Artist's impression of the E-ELT [Credit ESO-L. Cal�ada].

Young physicists go to 'infinity and beyond' with European Extremely Large Telescope

Physicists at Durham University are working alongside global scientific institutions on revolutionary new instruments for the European Extremely Large Telescope.

On 25 May 2016 the European Southern Observatory got the green light for constructing the dome and telescope structure of the E-ELT. Set atop the Cerro Armazones, a 3,000-metre peak in Northern Chile, on completion in 2024 the E-ELT will be the largest optical/near-infrared telescope in the world – with a footprint comparable in area to a football pitch and standing almost as tall as Big Ben.

Its 85m-diameter rotating dome will contain a telescope mounting and tube structure with a total moving mass of over 3,000 tonnes – and a primary mirror measuring over 39m in diameter.

The light-collecting area of the E-ELT will be bigger than all existing optical research telescopes combined and its adaptive optics (AO) system – employed by large observatories to correct the adverse effects of atmospheric turbulence on their image quality – will provide images about 15 times sharper than those from the NASA/ESA Hubble Space Telescope at the same wavelength.

Crucial to the project will be two new instruments: MOSAIC (the multi-object spectrograph) and HIRES (the high-resolution spectrograph). MOSAIC will enable astronomers to probe some of the deepest mysteries of the Universe, such as when the first galaxies formed and how they combined into large structures like the Milky Way.

HIRES will be used for extremely accurate studies of individual objects and will allow astronomers to study the atmospheres of planets around other stars in a search for signatures of life as well as probe the evolution of galaxies.

Durham’s CfAI is providing a co-principal investigator and AO expertise as well as a technical lead on the instrument core structure for MOSAIC, which enables corrections in atmospheric distortions. It’s also providing AO and fibre expertise for HIRES, and helping design the highly efficient optical fibre connections needed for the instrument.

Dr Andrew Reeves of Durham’s Department of Physics is a member of the CfAI research team. His PhD was based on adaptive optics.

AO reflect light gathered by a telescope off a 'deformable mirror' – a thin sheet of reflective material covering an array of push/pull actuators that can change the shape of the mirror with nanometre precision (the deformable mirror of the E-ELT will comprise around 800 hexagonal segments, each just 1.4m wide and only 5cm thick).

To calculate mirror alterations, physicists observe a bright star with a ‘wavefront sensor’ that detects the shifts in electronic signals from the light and sends data to compute the next mirror shape.

In the absence of a convenient bright star, physicists use ‘laser guide stars’ – basically lasers are beamed into the atmosphere to create artificial reference stars. But even laser guides can’t avoid disruptions caused by atmospheric turbulence, so they still require back up from a natural guide star.

“I worked on a method to gather all the data we require from lasers alone, removing the need for the natural guide star,” explains Dr Reeves. “Ideally this opens up more of the sky to the high-resolution observations that AO allows. I also developed a laboratory experiment and a computer simulation that have been used to validate this concept as well as for many other applications. The project was relatively successful, and resulted in an algorithm that could be used on the whole sky for some science cases, and hopefully relax the requirements on the natural guide star for others.”

Ollie Farley has just graduated with a master’s degree in physics at Durham, which included studying topics such as quantum mechanics, particle theory and condensed matter as well as astronomy. He is about to begin a PhD on atmospheric turbulence profiling: figuring out how the atmosphere is distributed along the line of sight of a telescope. 

As telescopes get larger, the effects of atmospheric turbulence become more pronounced. To combat this researchers use a system called ‘tomographic AO’ that employs multiple laser guide stars to measure the altitude of atmospheric turbulence to correct for a much larger field of view than conventional adaptive optics.

As part of Farley’s final year he worked in the CfAI on a research project to train an artificial neural network to perform tomographic AO – to be used by the E-ELT.

“I really enjoyed this project and it was the main reason I chose to pursue a PhD in instrumentation,” says Farley. “In addition to physics I've also always been interested in programming and software and hardware engineering, and I feel that astronomical instrumentation is a great mixture of both. Naturally, a very large part of current research in instrumentation is E-ELT related and so will be reflected in my PhD.”

Dr Reeves' initial involvement in the E-ELT project revolved primarily around building, aligning and running the laser guide systems.

“This is quite a complex job as it involves constantly monitoring the laser’s alignment and ensuring the system is healthy,” he explains. “As it developed I also got more involved in many of the algorithms that allow tomographic correction. This greatly helped my PhD work and eventually led me to the solution I adopted.”

For Farley, one of the most important parts of his PhD will be turbulence profile characterisation of the telescope site in Chile.

“An accurate profile can greatly increase the effectiveness of AO, which is especially important given that the scale of the E-ELT makes atmospheric turbulence effects much worse than for smaller telescopes,” he states. “There are several possible extensions to this such as forecasting: giving astronomers an advance idea of the strength of the turbulence for a particular night.”

Aside from working on the ground-breaking workhorse instruments, for both physicists the E-ELT project is thrilling for a multitude of other reasons.

“Finding exoplanets [a planet which orbits a star outside the solar system] with the resolution we can get from a 39m telescope will be incredible,” Dr Reeves says. “Other less obvious advances that I am excited about are the many engineering challenges associated with the telescope; AO in real-time control, for instance. To run such a large system quickly, we need new technologies.”

Dr Reeves is now involved in a collaboration with Paris Observatory to create an ‘E-ELT capable prototype real-time control system’ that can cope with the enormous data rates and processing requirements. This has leaned heavily on GPU and FPGA technologies, which are hot topics in many fields.

Farley echoes Dr Reeves’ enthusiasm.

“The E-ELT represents a massive leap in ground-based telescopes, particularly because all the technologies and instruments attached to it will help to accomplish many ambitious science goals that are simply impossible for smaller telescopes. It also has a knock-on effect as many of the new innovations can be applied to other current and planned telescopes.”

As for their future career paths both physicists plan to continue work on the E-ELT.

“I certainly hope I will be further involved in the project,” Farley says. “At the moment the plan is to stay in academia after my PhD, as I imagine that the E-ELT will still be a very big part of astronomical instrumentation research.”

“For the future I hope to stay at Durham,” adds Dr Reeves. “I really enjoy the laid back atmosphere. That’s not to say that we don’t work hard of course! But our time is not prescribed to specific jobs, so we can be flexible in what we work on and know our ideas will be taken seriously.”

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