Wednesday, 22 October 2014
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Your questions answered: Extremely Large Telescope

The UK team helping to design the world’s largest optical telescope answer your questions on how they’re doing it - and why.

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Artist’s impression of the European Extremely Large Telescope (EELT)

The world’s largest optical telescope is about to start taking shape on top of a mountain in Chile. The European Extremely Large Telescope (E-ELT), with a main (primary) mirror 39.3m in diameter, will gather 15 times more light than any other existing telescope, allowing it to not only detect planets around other stars but even to study the composition of their atmospheres; among other science goals is the direct detection of the acceleration of the expansion of the Universe, which could contribute to the explanation of dark matter.

E-ELT will be the size of a football stadium, and will present many engineering challenges, including the alignment of almost 800 hexagonal mirrors to form the primary mirror, and a fast-reacting adaptive mirror to remove the distortions to the image called by fluctuations in the atmosphere. The UK is a major contributor to both the technical team and the budget, contributing £88million to the £1billion total cost of the project.

We’ve put your questions to the leaders of the UK team on the E-ELT project, and two of the senior researchers, UK programme director Professor Colin Cunningham and Isobel Hook of the University of Oxford’s astrophysics department, have responded to them.

Why do we need another big telescope, especially when the UK is already involved in building the Square Kilometre Array?

IH: Telescopes operating at different wavelengths tell us about the Universe in different ways. The Square Kilometre Array will be the most powerful radio telescope in the world, and will be particularly sensitive to emission from neutral hydrogen, but it will not be sensitive to visible light, including starlight. The E-ELT however will operate at optical and infrared wavelengths where emission from stars, hot gas and warm ‘dust’ are visible. The E-ELT will allow us to observe these components all the way from our own solar system out to the farthest (and earliest) galaxies in the Universe. The E-ELT will also be able to observe planets beyond our solar system (known as exo-planets) both directly and indirectly from their effects on their parent stars. These observations are not possible at other wavelengths. Indeed to fully understand our Universe, we need to observe the sky at all wavelengths from radio to gamma rays, and even non-electromagnetic radiation from gravitational waves and cosmic rays. Limiting UK participation to only one of these facilities would leave UK astronomers with a very blinkered view.

Why should the UK be spending millions of pounds on a project that is so far removed from ordinary people’s lives at a time of austerity in the rest of society?

IH: The UK’s investment (which is significant but will be spent at a moderate rate over a period of about 10 years) keeps the UK at the forefront of exploration of the cosmos. Although most people don’t work on astronomy projects on an everyday basis, astronomy is one of the most exciting areas of science for the general population. Nearly four million people watched the 2012 BBC “Stargazing Live” broadcasts, while the National Space Centre in Leicester attracts 250,000 visitors a year. Beautiful images and discoveries (both expected and unexpected!) from the E-ELT will play their part in inspiring the next generation of scientists. In the shorter-term, the construction contracts that come back to the UK will support high-tech engineering in the UK, particularly in precision optics, detectors and control systems. The expected return is at least the cost of our participation, and possibly more.

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Assembled E-ELT mirror segments undergoing testing.

How will the E-ELT work differently in terms of its design and engineering from existing large telescopes?

CC: The largest optical/infrared telescopes in the world at the moment are the 10m diameter twin Keck telescopes in Hawaii, and the Gran Telescopio Canarias in La Palma, Canary Islands. These use segmented primary mirrors consisting of 36 hexagonal segments supported by highly accurate actuator and sensor systems to keep them aligned to each other and in the correct shape. The E-ELT is not fundamentally different, except it uses 798 segments to form the 39m mirror, so the difference is really one of engineering scale!

What is the most important innovation in the design and why?

CC: The principle innovation in this telescope is that it is designed as an Adaptive Telescope from the start, rather than having Adaptive Optics bolted on later as with all current telescopes. Adaptive Optics is the technology that makes it possible to cancel image distortions caused by the turbulence of the atmosphere. It works by measuring the distortions and calculating the size of the errors across the aperture of the telescope, which can then be applied in the opposite direction across a deformable mirror, so cancelling out the distortions. In the E-ELT, this is done with a 2.5m diameter flexible mirror, backed by 6000 actuators operating at up to 1000 times a second. This deformable mirror will be more than twice the diameter of those made to date, and only 2mm thick, so it is one of the major innovations. Another is the array of four to eight lasers that are used to project artificial guide stars into the upper atmosphere so that Adaptive Optics can be used even in those areas of the sky where there are no natural bright stars to use to measure the atmospheric distortion. These Laser Guide Stars will enable us to correct not just one layer of turbulence, but the whole column of air above the telescope, enabling observations over a wider field of view.

How is the design different from the cancelled Overwhelmingly Large Telescope and why was the decision taken to go down this route?

CC: The entertainingly titled Overwhelmingly Large Telescope (OWL) was planned to be 100m in diameter, and to use a spherical primary mirror, so that all the segments were identical, simplifying manufacture. However, when the cost was looked at in detail, it looked like it would be almost twice as much as a smaller 42m telescope, and it was not likely that we could raise that sort of money. Since them we have had to reduce the size of the primary mirror (and dome) in order to keep the cost close to €1billion. Other technical problems with the OWL were the large 8m tertiary mirror, the need to mount instruments 50m up in the structure and deal with flexure as they moved with the telescope (the E-ELT has fixed instrument mounts) and the lack of a ‘dome’ to protect the telescope from the wind. 

