The Centre for Sensors, Instruments and Systems Development (CD6) of the Universitat Politècnica de Catalunya · BarcelonaTech (UPC), based on the Terrassa Campus, is working on the design of the optical system that will be used to calibrate the 39-metre main mirror—the world’s largest one—of the Extremely Large Telescope (ELT). This revolutionary visible and infrared telescope will be the largest eye ever built to observe the sky and will pave the way for a new generation of ground-based optical telescopes.

With its unique five-mirror design, along with state-of-the-art technology to correct atmospheric distortions, the ELT will provide images 15 times sharper than those from the Hubble Space Telescope. It will allow deep exploration of the universe in great detail, which will lead to further advances in astrophysical knowledge.

Extremely large telescopes are considered top priorities for terrestrial astronomy worldwide. The construction project of the ELT was approved by the European Southern Observatory (ESO) in 2012 and is set to revolutionise modern astronomy. One of the goals of the telescope 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. It will also probe the farthest reaches of the cosmos, revealing the properties of the earliest galaxies and the nature of the dark universe.  

The telescope's five mirrors will have different shapes, sizes and roles, designed to work in perfect coordination, a pioneering optical design that will allow it to reveal the Universe in unprecedented detail. The primary mirror M1 will contain thousands of highly sophisticated components that will allow it to collect light from the night sky and reflect it onto the secondary mirror. Convex M2, the largest secondary mirror ever used in a telescope, about 4 meters in diameter, will hang above M1 and reflect light to M3, which in turn transmits it to a flat adaptive mirror, the M4, above. This fourth mirror will change the shape of its surface a thousand times per second to correct distortions caused by atmospheric turbulence, before sending the light to the M5, a tiltable flat mirror that will stabilize the image and send it to the ELT instruments.

Able to collect much more light than the human eye
The most spectacular mirror from a technological point of view is the primary mirror M1, which the CD6 researchers at UPC are involved. It is a concave mirror, 39.3 meters in diameter and a radius of curvature of 68.7 meters. Being too large to be made from a single piece of glass, the main mirror is made up of individual hexagonal segments, each about five centimeters thick, about 1.5 meters in diameter and 250 kg in weight, spaced apart for a distance of 4 mm. As a whole, the structure consists of six sectors made up of 133 segments of different shapes and functions. In total, there are 798 hexagonal segments that will act as a single mirror.

These hexagonal segments have to be perfectly positioned to reproduce the shape of the primary mirror. The alignment precision of these mirrors across the entire surface is 2 nanometers (10,000 times thinner than a human hair) and the goal is for these components to work together to form a perfect imaging system. Every day, two of these segments will be removed to be cleaned and to renew the coating, with the aim of guaranteeing the maximum efficiency of the telescope.

To ensure that the segments are positioned correctly, the telescope has almost 2,500 actuators that allow each individual segment to be positioned with nanometer precision. The task of these actuators is supervised by two systems: an optical interferometer, called Local Coherencer, and a network of approximately 9,000 sensors installed in the segments. The design, construction and validation of the Local Coherencer, a critical instrument for the proper functioning of the ELT, has been awarded to the company IDOM, which is leading the development in collaboration with CD6. The solution proposed by IDOM and CD6, and approved by ESO, is based on an original optical concept that is a variation of the interferometer that CD6 already created for the Gran Telescopio de Canarias 20 years ago.

As researcher Santiago Royo, director of the CD6 and project coordinator on behalf UPC, explains, "the challenge is to ensure that the mirror remains in position and in shape with a precision of tens of nanometers throughout its 39 meters extension as segments are replaced. Unlike other telescopes with segmented primary mirrors, the Local Coherencer will allow this task to be done during the day, thus maximizing observation time at night. One of the challenges with this system is ambient light, which adds backlight issues to the instrument”.

The structure of the telescope −which keeps it stable in all conditions, even in strong winds and in the event of earthquakes− is made up of a horizontal part, or azimuth structure, which supports the telescope tube, and a vertical part, or altitude structure, 50 meters high, which contains two huge platforms that house the five mirrors and other scientific instruments. The primary mirror is supported at the bottom of this structure, and the secondary mirror hangs high above the top of the telescope tube. The other three mirrors are located in the central tower of the 10-meter telescope tube, located in the center of the main mirror support structure.

Giant dome
The giant dome will house the telescope and its interior structure, providing protection from the extreme environment of the Atacama desert. The dome will be 80 metres high and have a diameter of about 88 metres, giving it a footprint roughly equivalent to that 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 it is fully equipped with optics and scientific instruments, the telescope is estimated to weigh about 3,700 tons.

Collaboration between cutting-edge research centres
From the construction of the immense structure of the telescope dome to the casting of the mirrors, the development of the ELT is the result of the work and collaboration of several leading European companies and research centers, such as IDOM and CD6 of UPC.

As work on manufacturing and designing elements of the ELT in Europe progresses steadily, the ELT is expected to deliver the first scientific observations in September 2027, about half a year after an initial “telescope technical first light”.