‘First light’ for ESO’s Extremely Large Telescope now pencilled in for November 2026. Construction of what will eventually become the world’s “biggest eye on the sky” is in a difficult period, but still making excellent progress. That was the message from Roberto Tamai, program manager for the European Southern Observatory’s Extremely Large Telescope (ELT), in his presentation at this week’s SPIE Astronomical Telescopes and Instrumentation Digital Forum. Tamai said that the combination of the Covid-19 pandemic and social unrest in Chile, where the ELT is being built, meant that “first light” was now pencilled in for November 2026. Prior to the latest disruption, that had been expected a year earlier. €127M budget boost The biennial SPIE event, which had been due to take place in person in Yokohama earlier this year, brings together much of the world’s expertise in telescope development and construction. Among the highlights at the event this time around are updates on the largest telescope construction projects currently underway, including the Vera C. Rubin Telescope, Giant Magellan Telescope, and the Thirty Meter Telescope (TMT), alongside ELT. With its 798-segment primary mirror, set to measure a colossal 39 meters in diameter, ELT represents the single largest of those projects in terms of its collection optics, and is expected to generate imagery way beyond what is currently possible with either terrestrial or space observatories. Tamai said that ELT operations would be helped by the additional €127 million recently approved for the observatory’s budget by the ESO Council. Earlier this month, ESO said that the increase - representing an overall rise of 10 per cent and bringing the total cost of the project to €1.3 billion - included the procurement of components originally deferred to the second phase of the project. Those include ELT’s second pre-focal station, two more laser guide star systems, astronomy-relevant atmospheric monitoring equipment, and a small technical building at the Armazones observatory site to optimize operations and maintenance activities. A rough sea Likening the giant project’s current challenges to that of “a boat heaving on a rough sea”, Tamai outlined setbacks including a key contractor becoming insolvent, social unrest in Chile last year, and inevitably the impact of the Covid-19 pandemic and travel restrictions on site activities, inspections, and interaction between the hundreds of contractors and sub-contractors working on ELT. At the Armazones summit site, ELT’s foundations were being built when the pandemic arrived in Chile and work halted as a result. Tamai said that it was still not certain when that work would continue, but that a restart in early 2021 did look possible. On the plus side, Schott has been able to continue production of the mirror segment blanks that make up ELT’s huge primary optical element, with the first of those segments delivered in July 2020. After being cast at Schott, the blanks are sent to Safran-Reosc in France for polishing. Adaptive optics progress Among the other major elements that make up the telescope’s optical design, assembly and integration of the 2.4 meter-diameter “M4” adaptive optical unit is underway at AdOptica in Spain. This highly complex mirror, fabricated from silicon carbide, will use 5300 actuators to help correct for turbulence in the Earth’s atmosphere. Other recent developments include completion of the ELT Technical Facility near the summit site earlier this year, while Tamai also noted that the first four lasers from Toptica that will be used to generate ELT’s guide stars have passed factory acceptance tests. Wrapping up the overview, Tamai said that the schedule for ELT’s completion remained unclear, especially with regard to the situation at the Armazones site, while a large number of delays to intermediate milestones had materialized at the contractor level. “The program schedule can only be released after Armazones is re-opened,” he concluded. “These are difficult times, but we very much believe we are maintaining the objective of constructing the world’s biggest eye on the sky - and there is still excellent progress and team cohesion, adapted to Covid.” • The SPIE Astronomical Telescopes and Instrumentation Digital Forum includes a number of other presentations detailing progress on ELT's optical subsystems and components. For further information, check out conference sessions 15-18 here.

