ESO’s Extremely Large Telescope Casts Secondary Mirror of 4.2 Meters

The European Southern Observatory’s (ESO’s) 39-m Extremely Large Telescope (ELT) has reached a new milestone with the casting of the telescope's secondary mirror (M2), which is larger than the primary mirror of many of today's research telescopes.

Opening of the ELT M2 ZERODUR blank mold containing the still very hot ZERODUR glass at first annealing at the SCHOTT 4-meter blank annealing facility in Mainz, Germany. The completed mirror will be 4.2 meters in diameter and weigh 3.5 tons. It will be the largest secondary mirror ever employed on a telescope and also the largest convex mirror ever produced. Courtesy of SCHOTT/ESO.

The mirror blank is the cast block of Zerodur glass-ceramic, which will be ground and polished to produce the finished mirror. In January, ESO awarded Schott the contract to manufacture the M2 mirror blank. Schott also produced the 8.2-m meniscus main mirrors for the Very Large Telescope at ESO's Paranal Observatory. A manufacturer of astronomical products, Schott has already delivered the blanks of the deformable thin shell mirrors that will make up the ELT's quaternary mirror, M4, and will also provide the blank of the tertiary M3 mirror.

The blank of the secondary mirror now has to go through a slow cool down, machining and heat treatment sequence over the next year. It will then be ready to be ground to precisely the right shape and polished. The blank will be shaped and polished to a precision of 15 nm across the entire optical surface.

When completed and installed, the M2 mirror will hang upside down above the telescope's huge primary mirror and forms the second element of the ELT's novel five-mirror optical system. The mirror is strongly curved and aspheric and is a major challenge to make and test.

When it achieves its first light in 2024, the ELT will be the largest of its kind.

ESO is an intergovernmental astronomy organization in Europe and a ground-based astronomical observatory supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries.