The first eighteen segments of the European Southern Observatory’s Extremely Large Telescope’s (ESO’s) primary mirror (M1) have been delivered to ESO and subsequently shipped to Chile, marking a significant milestone in the ELT’s construction. After arriving in Chile, the segments will be transferred to the ELT Technical Facility at the Atacama Desert’s ESO’s Paranal Observatory. There, they will undergo a coating process to prepare them for their subsequent integration into the primary structure of the telescope. M1, which cannot be manufactured in a single piece, will comprise 798 distinct segments organised in a substantial hexagonal configuration; an extra 133 segments are being produced to aid in the recoating process. It will surpass 39 metres in diameter and become the most sizable telescope mirror globally.
Polishing, the last step in the manufacturing process of M1 segments, was conducted by Safran Reosc, a globally renowned optical systems manufacturer, in a building that had been entirely renovated for this intricate undertaking, close to Poitiers in central France. Safran Reosc developed novel automation workflows and measurement techniques to guarantee that the polishing adhered to the stringent requirements of ESO’s ELT. The mirror exhibits surface irregularities measuring no more than 10 nanometers, equivalent to a thousandth of the width of a human hair. Safran Reosc achieved this performance level through ion-beam figuring, a process in which irregularities are eliminated atom by atom from the mirror surface using a beam of ions.
Although only 18 segments have been dispatched thus far, Safran Reosc is scheduled to deliver many more to ESO soon. The 100th segment was removed from the assembly line on November 1, 2023, and the rigorous inspection process preceding final delivery immediately began. In addition, Safran Reosc has surpassed four segments per week in production and is on track to reaching its goal of five segments per week soon. This is a tremendous accomplishment considering the series production nature of extremely precise optics.
ESO’s teams and multiple European and Chilean corporations were intimately involved in constructing the ELT, demonstrating that the telescope is a truly international undertaking. The mirror segments were manufactured at the facility of the German company SCHOTT in Mainz, Germany, before their shipment to Safran Reosc in France for refining. The delicate segment supports were manufactured by the Dutch company VDL ETG Projects BV, the 4500 nanometric-accuracy sensors monitoring the relative position of each segment were designed and manufactured by the German-French FAMES consortium, and the 2500 actuators capable of positioning the segment with nanometric precision were designed and manufactured by the German company Physik Instrumente. Danish firm DSV was entrusted with the delicate responsibility of transporting the segments.
The eighteen polished mirror segments, which departed France a week ago, are currently en route to the ELT construction site in the Atacama Desert, a distance of over 10,000 kilometres. Once it begins operations later this decade, ESO’s ELT will proceed to confront the most formidable astronomical challenges of our time and make discoveries that are still inconceivable.
