Rise of the super telescope
As work continues on its design and construction, we take you inside the Extremely Large Telescope as it prepares for first light
As work continues on its design and construction, we take you inside the Extremely Large Telescope as it prepares for first light in 2025
Looking further and deeper than ever before… that’s the central goal that’s driven astronomy since its inception. Studying the night sky and the universe that frames our world in ever-increasing detail, dissecting the light that’s reaching our little world and discerning the grand legacy of the cosmos. For centuries humans have been building more and more powerful terrestrial telescopes that can see further into the void of space, each one expanding in size and breadth of vision.
That ever-increasing scale and desire to know more has brought us to the Extremely Large Telescope (ELT). A global initiative centralised by the European Southern Observatory (ESO) Council, the ELT project aims to construct the largest optical to near-infrared Earth-based telescope ever created with the intention of studying the furthest reaches of the universe, for the first time studying the properties and physics of the first galaxies and the behaviours of distant planets that orbit other stars.
The genesis of the project took place back in 2000, when European astronomers and scientists began discussing the desire to see the furthest reaches of the universe in more detail. Some of the largest telescopes in operation at the time, such as the Gran Telescopio Canarias – based in the Canary Islands – or the Very Large Telescope (VLT) – based in Chile just 20 kilometres (12.4 miles) from the future ELT site – were capable of identifying these far-flung points in space, but were too primitive to study them in depth.
So began a planning and preproduction stage that lasted over ten years as the ESO Council began calculating how much and how long it would take to build the ELT. Over this period, each of the 15 – 16 following the admittance of Brazil in 2010, though the nation is awaiting ratification of the Accession Agreement – member states of the ESO Council began drawing up Phase 1 plans, which presented research and development plans in order to secure different contracts for each element of the ELT. In 2018 the cost of telescope was around £1 billion
($1.3 billion) with a planned ‘first light’ set for 2025.
For Simon Morris, a professor of physics at the University of Durham; a member of the team working on the ELT’s Multi-Object Spectrograph for Astrophysics, Intergalactic-medium studies and Cosmology (MOSAIC) instrument and the UK astronomy representative for the ESO Council, studying those distant glimpses of the first galaxies is exactly why something as grand as the ELT is needed. “I’ve always done research on how galaxies form, and I feel that’s the most interesting thing we’ll be able to do with the ELT. The galaxies that we can see at the furthest distance are showing us the point where the universe went from being neutral to being ionised, so the things that formed in this period essentially cooked the universe,” he says. “We can see some galaxies at the point when that happened, but it’s very difficult to study them in great detail. The current class of telescopes can only just barely detect them, and can barely perform spectroscopy. With the ELT we will finally
be able to study the distant cosmos with a new sense of clarity.”
The ELT will have the potential to provide data for the entire astronomical community, performing hundreds of tasks with six currently planned instruments. As a telescope with the power to see light from around 1 billion years after the Big Bang, the ELT will focus its attention on the first galaxies of the universe and how they evolved and transformed across the cosmic timeline. We’ll have the ability to study the ancient elements at their heart – primordial stars, planets and black holes that helped shape the space around them.
We’ll also have the opportunity to study another fact of astronomy that continues to fascinate scientists: planets that orbit stars outside our own system, or exoplanets. In the eternal search for planets that share the characteristics of Earth – and by proxy the potential to support life as we know it – the ELT will be able to study these distant planets as they orbit far-flung stars, discerning their composition and the nature of their suns.
But how big does a telescope have to be to capture these distant times, and do so with an almost unheard of clarity? The ELT will have a huge 39.3-metre (128.9-foot) wide primary mirror, with a 4.2-metre (13.8-foot) wide secondary mirror, a third supporting mirror 3.75 metres (12.3 feet) in diameter and two additional mirrors that can be adjusted to remove atmospheric blurring. It will be able to gather more light than all the eight to ten metre (26.2 to 32.8 foot) telescopes on Earth combined
– to put that into perspective, the ELT will be able to capture 100 million times more light than the human eye and detect objects millions upon millions times fainter than that.
