ESO: first segments of world's largest telescope mirror shipped to Chile…

20 Dec 2023

…and James Webb telescope releases new view of “rings of Uranus”.

The construction of the European Southern Observatory’s Extremely Large Telescope (ESO's ELT) has announced the delivery to ESO and shipment to Chile of the first 18 segments of the telescope’s main mirror known as M1. Once they arrive in Chile, the segments will be transported to the ELT Technical Facility, at ESO’s Paranal Observatory in the country’s Atacama Desert, where they will be coated in preparation for future installation on the telescope main structure.

Unable to be physically made in one piece, M1 will consist of 798 individual segments arranged in a large hexagonal pattern, with an additional 133 being produced to facilitate the recoating of segments. With a diameter of more than 39 meters, it will be the largest telescope mirror in the world.

The final stage in the production process of M1 segments — polishing — was carried out by optical systems manufacturer Safran Reosc, near Poitiers, France, at a building completely refurbished to work on this delicate task.

As part of the process, Safran Reosc developed new automation workflows and measurement techniques to ensure that the polishing met the high standards required for ESO’s ELT. The surface irregularities of the mirror are less than 10 nm. To reach this level of performance, Safran Reosc used ion-beam figuring, in which a beam of ions sweeps the mirror surface and removes irregularities atom by atom.

While only 18 segments have been shipped thus far, many more will soon be delivered by Safran Reosc to ESO. On 1 November 2023, the 100th segment went out of the production line and entered into the extensive inspection phase that takes place before final delivery.

Furthermore, Safran Reosc has achieved a production rate in excess of four segments per week, with a target of five a week expected soon, a remarkable achievement for the series production of incredibly high-accuracy optics.

The construction of ESO’s ELT has required the close involvement of multiple companies in Europe and Chile with ESO’s teams, highlighting how the telescope is a true international endeavour. The mirror segments were cast by glass maker Schott at its facility in Mainz, Germany, before being delivered to Safran Reosc in France for polishing.

James Webb releases new view of rings of Uranus

NASA’s James Webb Space Telescope (“Webb”) recently trained its sights on Uranus, an ice giant that spins on its side. Webb captured this dynamic world with rings, moons, storms, and other atmospheric features – including a seasonal polar cap. The image expands upon a two-color version released earlier this year, adding additional wavelength coverage for a more detailed look.

Webb captured Uranus’ dim inner and outer rings, including the elusive Zeta ring – the extremely faint and diffuse ring closest to the planet. It also imaged many of the planet’s 27 known moons, even observing some small moons within the rings.

In visible wavelengths as seen by Voyager 2 in the 1980s, Uranus appeared as a placid, solid blue ball. In infrared wavelengths, Webb is revealing a strange and dynamic ice world filled with exciting atmospheric features.

One of the most striking of these is the planet’s seasonal north polar cloud cap. Compared to the Webb image from earlier this year, some details of the cap are easier to see in these newer images. These include the bright, white, inner cap and the dark lane in the bottom of the polar cap, toward the lower latitudes.

The polar cap appears to become more prominent when the planet’s pole begins to point toward the Sun, as it approaches solstice and receives more sunlight. Uranus reaches its next solstice in 2028, and astronomers are eager to watch any possible changes in the structure of these features.