Construction of the European Southern Observatory’s Extremely Large Telescope has reached a significant milestone with the successful installation of one of the dome’s two massive doors.
The European Southern Observatory (ESO) Council officially selected the Cerro Armazones site in Chile for the construction of the Extremely Large Telescope (ELT) in April 2010. The green light to begin construction of the ELT was given in December 2014, with the ceremonial “first stone” being placed on May 26, 2017. In June 2023, ESO announced that the ELT project had reached an important milestone with more than 50% of the project completed.
On 16 April, the ESO announced another significant milestone in the construction of the ELT, with one of a pair of dome doors being fully installed. This achievement marked the dome reaching its highest point, a milestone celebrated with a traditional ‘Topping Out’ ceremony. According to the ESO, the milestone also marked the project being more than 60% complete.
In 2018, the European Southern Observatory (ESO) announced that the ELT’s first test observations, known as “first light,” were expected by the end of 2025. In June 2021, this timeline was adjusted to September 2027, with ESO citing disruptions caused by the COVID-19 pandemic as the primary reason for the delay. The most recent update, published in March 2025, further postponed the “first light” to March 2029 due to construction delays. If all proceeds according to this revised schedule, ESO anticipates commencing scientific observations in December 2030.
The ELT will feature a 39-metre segmented primary mirror made up of 798 individual hexagonal segments and a five-mirror optical design that features adaptive optics. Once operational, the ELT is expected to collect more light than any other optical telescope and deliver images significantly sharper than those from some space-based observatories. Its primary scientific goals include the study of exoplanets, galaxy formation, and the nature of dark matter and dark energy.