“The ELT will be able to study objects that are much fainter with just as much depth as far brighter celestial objects,” says Niranjan Thatte, a professor of astrophysics at the University of Oxford and project leader on the High Angular Resolution Monolithic Optical and Near-infrared Integral field spectrograph (HARMONI). “Fainter doesn’t always mean further away – for instance, planets are usually fainter in comparison to stars – but in most cases reduced light almost always means it’s at a considerable distance. Since the light has taken a long time to get to us, we need a telescope that has the potential to study light originating around 1 billion years after the Big Bang. Being able to study these galaxies in their youth, we can learn when a galaxy’s given stars were created or what the properties of these infantile areas of space were in this formative cosmic period.”
Originally envisioned with a 100-metre (328-foot) diameter mirror – though this idea was eventually scrapped as it was both unfeasible and far too costly to construct – the ELT will be built on a specially chosen location in northern Chile. The site itself, a mountain known as Cerro Armazones, was chosen for its high rate of clear skies, making it an ideal location for gazing into the stars.
The mountain itself has already been prepared for the first stage of the telescope’s on-site
construction, with a controlled explosion in June 2014 blowing millions of rocks from its peak. The controlled demolition reduced the mountain by 40 metres (131.2 foot) and will see the ELT sitting at a final altitude of about 3,046 metres (9,993 feet).
The main mirror of the telescope, which was downgraded from the original 100-metre (328-foot) design envisioned for the first version of the ELT, was adjusted in 2005 in order to keep the whole project within the original budget. The question is, however, how much of an effect will these seemingly minor adjustments have on the wider capabilities of the finished project? “The reduction in size for the primary mirror from 42 metres
(138 feet) to 39.3 metres (129 feet) reduces both the sensitivity of the telescope and the angular resolution – the detail that can be observed in the sky,” explains Colin Cunningham, director of the E-ELT UK Project Office.
“The angular resolution reduces linearly with the diameter of the primary mirror, so the reduction did not have a huge impact on the detail we will see. However, the reduction in mirror area makes a big difference to the sensitivity by reducing the number of photons collected. For some science the reduction in sensitivity goes with the fourth power of mirror diameter or more. This means that it will be more difficult to directly observe exoplanets, for example. However, the ELT will still be much more sensitive than its nearest rival, the US-led Thirty Meter Telescope,” Cunningham adds.
Even with that slight reduction in size, the primary mirror still forms part of a unique fivemirror set-up that will prove vital in enabling astronomers to study the earliest light in the universe with an untold clarity. Those extra mirrors will enable astronomers to look through incredibly fine angular scales across a large angle of the sky.
“In this case it means that we can study the sky to a scale of ten arcminutes – a unit of angular measurement equal to 160th of one degree – and for a 39.3-metre (129-foot) diameter telescope, that’s an incredible field of view,” reveals Morris. “Apart from blurring from the atmosphere, the images will be measured in micro-arcseconds [millions of a 60th of a 60th of a degree], so incredibly tiny angles will be detectable.”
There’s also the introduction of an already established astronomical technology known as ‘adaptive optics’. Looking so far into the cosmos leaves such a gaze open to ‘atmospheric turbulence’, the natural phenomena caused by the atmosphere
“With the ELT we will finally be able to study the distant cosmos with a new sense of clarity”
Simon Morris
of our world partially obscuring stars and causing them to twinkle. This disturbance causes blurring to a telescopic eye, so a countermeasure is required to remove it.
One of the mirrors in the ELT will be deformable, so it can actively change its shape around a thousand times per second. In order to pull this off the mirror needs additional ones to work on this scale, hence the decision to have a total of five in effect. Another one of these five mirrors will tip and tilt on a similar timescale and compensate for this blurring. On top of this the ELT will use a laser guide system that will project beams of light towards the chosen point of observation – since not every point of study will have a suitable star present to generate enough light, these ‘artificial stars’ will provide the perfect conditions for study.
“The five-mirror design enables a huge, deformable mirror – which is 2.5 metres (8.2 feet) in diameter – to be incorporated,” comments Cunningham. “This is controlled by over 5,000 actuators to compensate for atmospheric distortions and wind shake to ensure that the telescope images are the best possible. The other key technologies include the active support structure for the 798 hexagonal mirrors forming the 39.3-metre (129-foot) wide primary mirror and the laser guide system that will enable adaptive optics to be carried out anywhere in the sky, not just near to bright ‘natural’ guide stars.”
