Telescope Mirror Polishing: Herschel to ABB Robots

Telescope mirror polishing asks for a surface that matches its intended curve to within a few nanometres. For most of its history opticians got there by hand, pushing a tool and abrasive over the glass stroke after stroke. Computer-controlled machines took over in the 1970s and 1980s. Today the 931 hexagonal segments of Europe's Extremely Large Telescope come off an automated line with robots in France at a rate of one a day. This post covers that history and this month's ELT news, then looks at where ABB robots have already appeared in the work and what is still hard.

ELT primary mirror segments take shape in the Safran Reosc polishing plant, 2023. Photo: Safran / ESO via Wikimedia Commons (CC BY 4.0)

How astronomers figured mirrors by hand

William Herschel made his own mirrors in Bath in the late eighteenth century. They were cast from speculum metal, a hard alloy of copper and tin, then ground and polished by hand on a bench with tools like the replica now shown in his house. His 40-foot telescope of 1789 had a mirror about 1.2 metres across, and the original is kept at the Science Museum in London. Speculum tarnished quickly, so the mirrors had to be repolished again and again, which meant more weeks of handwork each time.

A replica mirror polisher and tools on the workbench at the William Herschel Museum in Bath. Photo: Mike Peel / Wikimedia Commons (CC BY-SA 4.0)

Glass made the job easier. In the 1850s Léon Foucault in Paris coated glass mirrors with a thin layer of silver and invented a simple knife-edge test that showed the opticians where the surface was still wrong. Mirrors kept growing. Corning cast the Pyrex disc for the 200-inch Hale Telescope in 1934, and opticians in Pasadena ground and polished it on a jig that could move in five different ways. The Second World War interrupted the work, and the telescope was only dedicated in 1948.

The original speculum mirror of Herschel's 40-foot telescope at the Science Museum, London. Photo: Mike Peel / Wikimedia Commons (CC BY-SA 4.0)

Computers entered the shop in the following decades. For the Hubble Space Telescope, Perkin-Elmer used custom computer-controlled polishing machines and kept polishing until May 1981, while Kodak built a backup mirror with traditional methods in case the new machines failed. A test instrument had been set up wrongly, so the mirror came out the wrong shape, and astronomers found the flaw only after launch in 1990. Segmented mirrors changed the scale of the problem. Each of the two Keck telescopes in Hawaii uses 36 hexagonal segments, and the first Keck saw first light on 24 November 1990 with only nine of them.

Polishing the Hubble Space Telescope primary mirror at Perkin-Elmer, 1979. Photo: NASA via Wikimedia Commons (public domain)

Robots polishing ELT segments today

The Extremely Large Telescope needs 798 segments for a 39-metre mirror, plus 133 spares, so 931 in all. SCHOTT in Germany casts the blanks in Zerodur. Safran Reosc then polishes them in a 5,000 square metre plant near Poitiers that opened in 2020 with several robots and linked, automated polishing cells. On 6 October 2026 Safran said the last of the 931 segments will be finished by the end of the year, on a line that combines ion beam polishing, digital measurement and RFID tracking of each part.

ELT segment blanks queuing in the Safran Reosc polishing facility, 2023. Photo: ESO via Wikimedia Commons (CC BY 4.0)

ESO followed on 14 October 2026. Nearly 500 segments have now reached the ELT Technical Facility at Paranal in Chile, after a 10,000 kilometre trip by road and sea, and the final segment has entered production. Safran Reosc now finishes one segment a day. Production ends this year, but handling goes on: once the telescope opens, ESO plans to take two segments a day out of the mirror for fresh coating, which is why the spares exist. ESO says the project is more than 75 percent complete, with first science observations planned for late 2030.

A finished ELT mirror segment wrapped in its crate for delivery to Chile. Photo: ESO/A. Centeio via Wikimedia Commons (CC BY 4.0)

An ABB robot has already worked on ELT style segments. In 2015 researchers at the National Facility for Ultra Precision Surfaces in St Asaph, Wales, which had made prototype 1.4 metre ELT segments accepted by ESO, described a robot lab with two Fanuc arms and an ABB arm of 2.55 metre reach and 40 kg payload, the size of an IRB 4600. They connected the robots to Zeeko's polishing software and used the ABB and the large Fanuc for loose abrasive smoothing between grinding and polishing. A smaller robot loaded and unloaded parts on a Zeeko polisher.

An ABB IRB 4600 working in a packaging cell, the robot class of the ABB arm used in the St Asaph optics lab. Photo: ABB

What comes next for telescope mirror polishing

The next giant mirrors are on the way. The Giant Magellan Telescope's 8.4 metre mirrors are cast and polished at the Richard F. Caris Mirror Lab in Tucson, and the project was preparing contractor selection in September 2026. Future observatories will need thousands more segments, plus spares that are recoated for decades. The St Asaph team pointed out that the ELT needed nearly thirty times as many segments as each Keck telescope on a similar schedule, so making one prototype after another by hand no longer works.

A Giant Magellan Telescope mirror in production at the Richard F. Caris Mirror Lab, University of Arizona, 2026. Photo: Jengod / Wikimedia Commons (CC BY-SA 4.0)

Much of the remaining manual work happens around the polisher. The 2015 paper listed manual steps such as changing tools, mounting and aligning the part, washing it down and starting the next run. A heavy arm like an IRB 6700 or IRB 4600 can carry segments and grind with force control, a GoFa CRB 15000 can change tools beside a technician, and such a cell can be planned in RobotStudio first. For homes, the change shows up in cheaper optics, such as the small mirrors inside a smart telescope.

An ABB IRB 6700 grinding a steel part in Teqram's EasyGrinder cell at Ancofer. Photo: ABB

Cost comes first. A polishing line built for one telescope is a large investment, and Safran has said it will use its ELT experience to scale up production of space and defence optics. Accuracy is another, since a general purpose arm is far less stiff than a dedicated polishing machine, so robots still hand the final nanometres to specialised machines and ion beams. Then there are skills: opticians now need to understand metrology software and robot programs as well as glass.

An ABB GoFa cobot loading parts into a machine at a small factory. Photo: ABB

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