“Council’s decision means ESO has the funds to build an ambitious and extremely powerful science machine, fully integrated with ESO’s Paranal Observatory, that meets the longer-term aspirations of the astronomy community,” says ESO Director General Xavier Barcons. Overall, 80% of the ELT’s budget is being invested in contracts with industry in ESO member states and in Chile. The funding boost will strengthen the scientific capabilities of the under-construction telescope, bringing them in line with those envisioned in the original ELT programme approved by Council in 2012. Two years later, ESO Council gave green light for ELT construction but stipulated that it should occur in two phases, with funding only committed for a fully working but less-powerful ‘Phase 1 ELT’. The revised budget includes the procurement of components originally deferred to the second phase of the project, such as the telescope’s second prefocal station, two more laser guide star systems, a set of astronomy-relevant atmospheric monitoring equipment and a small technical building at Armazones to optimise operations and maintenance activities. The new budget incorporates the impact on cost and schedule of known technical risks and includes the cost of activities needed to bring the ELT into operation as part of ESO’s Paranal Observatory. The funding boost follows an ELT total cost exercise that started in 2019. The exercise is an example of ESO’s continuous monitoring of the project and dedication to delivering a pioneering telescope that will tackle the biggest astronomical challenges of our time and make yet unimaginable discoveries. A truly international endeavour, this ambitious and exciting ESO project is made possible thanks to the organisation’s staff and governing bodies, the astronomy community, industry and scientific institutions in member states, as well as to the host state of Chile.

The primary mirror on the ELT, M1, is composed of 798 hexagonal segments, made of a thin glass-ceramic material. To ensure the optical quality of the mirror, its segments have to work together to maintain the mirror’s shape, despite changes in the telescope's position and in external conditions, such as temperature, pressure and wind. Movable supports and sensors for the mirror, namely the M1 position actuators and edge sensors, allow each mirror segment to move independently and with nanometric (1 millionth of a millimetre) precision and are crucial for the telescope to achieve its potential. M1 position actuators Lying underneath each of the 798 mirror segments, the M1 segment supports connect each mirror to the primary mirror support cell. The first set of M1 segment supports, which are being produced and tested to high precision by VDL ETG Projects BV in the Netherlands, was recently delivered to Safran Reosc in Poitiers, France, where the mirror segments are being polished and assembled. Once in Chile, each individual mirror segment, some 1.4 metres across and weighing about 250 kg with its support structure, will be installed on three position actuators. These actuators constantly adjust the position of the mirror segments through tiny, precise movements. The position actuators are built by the German company Physik Instrumente (PI). Specialising in precision motoring and positioning, the company will produce the required 2394 position actuators to adjust the segments of the ELT’s main mirror with pinpoint accuracy. The final design for these high-precision actuators is now nearing approval, with series manufacturing starting once the final design review is completed. M1 edge sensors Another important component of the main mirror system, the edge sensors, has already passed its final design review. The M1 edge sensors, which are being designed and manufactured by the FAMES consortium (comprising Fogale Nanotech in France and Micro-Epsilon in Germany), are the most precise edge sensors ever designed for a telescope. Two on each side of each segment, the sensors can detect when the segments move out of position, even if only by a millionth of a millimetre. A total of 4608 edge sensors will measure the relative positions of all M1 segments, allowing the overall shape of the mirror to be corrected when the segments move out of place. Given the high number of edge sensors and position actuators, a consequence of the size of the ELT, the final design phase required the companies to produce dozens of units to carefully test and validate not only the products themselves, but also their manufacturing and production processes. With the completion of the final design review for the edge sensors, FAMES will begin in-series production of these crucial components of the telescope. In mid 2021, the first batch of edge sensors and position actuators is anticipated to be delivered to the ELT Technical Facility in the Chilean Atacama Desert for future assembly into the M1 segments.