The ELT won’t just have a unique mirror system, but also a large selection of precise instruments for studying the properties and physics of the wider universe. The ELT needs to be able to support the wider astronomical community, so a series of tools have been placed into an order of importance that will see each one systematically added to the observatory over time as it becomes more and more capable. For instance, the UK is part of two vitally important instruments for the ELT: the aforementioned MOSAIC multi-object spectrometer and the HARMONI visible/near-infrared
spectrometer. HARMONI will be one of the first two instruments added to the ELT, and will be there to ensure the telescope receives first light in 2025.
The MOSAIC multi-object spectrometer, which is being researched and designed in cooperation with astronomers and engineers in France and the Netherlands, is designed specifically for use on a telescope as big as the ELT. At its core a spectrograph is used to separate out and measure the wavelengths present in electromagnetic radiation – it determines the spectrum of light around and emitted by a given object in space. That same principle applies to MOSAIC, only this time the spectrograph needs to make multiple copies of the same area of space in order to study the different elements.
“With the ELT we want to look at multiple objects spread across a considerably large field of view,” says Morris, part of the UK team working on the MOSAIC component. “And we want to do that using adaptive optics, so there’s the challenge there of making these two fields work in harmony. The UK has actually been taking a lead on these tests, known as multi-object adaptive optics, and we’ve actually been testing that in collaboration with a number of engineers from Paris to show that these principles will work when the ELT hits first light.”
Alongside MOSAIC, the ELT will also house another significant piece of astronomical equipment: the HARMONI optical and nearinfrared spectrograph. The contract to design and construct this instrument was awarded to the UK in September 2015 and will reportedly cost £50 million ($62.2 million). The project is being led by a team of researchers at the University of Oxford. A near-infrared spectrograph is designed to measure the cool atmospheres of stars where new molecules form. By studying the rotational and vibrational nature of these molecules, astronomers can then gain a greater understanding of the nature of the star that bore them.
“HARMONI will be a four-metre (13-foot) tall cryogenic instrument, which requires us to cool every element of the tool down to around
-153 degrees Celsius (-243 degrees Fahrenheit),” comments Thatte. “We do this because any amount of heat produces radiation. Any kind of radiation, even the smallest amounts, can be detected by a near-infrared spectrograph, so unless we cool everything down to a significantly cold temperature, these pockets of radiation will distort the readings we’d be attempting to make.”
Another challenge faced by the HARMONI spectrograph, and the ELT as a whole, is seismic activity found in the Chilean region selected for the telescope. The VLT, based at Cerro Paranal, has already experienced and endured a magnitude 8.0 earthquake – considered ‘Great’, the most severe band for seismic classification – so the ELT and its instruments will need to be reinforced in order to withstand these natural occurrences. “In that sense the ELT will have something in common with a space telescope,” adds Thatte. “Just as a telescope will have to survive the launch process, so will our instruments have to withstand considerable destructive forces.”
Alongside its six planned instruments, there’s also the potential to build an imager powerful enough to take pictures of identified exoplanets. “There’s currently a discussion to build something along those lines for the ELT,” says Morris. “But the general consensus is that the technology needed to make it a reality isn’t quite ready yet, so there’s a rolling technology development going on now and teams are working on proving it can be done. If those results prove positive, then it will be included in the final telescope set-up.”
The potential to study these distant worlds in greater detail remains one of the most incredible prospects for the ELT and has the potential to change the way we see the wider cosmos forever. With thousands of exoplanets having been detected across our galaxy by pioneering missions such as Kepler and the Transiting Exoplanet Survey Satellite (TESS), finding out more about these worlds is a hot topic in astronomy. “The most exciting thing about not just detecting and locating exoplanets in the universe, but also measuring their properties is that it may bring us much closer to understanding if Earth is unique,” muses Cunningham. “Again, with the first galaxies, it is not just about detection, but understanding their astrophysical properties using spectroscopy that will open up new understanding about how the early universe evolved.”
With less than five years to go until the ELT’s proposed first light detection sometime in 2025, the road to a fully operational Extremely Large Telescope is well underway. With 16 countries contributing to its inception, design and construction, it’s set to become a truly world-class example of modern engineering. Through its many lenses astronomers will be able to see into the cosmos like never before, studying the properties of exoplanets, the formation of the first galaxies, the genesis of primordial systems and the expansion of the very universe itself.
“THE ELT’S HUGE LIGHT-GATHERING CAPABILITIES WILL ENABLE US TO STUDY HOW GALAXIES AND STARS WERE BORN”
NIRANJAN THATTE