Secondary mirror of ELT successfully cast—largest convex mirror blank ever created

May 22, 2017
Opening of the ELT M2 ZERODUR® blank mould containing the still very hot ZERODUR® glass at first annealing at the SCHOTT 4-metre blank annealing facility in Mainz, Germany in May 2017. The completed mirror will be 4.2 metres in diameter and weigh 3.5 tonnes. It will be the largest secondary mirror ever employed on a telescope and also the largest convex mirror ever produced. Credit: SCHOTT/ESO

The casting of the secondary mirror blank for ESO's Extremely Large Telescope (ELT) has been completed by SCHOTT at Mainz, Germany. The completed mirror will be 4.2 metres in diameter and weigh 3.5 tonnes. It will be the largest secondary mirror ever employed on a telescope and also the largest convex mirror ever produced.

ESO's 39-metre Extremely Large Telescope (ELT - http://www.eso.org/public/teles-instr/elt/) will be the largest telescope of its kind ever built when it achieves first light in 2024. A new milestone has now been reached with the casting of the telescope's secondary mirror (M2), which is larger than the of many of today's research telescopes.

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.

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 15 nanometres (15 millionths of a millimetre) across the entire optical surface.

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

Explore further: Image: Looking into the Cheops telescope tube

Related Stories

Image: Looking into the Cheops telescope tube

May 8, 2017

Seen here is a Cheops team member reflected in the satellite's main mirror, and framed by the black internal surface of the telescope tube. The back of the secondary mirror is seen at the centre of the image, held in place ...

Contracts signed for ELT mirrors and sensors

January 18, 2017

At a ceremony today at ESO's Headquarters four contracts were signed for major components of the Extremely Large Telescope (ELT) that ESO is building. These were for: the casting of the telescope's giant secondary and tertiary ...

Being 'secondary' is important for a Webb Telescope mirror

July 20, 2011

(PhysOrg.com) -- "Secondary" may not sound as important as "primary" but when it comes to the next-generation James Webb Space Telescope a secondary mirror plays a critical role in ensuring the telescope gathers information ...

Cryogenic testing completed for James Webb telescope mirrors

December 21, 2011

Cryogenic testing is complete for the final six primary mirror segments and a secondary mirror that will fly on NASA's James Webb Space Telescope. The milestone represents the successful culmination of a process that took ...

Recommended for you

NASA telescope studies quirky comet 45P

November 22, 2017

When comet 45P zipped past Earth early in 2017, researchers observing from NASA's Infrared Telescope Facility, or IRTF, in Hawai'i gave the long-time trekker a thorough astronomical checkup. The results help fill in crucial ...

Uncovering the origins of galaxies' halos

November 21, 2017

Using the Subaru Telescope atop Maunakea, researchers have identified 11 dwarf galaxies and two star-containing halos in the outer region of a large spiral galaxy 25 million light-years away from Earth. The findings, published ...

Cassini image mosaic: A farewell to Saturn

November 21, 2017

In a fitting farewell to the planet that had been its home for over 13 years, the Cassini spacecraft took one last, lingering look at Saturn and its splendid rings during the final leg of its journey and snapped a series ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.