Astronomers scale new summit

29 November 2019

The world’s largest optical/near-infrared telescope, the Extremely Large Telescope, under construction in Chile, will bring mysteries such as dark energy into focus.

The foundations of ESO’s Extremely Large Telescope

The 3 km-high summit of Cerro Armazones, located in the Atacama desert of Northern Chile, is a construction site for one of most ambitious projects ever mounted by astronomers: the Extremely Large Telescope (ELT). Scheduled for first light in 2025, the ELT is centred around a 39 m-diameter main mirror that will gather 250 times more light than the Hubble Space Telescope and use advanced corrective optics to obtain exceptional image quality. It is the latest major facility of the European Southern Observatory (ESO), which has been surveying the southern skies for almost 60 years.

The science goals of the ELT are vast and diverse. Its sheer size will enable the observation of distant objects that are currently beyond reach, allowing astronomers to better understand the formation of the first stars, galaxies and even black holes. The sharpness of its images will also enable a deeper study of extrasolar planets, possibly even the characterisation of their atmospheres. “One new direction may become possible through very high precision spectroscopy – direct detection of the expansion rate of the universe, which would be an amazing feat,” explains Pat Roche of the University of Oxford and former president of the ESO council. “But almost certainly the most exciting results will be from unexpected discoveries.”

Technical challenges

Approved in 2006, civil engineering for the ELT began in 2014. Construction of the 74 m-high, 86 m-diameter dome and the 3400-tonne main structure began in 2019. In January 2018 the first segments of the main mirror were successfully cast, marking the first step of a challenging five-mirror system that goes beyond the traditional two-mirror “Gregorian” design. The introduction of a third powered mirror delivers a focal plane that remains un-aberrated at all field locations, while a fourth and a fifth mirror correct distortions in real-time due to the Earth’s atmosphere or other external factors. This novel arrangement, combined with the sheer size of the ELT, makes almost every aspect of the design particularly challenging.

Concepts of the ELT at work

The main mirror is itself a monumental enterprise; it consists of 798 hexagonal segments, each measuring approximately 1.4 m across and 50 mm thick. To keep the surface unchanged by external factors such as temperature or wind, each segment has edge sensors measuring its location within a few nanometres – the most accurate ever used in a telescope. The construction and polishing of the segments, as well as the edge sensors, is a demanding task and only possible thanks to the collaboration with industry; at least seven private companies are working on the main mirror alone. The size of the mirror was originally 42 m, but it was later reduced to 39 m, mainly for costs reasons, but still allowing the ELT to fulfill its main scientific goals. “The ELT is ESO’s largest project and we have to ensure that it can be constructed and operated within the available budget,” says Roche. “A great deal of careful planning and design, most of it with input from industry, was undertaken to understand the costs and the cost drivers, and the choice of primary mirror diameter emerged from these analyses.”

The task is not much easier for the other mirrors. The secondary mirror, measuring 4 m across, is highly convex and will be the largest secondary mirror ever employed on a telescope and the largest convex mirror ever produced. The ELT’s tertiary mirror also has a curved surface, contrary to more traditional designs. The fourth mirror will be the largest adaptive mirror ever made, supported by more than 5000 actuators that will deform and adjust its shape in real-time to achieve a factor-500 improvement in resolution. 

Currently 28 companies are actively collaborating on different parts of the ELT design; most of these companies are European, but also include contracts with the Chilean companies ICAFAL, for the road and platform construction, and Abengoa for the ELT technical facility. Among the European contracts, the construction of the telescope dome and main structure by the Italian ACe consortium of Astraldi and Cimolai is the largest in ESO’s history. The total cost estimate for the baseline design of the ELT is 1.174 billion, while the running cost is estimated to be around 50 million per year. Since the approval of the ELT, ESO has increased its number of member states from 14 to 16, with Poland and Ireland incorporating in 2015 and 2018, respectively. Chile is a host state and Australia a strategic partner.

European Southern Observatory’s particle-physics roots

ESO’s Telescope Project Division and Sky Atlas Laboratory in the 1970s

The ELT’s success lies in ESO’s vast experience in the construction of innovative telescopes. The idea for ESO, a 16-nation intergovernmental organisation for research in ground-based astronomy, was conceived in 1954 with the aim of creating a European observatory dedicated to observations of the southern sky. At the time, the largest such facilities had an aperture of about 2 m; more than 50 years later, ESO is responsible for a variety of observatories, including its first telescope at La Silla, not far from Cerro Armazones (home of the ELT).

Like CERN, ESO was born in the aftermath of the war to allow European countries to develop scientific projects that nations were unable to do on their own. The similarities are by no means a mere coincidence. From the beginning, CERN served as a model regarding important administrative aspects of the organisation, such as the council delegate structure, the finance base or personnel regulations. A stronger collaboration ensued in 1969, when ESO approached CERN to assist with the powerful and sophisticated instrumentation of its 3.6 m telescope and other challenges ESO was facing, both administrative and technological. This collaboration saw ESO facilities established at CERN: the Telescope Project Division and, a few years later, ESO’s Sky Atlas Laboratory. A similar collaboration has since been organised for EMBL and, more recently for a new hadron-therapy facility in Southeast Europe.

Unprecedented view

A telescope of this scale has never been attempted before in astronomy. Not only must the ELT be constructed and operated within the available budget, but it should not impact the operation of ESO’s current flagship facilities (such as the VLT, the VLT interferometer and the ALMA observatory).

The amount of data produced by the ELT is estimated to be around 1-2 TB per night, including scientific observations plus calibration observations. The data will be analysed automatically, and users have the option to download the processed data or, if needed, download the original data and process it in their own research centres. To secure observation time with the facility, ESO makes a call for proposals once or twice a year, at which researchers propose desired observations according to their own fields. “A committee of astronomers then evaluates the proposals and ranks them according to their relevance and potential scientific impact, the highest ranked ones are then chosen to be followed,” explains project scientist Miguel Pereira of the University of Oxford.

Currently, 28 companies are actively collaborating on different parts of the ELT design, mostly from Europe

In addition to its astronomical goals, the ELT will contribute to the growing confluence of cosmology and fundamental physics. Specifically, it will help elucidate the nature of dark energy by identifying distant type 1a supernovae, which serve as excellent markers of the universe’s expansion history. The ELT will also measure the change in redshift with time of distant objects – a feat that is beyond the capabilities of current telescopes – to indicate the rate of expansion. Possible variations over time of fundamental physics constants, such as the fine-structure constant and the strong coupling constant, will also be targeted. Such measurements are very challenging because the strength of the constraint on the variability depends critically on the accuracy of the wavelength calibration. The ELT’s ultra-stable high-resolution spectrograph aims to remove the systematic uncertainty currently present in the wavelength calibration measurements, offering the possibility to make an unambiguous detection of such variations.

The ELT construction is on schedule for completion, and first light is expected in 2025. “In the end, projects succeed because of the people who design, build and support them,” Roche says, attributing the success of the ELT to rigorous attention to design and analysis across all aspects of the project. The road ahead is still challenging and full of obstacles, but, as the former director of the Paris observatory André Danjon wrote to his counterpart at the Leiden Observatory, Jan Oort, in 1962: “L’astronomie est bien l’ecole de la patience.” No doubt the ELT will pay extraordinary scientific rewards.


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