diameter optical-infrared telescope being developed by the European Southern Observatory (ESO). The other is the Square Kilometer Array (SKA), a huge radio telescope (see box).

Guy Monnet, the E-ELT's international collaboration coordinator, believes that the ASTRONET endorsement was vital. "The funding agencies in the member states are essentially giving their support to E-ELT via ASTRONET," he told OLE. "It is these same agencies that will say yes or no to move the project forward. Not to be top-ranked at this stage would have been a disaster. ASTRONET was a crucial hurdle but we have many more to cross to build the E-ELT."

Why do we need a 20 year roadmap?
Today, Europe is at the forefront of astronomy, largely thanks to the level of co-operation in the community. In order to maintain this position, Europe required a comprehensive long-term strategy. Enter ASTRONET.

ASTRONET was created by the major European funding agencies and supported by the European Commission. The roadmap prioritizes both ground- and space-based projects and groups them into small-, medium- and large-scale depending on the level of investment required. Over 100 existing and proposed infrastructure projects were reviewed, primarily those requiring new funds of €10m or more from European sources and on which spending decisions are required after 2008.

With this in mind, it is worth noting the cost implications. According to ASTRONET, Europe currently spends approximately €2 bn a year on astronomy. Implementing the roadmap in full will require an increase of around 20%. In quantifiable terms, this is less than €1 per year per European citizen.

Crunch time for the E-ELT
The E-ELT project has been running for around 10 years now and approximately €50m has been invested in research and development to establish the enabling technologies. 2010 will be a crucial year for the E-ELT project as the design phase will have come to an end and, once and if approved, the seven year build can begin.

"Our plan is to have the telescope fully designed by the end of 2009," said Monnet. "By the end of the year we will have demonstrated the technologies that are needed to make the telescope in a cost-effective manner. We will also be able to anticipate the performance of the telescope."

Monumental mirrors

Two areas where R&D teams in both academia and/or industry have pushed the boundaries of technology are the segments that make up the primary mirror and adaptive optics.

"The primary mirror will comprise just under 1000 1.42 m wide hexagonal segments," said Monnet. "With a total area of 1300 m2, there is a real need to make the mirror financially viable. The challenge is having the technology that can do the job in a reasonable time that is affordable."

One company involved in the preliminary development and sampling of the mirror segments is UK-based precision polishing expert Zeeko. Zeeko is part of a consortium that includes optics specialists at QioptiQ and measurement experts at TNO in the Netherlands. A second consortium involving French company SAGEM also has a contact to sample mirror segments.

"Our consortium produced a detailed bid running to 400 pages and was awarded the contract in November 2007," Zeeko's managing director Richard Freeman told OLE. "The contract started in January 2008 and runs for 28 months. At the end of that time we should be delivering seven segments and a fully costed bid for making 1100."

Needless to say this is easier said than done. The two key challenges facing Zeeko and its collaborators were firstly how to polish the segment and then how to measure it. For Zeeko, one of the biggest challenges was meeting the stringent edge control specification.

"When you rub sandpaper on wood, the edge becomes rounded," explained Freeman. "Imagine the same thing on a series of optics that fit together as a mosaic. The last thing you want is an edge that is rolled over as you lose reflectivity. We have been developing large-scale machines to meet these requirements."

It doesn't end there however as measuring the optics is equally demanding. "The 42 m mirror is essentially a large aspheric mirror," said Freeman. "If you break that down into separate tiles, then each one is an off-axis asphere. When you take a segment out of an asphere it becomes something the likes of which people have not had to measure in the past, or certainly haven't had to measure quickly or in volume."

And when you consider the timescales involved, the enormity of the task is eye-opening. "A few years ago, no one would quote less than six months to make one segment," said Freeman. "Say you have 200 working days in a year then even doing one segment per day means that it will take years to make all of the segments. We expect to produce segments in days rather than months."

The E-ELT also requires an adaptive optics system to compensate for wavefront distortions, induced when light passes through the Earth's atmosphere, in order to meet the precise requirements on image quality.

"We use detectors with pixels around 15 µm in size and you simply cannot put light from a star on to this size of pixel using a 42 m collector," noted Monnet. "This means that the telescope has to use adaptive optics to reduce the seeing disk of the object. We have had to make enormous advances in adaptive optics such as the development of deformable mirrors with many more actuators and large fast detectors to sense the wavefront."

Another key area has been reducing the time that it takes to compute the required correction. "The atmosphere changes on the order of a millisecond," explained Monnet. "The correction requires an applied voltage – and this cannot be done in nanoseconds. This means that you have very little time to do the computation but there has been spectacular progress in this area."

For all involved, the daily task of meeting the requirements of the E-ELT is incredible. "It's a very big project no matter what way you look at it," said Freeman. "There has never been so much low-expansion glass made, such large areas polished or such a big construction project. What brings you back down to Earth is the realization that what we have to do is being multiplied many times throughout the entire project."

Getting industry involved
One of the key stumbling blocks identified by ASTRONET is making industry aware of the opportunities that arise to get involved in astronomy projects. As technological development is essential to future facilities, boosting this link is seen as vital.

"There is no central bank or repository easily found or accessible to promote successful commercial transfer between the two communities on a regional or EC-wide level," stated the final ASTRONET roadmap report. "The promotion of successful astronomy technology transfer activities would be most helpful in rectifying this situation. The creation of an easily accessible European repository of astronomy technology transfer would greatly enhance visibility of European success stories in astronomy."

The ASTRONET roadmap emphasizes that it is essential to fund research into enabling technologies. It also goes as far as listing some of the technologies that will be required by prioritized facilities such as high quantum efficiency photodetectors and lightweight X-ray mirrors.

For further details, including the executive summary and the full roadmap report, see www.astronet-eu.org.

• This article originally appeared in the February 2009 issue of Optics & Laser Europe magazine.

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