While the construction of ESO's Extremely Large Telescope (ELT) in Chile's Atacama Desert is on hold due to the COVID-19 pandemic, important progress on the project is being made elsewhere. In the past few months, design activities for the telescope’s dome have been completed (in a process called final design review) and manufacturing of the dome components is currently ongoing. The extensive design review solved several complex problems inherent to building a top-performing science machine, with strict scientific requirements, in the extreme environment of the Atacama Desert. Engineers also had to ensure the operational requirements were met: high reliability, low maintenance, and resistance to earthquake and strong winds, for example. The latest renderings show what the dome will look like, and how subtle but important changes were made from the older designs. The ELT dome and telescope structure contract was placed with the Italian consortium ACe (Cimolai, Astaldi). It covers not only the design, but also the manufacture, transport, construction, on-site assembly and verification of the dome and telescope structure. The company is currently working on finalising the design for the telescope structure, with the final design review anticipated to happen in the first quarter of 2021. ACe is also manufacturing key hardware components for the dome that will protect the ELT. As part of the design process, ACe manufactured and extensively tested several critical components to qualify the design as well as the manufacturing and assembly procedures. This included prototyping and manufacturing the seismic isolation systems and testing a full-scale dome ventilation louver. It also involved testing the performance of the dome cladding panels, including a trial installation on a 10m-high partial dome structure, manufacturing the full set of dome rotation trolleys (36 units in total, each weighting 27 tons) and manufacturing the structural components for the first bottom ring of the dome lattice structure. Some of those components have already been shipped to Chile. ESO's ELT, with a main mirror 39.3 metres in diameter, will be by far the largest optical/near-infrared telescope in the world once it sees first light later this decade. When completed, the ELT dome and telescope structure will dwarf other similar constructions around the world. The telescope structure, weighing some 3700 tonnes, will be equipped with the five telescope mirrors, which will collect and direct the light from astronomical targets to the various instruments. The instruments themselves will be placed in two platforms, about the size of a tennis court each, located on either side of the telescope structure. The giant ELT dome will house all of these structures, protecting them from the desert elements. The dome will be about 80 metres high and have a diameter of about 88 metres, giving it a footprint roughly equivalent to a football pitch. The upper part of the dome structure, out of steel and weighing about 6100 tonnes, will rotate on top of the concrete cylindrical base of the dome, to allow the telescope to point in any direction through its large observing slit. ESO’s ELT will apply its unique angular resolution to answer the biggest astronomical questions of our time, exploring the past and present of the universe, the locations and compositions of exoplanets, and the nature of dark matter and dark energy.

The two ELT prefocal stations, which are designed and manufactured by IDOM, are large structures measuring over 9 metres tall and each standing on supporting platforms, located on opposite sides of the telescope main structure. Working at the interface between the telescope’s five-mirror system and its instruments, they have a triple function: control the telescope mirror alignment (including the precise pointing on-sky of the telescope structure), ensure the segmented main mirror continues to act as one, and distribute light to the telescope instruments. Sensing starlight to check alignment The ELT is extremely sensitive to very small changes in environmental conditions, which will shift the mirrors out of alignment, degrading observations. Gravity can pull the mirrors out of shape as the telescope moves. In addition, small changes in temperature cause the telescope’s metal structure to expand slightly by hundreds of micrometres, again shifting the positions of the various ELT mirrors. The PFS-A and PFS-B analyse the light from ‘guide stars’ — natural, bright stars close to the object of study in the sky. The stations, which do not work simultaneously, will monitor up to three guide stars during observations, assisted by its three sensor arms positioned with accuracies of a few hundred micrometres. Information from the guide stars is used to actively position the mirrors and telescope main structure to control the telescope’s alignment during observations — a process known as active optics. This ensures that light remains properly focused and that the telescope remains pointed accurately at the target throughout observations. 798 segments acting as one mirror An additional function of the PFS is to ensure that the hundreds of segments of the main mirror (M1) continue to act as one giant mirror. To achieve this, the relative positions of the 798 mirror segments must be accurate to tens of nanometres. However, the positions of the segments drift over time due to changes in temperature and gravity loads so they need to be corrected periodically. The PFS will regularly control the shape of M1 using the guide star measurements to maintain its optimal shape. Distributing light Finally, the PFS is also used to distribute the light collected by the telescope to the various scientific instruments and auxiliary equipment, depending on which system is being used at any given moment. The platforms where the PFS A and B stand will host multiple scientific instruments, specialising in analysing the light collected by the telescope to answer our questions about the Universe. The PFS will direct the light to the select scientific instrument using a large flat mirror. It is also the last component before the light arrives at the telescope focus, giving the station its name. A successful collaboration Following two years of work by engineers at IDOM and at ESO on the complex PFS-A, the teams are now very close to successfully completing the design activities and will soon begin manufacturing this crucial telescope component. In addition, ESO, through its Director General Xavier Barcons, and IDOM, through their president Luis Rodríguez, have now signed a contract for the Spanish company to develop and supply the second prefocal station for the ELT. ESO’s ELT, made possible by high-tech engineering solutions and many talented people, will answer the biggest astronomical questions of our time, from investigating the Universe’s history to finding and studying Earth-like planets outside our Solar System. The telescope will sit atop Cerro Armazones in the Chilean Atacama Desert and will start operations later this decade.