The Centre for Sensors, Instruments and Systems Development (CD6) of the Universitat Politècnica de Catalunya - BarcelonaTech (UPC), located on the Terrassa Campus, has completed its contribution to the Extremely Large Telescope (ELT) construction project: the design and manufacture of the optical system that will calibrate the telescope’s primary mirror.

Of the five mirrors that will make up the telescope, the main mirror, known as M1, is the most technologically spectacular. It is a concave mirror with a diameter of 39.3 metres (the largest currently in existence) and a radius of curvature of 68.7 metres. As it is too large to be cast from a single piece of glass, it consists of individual hexagonal segments. Each segment is around 5 cm thick, measures 1.5 m across, weighs 250 kg and is separated from its neighbours by a mere 4 mm gap. Together, the structure comprises six sectors made up of 133 segments of varying shapes and functions. In total, 798 hexagonal segments will operate as a single mirror, capable of gathering tens of millions of times as much light as the human eye.

The hexagonal segments must be perfectly aligned to maintain the shape of the primary mirror to an accuracy of two nanometres—10,000 times thinner than a human hair—across its entire surface, allowing them to work together to form a perfect imaging system. To maintain the maximum efficiency of the telescope, two segments will be removed every day for cleaning and recoating.

The telescope features almost 2,500 actuators to ensure that each segment is positioned correctly with nanometric accuracy. The actuators are monitored by two systems: an optical interferometer, called Local Coherencer, and a network of some 9,000 sensors installed in the segments.

A critical instrument for the proper operation of the ELT, the Local Coherencer was awarded to IDOM, which led its development in collaboration with the CD6. Created by IDOM and the CD6, and approved by ESO in an international competitive call for proposals, the solution is based on an original optical concept that is a variant of the interferometer that the CD6 created for the Gran Telescopio Canarias 20 years ago. This non-contact metrology system offers a lightweight, compact and robust solution, enabling simultaneous measurements of position difference (piston) and two-axis tilt (tip and tilt) between a segment and its six immediate neighbours. It boasts an accuracy of under 300 nanometres within a range of ±250 microns and operates while mounted on the telescope’s segment manipulator at tilts of up to 15°. The system was delivered to ESO in Munich this May for validation, prior to its final installation at the ELT in Chile.

As CD6 director and UPC project coordinator Santiago Royo explains, “The system we built is based on a completely innovative concept, which allowed us to win an international bidding process alongside IDOM and secure an international patent. The instrument measures not only the height difference between two segments but also their relative tilts, all within a single device.”

For technical manager Noel Rodrigo, “participating in the Extremely Large Telescope with critical instrumentation is highly exciting for us as optical engineers. It is particularly rewarding to propose a completely original system and see it through every stage of implementation, including the detailed system design, component selection, instrument assembly, processing software development and confirming that all models and hypotheses meet the desired specifications.”

According to Royo, “we didn’t just propose the system; we built it and commissioned it exactly as planned, in a highly positive collaboration between the UPC and a top-tier industrial partner with whom we are already exploring future project collaborations.”

The telescope structure keeps it stable under all conditions, including in high winds and during earthquakes. It consists of a horizontal azimuth structure supporting the telescope tube and a 50-metre-high vertical altitude structure housing two massive platforms for the five mirrors and other scientific instruments. The primary mirror rests at the bottom of this structure, while the secondary mirror hangs well above the top of the telescope tube. The remaining three mirrors are in the 10-metre central tower of the telescope tube, located at the centre of the main mirror’s support structure.

With different shapes, sizes and roles, the telescope’s five mirrors are designed to work together seamlessly. This pioneering optical design will allow it to reveal the Universe in unprecedented detail. The M1 primary mirror will contain thousands of highly sophisticated components to collect light from the night sky and reflect it onto the secondary mirror. With a diameter of some 4 metres, the convex M2 mirror is the largest secondary mirror ever employed on a telescope; it will hang above M1 and reflect light towards M3, which will then transmit it to an adaptive flat mirror (M4) above it. This fourth mirror will adjust its surface shape a thousand times per second to correct distortions caused by atmospheric turbulence before sending the light to M5, a tiltable flat mirror that will stabilise the image and direct it to the ELT instruments.

The largest eye ever built
The ELT is a groundbreaking visible and infrared light telescope that will become the world’s largest eye on the sky, paving the way for a new generation of ground-based optical telescopes. With its unique five-mirror design and cutting-edge technology to correct atmospheric distortion, the ELT will provide images 15 times sharper than those of the Hubble Space Telescope. It will enable deep exploration of the universe in great detail, leading to major breakthroughs in astrophysical knowledge.

Extremely large telescopes are considered worldwide a top priority in ground-based astronomy. The ELT construction project was approved by the European Southern Observatory (ESO) in 2012 and is set to revolutionise modern astronomy. One of the telescope’s goals is to detect and study Earth-like planets around other stars, and it could become the first telescope to find evidence of life outside our solar system. The ELT will also probe the furthest reaches of the cosmos, revealing the properties of the very earliest galaxies and the nature of the dark universe.

The telescope and its internal structure are protected from the extreme environment of the Atacama Desert by a giant dome 80 metres high and 88 metres in diameter—roughly the size of a football pitch. The upper part of the dome will rotate to allow the telescope to point in any direction through its large observing slit.

Once fully equipped with optics and scientific instruments, the telescope is estimated to weigh around 3,700 tonnes.

Collaboration between leading research centres
From the construction of the massive dome structure to the casting of the mirrors, the development of the ELT is the result of collaboration between several leading European companies and research centres, including IDOM and the UPC’s CD6.

As work on manufacturing and designing ELT components progresses steadily in Europe, the ELT is scheduled to conduct its first scientific observations in September 2027, roughly half a year after the telescope’s first technical light.