ESO signs contract for the ELT’s M5 commissioning mirror

Today, ESO’s Extremely Large Telescope (ELT) has taken another step towards timely completion. At ESO’s Headquarters in Garching, Germany, and with remote participation from the UK, the organisation signed a contract with Glyndwr Innovations Limited (a wholly-owned subsidiary of Wrexham University) in Wales, UK, for the production of the ELT’s M5 commissioning mirror. In case of delays in producing M5, this commissioning mirror will be used as the fifth mirror of the ELT’s innovative five-mirror design when the telescope sees first light.

The M5 mirror is a crucial part of the ELT’s adaptive-optics system design: it will adjust its position up to 10 times per second without bending to allow the telescope to obtain the sharpest possible images. Like the rest of the ELT, M5 is pushing technology to its limits: at 2.7 by 2.2 metres, it is the largest tip-tilt mirror ever produced. It is made from silicon carbide, a very stiff yet very lightweight material. The M5 blank — the shaped piece of material that is later polished to become the mirror — was finalised recently by brazing together six silicon-carbide segments and then topping them with a very thin layer of the same material, deposited atom by atom. This deposition layer was necessary to allow the mirror to be polished to an accuracy of less than a hundredth of the width of a human hair, the next stage in the manufacturing process.

Since such a large silicon-carbide mirror, with such strict polishing requirements, has never been produced before, there are risks associated with manufacturing it. In particular, the ELT team at ESO have inferred that the current production risk is high enough to potentially delay the schedule of the entire ELT project.

To ensure that the ELT is ready to observe the skies before the end of the decade, ESO has partnered with Glyndwr Innovations to create an alternative M5 mirror, called M5 commissioning mirror, to be used to commission the telescope and demonstrate its performance. The M5 commissioning mirror will be made of a single piece of the more conventional ZERODUR©, a glass-ceramic material that has been used in astronomical telescopes for decades. Because the properties of ZERODUR© are different to those of silicon carbide, the commissioning mirror will be smaller than M5, measuring 2.2 by 1.8 metres, and won’t be able to demonstrate all the adaptive-optics capabilities of the telescope. But it is an excellent replacement for the first few months of ELT operation, when several observations will be done to test and calibrate the various ELT systems. 

The team at Glyndwr Innovations now has the exciting task of grinding and polishing the M5 commissioning mirror blank — which is being produced by German company SCHOTT — as well as mounting interfaces onto the mirror and safely delivering the completed product, on time, to the ELT’s construction site in Cerro Armazones, in the Chilean Atacama Desert. Glyndwr Innovations will also design, manufacture, and supply the auxiliary equipment needed to handle, store, and transport the mirror, and develop a test bench to precisely measure the shape of the mirror during manufacturing.

When the ELT begins observations later this decade, it will be the world’s biggest eye on the sky, and is expected to advance our understanding of dark energy, exoplanets, and the earliest epochs of the Universe.

Courtesy of European Southern Observatory