Mirror, Mirror on the Truck …

Loading the still glowing hot cast ELT M2 Zerodur blank onto a truck trailer at Schott melting facility in Mainz, Germany, in May 2017. It was then transported at walking-pace through Mainz with a police and fire escort, and arrived safely at the Schott annealing facility, ready to slowly cool and be shipped by boat to France for shaping and polishing. (Source: Schott / ESO)

The European Southern Observatory ESO’s 39-meter Extremely Large Telescope, E-ELT, will be the largest telescope of its kind ever built when it achieves first light in 2024/25. A new milestone has now been reached with the casting of the telescope’s secondary mirror, M2, which is larger than the primary mirror of many of today’s research telescopes.

At 3.5 tonnes and 4.2 meters, ESO’s M2 is the biggest secondary mirror in existence. Here, the still glowing hot mirror blank is transferred carefully through the streets of Mainz, Germany, to the Schott 4-m blank annealing facility. (Source: Schott / ESO)

The mirror blank is the cast block of material – in this case Zerodur glass-ceramic – that will then be ground and polished to produce the finished mirror. In January 2017, ESO awarded the contract to manufacture the M2 mirror blank to Schott, who also produced the 8.2-meter meniscus main mirrors for the Very Large Telescope at ESO’s Paranal Observatory. A long-time 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 French company Safran-Reosc will carry this out, along with additional testing. The blank will be shaped and polished to a precision of fifteen nanometers across the entire optical surface.

Opening of the ELT secondary mirror blank mould, containing the still very hot Zerodur glass at first annealing. (Source: Schott / ESO)

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. (Source: ESO)

Links: Astronomy, SCHOTT AG, Mainz, GermanyAstronomy, Safran Reosc, Saint Pierre du Perray – FranceExtremely Large Telescope (E-ELT), European Southern Observatory ESO

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