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The telescope’s internal structure will contain five different mirrors.

What will be the biggest technical challenges in building the telescope and how do you plan to go about addressing these?

CC: The biggest technical challenges are the previously mentioned large deformable mirror, manufacturing nearly 1000 highly precise mirror segments, and developing new generations of adaptive optics techniques with the associated high speed control systems. The way we are addressing these is by a process of prototyping and competitive industrial design studies that have been going on for nearly a decade now. New modes of adaptive optics are being very successfully prototyped and tested on the William Herschel Telescope in La Palma in a collaboration between UK and French scientist and engineers.

Where do you think the biggest opportunities are for UK companies to get involved with the project?

CC: We believe that the biggest opportunity areas for UK companies are in the optics and opto-mechanics manufacturing; the telescope control systems for both hardware and software; engineering consultancy; detectors for the adaptive optics systems; and for engineering equipment and specialist service supplies. The industrial return during initial design studies and prototyping has already had an impact to UK industry in excess of £9 million. One of these early industrial contracts was won by OpTIC Glyndwr in North Wales for the production of prototype primary mirror segments. We expect that the return to UK Industry could exceed £100 million by the end of the project. To help ensure that, STFC are leading a programme of Industry liaison activities. These have concentrated in regional roadshow events highlighting ‘industrial opportunities’ with the E-ELT. Four events have been held in the last year (Durham, Belfast, Heathrow, and Cardiff) with a total of around 100 companies participating and many registering their serious interest in the future E-ELT opportunities.

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A complex strucuture underpins the adaptive mirror.

In order to fund the E-ELT, the UK (and other ESO countries) has had to pull out of almost all of its other optical telescopes, and drastically reduce funding for the postdoctoral researchers who actually do the science.

Why is the E-ELT worth this, especially as it will take over a decade to build? Will it produce more science (and scientists) than several more modestly-sized telescopes would have done? And won’t the huge pressure to get time prevent most astronomers from ever using it?

IH: The balance between current and future science is indeed difficult to get right, and the UK’s funding agency for such projects (the Science and Technology Facilities Council) has set up panels of experts to provide guidance. My own view is that while savings are needed elsewhere in order to participate in E-ELT, our membership of ESO ensures continued access to 4-meter and 8-meter optical-infrared telescopes with advanced instrumentation, and access to ALMA (the new sub-millimetre telescope array in Chile). Beyond ESO, UK scientists and engineers will also participate in future projects including the Square Kilometre Array (radio telescope), the James Webb Space Telescope (the successor to Hubble), a range of space science missions developed by ESA and possibly also the Large Synoptic Survey Telescope (a future 8-meter survey telescope). Such a range of facilities with complementary capabilities is needed in order to fully understand our universe, and there is no doubt that these new facilities will be capable of completely new science that is not possible with current telescopes. Even with an infinite amount of observing time, a smaller telescope can never match the sharpness of images that the E-ELT will deliver (about 5 times sharper than from an 8m telescope) because of the fundamental diffraction limit. Without participating in E-ELT we’ll be left behind both in scientific exploration and technological innovation.

Competition for observing time on E-ELT will be strong and it seems likely that many projects will involve large groups of scientists. The information coming from E-ELT will be enormous – it will gather more light than all of the existing 8–10-metre class telescopes on the planet combined. I believe it will be essential to continue funding the scientists to analyse the data, in order to capitalise on the current investment in construction.

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What are the most exciting things you think (or hope) you might be able to see for the first time or discover using the telescope?

IH: The E-ELT’s ability to observe planets in other solar systems (“exo-planets”) is one of its most exciting prospects. The E-ELT will be able to directly observe these planets, by separating them from the glare of their parent stars, and will even find out about their atmospheres. Ultimately, finding evidence for exo-planets similar to Earth and that are capable of supporting life would be a profound breakthrough.

On a completely different topic, I would be fascinated to see resolution of the current debate about variations in the “fundamental constants” of physics. Existing observations show hints of variations with cosmic time in the fine structure constant. The E-ELT will have a much larger collecting area (about 15 times larger than that of existing optical telescopes), which, with appropriate instrumentation, will allow it to confirm or refute these claims. If variations are confirmed, it would have implications for our understanding of “Dark Energy” that is responsible for the accelerating expansion of the Universe, and would give support to theories that the Universe has many more space-time dimensions than most of us are aware of!


Readers' comments (3)

  • With the E-ELT, why is anyone investing in TMT? Also, since the U.K. is involved with both, why both ALMA and SKA?

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  • ALMA and SKA are looking at different radiation wavelengths — sub-millimetre for ALMA and radio for SKA. They're used for different types of astronomy, studying different objects and phenomena, and are complementary to each other.

  • Thanks. What about TMT versus E-ELT?
    Is this just because ESO countries will take up so much of the E-ELT, freezing out many astronomers from non-ESO countries?

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  • Will it be possible to combine the ELT and VLT data for enhanced resolving power?

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