In the ELT, M2 will reflect the light from the 39-m main mirror extremely precisely onto the downstream mirror

SCHOTT recently announced that it has manufactured a curved secondary mirror substrate made of ZERODUR® glass-ceramic in Mainz, Germany, for the European Southern Observatory’s (ESO) Extremely Large Telescope (ELT). The substrate is 10 cm thin, with an extreme curvature, a diameter of 4.25 m and a weight of 3 tons. The substrate was manufactured for the ELT’s secondary mirror (Mirror 2, or M2).

In France it will receive its “final touch” there, comprising fine polishing at REOSC, a SAFRAN Group company. The actual mirror layer made of silver with a wafer-thin layer of silicon oxide as a protective film will be applied at the coating facility of the Paranal Observatory in Chile before it is mounted in the telescope.

The requirements for the casting of the 4.25 m mirror substrate were extremely high due to very narrow specifications for the glass quality. Last year, a highly convex mirror substrate 10 cm thin was ground from the blank weighing more than 10 tons on state-of-the-art CNC machines.

“It was a great challenge for our melting team and the experts running the CNC machines,” said Thomas Westerhoff, Ph.D., head of the strategic business field for ZERODUR at SCHOTT. “A workpiece with this geometry had never been manufactured before: The M2 will be the largest convex mirror ever made.”

In the ELT, M2 will reflect the light from the 39-m main mirror extremely precisely onto the downstream mirror. After the “First Light,” the commissioning of the ELT in 2025 on the mountain Cerro Armazones in Chile, its performance will provide the sharpest view into space compared to previous telescopes.

SCHOTT is manufacturing the substrate material for four of the five mirror components of the ELT optics: for the ESO’s ELT 39-m primary mirror with 798 hexagons (plus 131 replacement segments), the 4.25-m secondary mirror that is now being delivered, the 4-m tertiary mirror, and the segmented fourth mirror (M4)

The first set of 18 blanks for the primary mirror of ESO’s Extremely Large Telescope have arrived safely at Safran Reosc in Poitiers, France. The contracts for casting the blanks of the mirror segments, as well as polishing, mounting and testing them, were signed in 2017 with respectively the German company SCHOTT and the French company Safran Reosc, a subsidiary of Safran Electronics & Defense. 

Mirror blanks are packed by six into a special wooden transport crate and six of these crates can fit inside a regular shipping container. Sophisticated shock sensors travel with the blanks to measure any sudden acceleration or shock that could affect the blanks. 

The shipping container with the first 18 blanks was sent from SCHOTT in Mainz, Germany, and arrived at Safran Reosc the following day. The ELT mirror blanks are made of the low-expansion glass-ceramic Zerodur^© and are circular, measuring 1.5 metres across and about 5 centimetres thick. The back of each blank is flat and the front surface is concave. The blanks come in three types with slightly differently shaped front surfaces, depending on the planned location of the segment in the primary mirror. The first primary mirror segments were cast by SCHOTT in 2018. 

SCHOTT manufactures the blanks to approximate shape so that Safran Reosc does not need to remove too much material during the polishing process. Safran Reosc will polish the blanks before they are cut into hexagons and receive a final precise polishing using Ion Beam Figuring.

When it is completed, the primary mirror will comprise 798 hexagonal segments and will have a total light-collecting area of 978 m2. In total, more than 900 segments will need to be cast and polished (including a spare set of 133 segments). 

Once built the ELT will be the largest ground-based telescope in operation. The telescope is being built at Cerro Armazones in the Atacama Desert of northern Chile. With a primary mirror almost 40 metres in diameter, the ELT will gather almost 100 million times more light than the human eye, which will allow scientists to tackle some of the biggest challenges in current astrophysics, such as the formation of the first stars, the assembly of the first galaxies, the characterisation of the atmospheres of Earth-like exoplanets, or the nature of dark matter and dark energy, among many other questions.

The Extremely Large Telescope

The ELT has a main mirror 39 metres in diameter, covering a field of view on the sky about a tenth of the size of the full Moon. The telescope’s “eye” will be almost half the length of a soccer pitch in diameter and will gather 15 times more light than the largest optical telescopes operating today. The mirror design itself is revolutionary, and is based on a novel five-mirror scheme which results in exceptional image quality. The primary mirror consists of 798 segments, each 1.4 metres wide and only 50 mm thick. The optical design calls for a secondary mirror four metres in diameter — the largest secondary mirror ever employed on a telescope, and the largest convex mirror ever produced. 

Adaptive mirrors are incorporated into the optics of the telescope to compensate for the twinkling of astronomical sources caused by atmospheric turbulence. One of these mirrors, M4, is supported by more than 5000 actuators that can correct its shape a thousand times per second.

The telescope will have several science instruments, and it will be possible to switch quickly from one instrument to another. The telescope and dome — which together will weigh more than 8000 tonnes  — will also be able to change positions on the sky and start a new observation within minutes.

The ability to observe over a wide range of wavelengths from the optical to the mid-infrared will allow scientists to exploit the telescope’s size to the fullest extent, allowing astronomers to tackle the biggest scientific challenges of our time. (ELT) is currently under construction in the remote Chilean Atacama Desert.

AN EXTRAORDINARY TELESCOPE CALLS FOR AN EXCEPTIONAL SITE.

Great care was therefore taken to ensure that the home of the ELT was the best possible. This required a high, dry site — perfect for observations.

On 2–3 March 2010, the ELT Site Selection Advisory Committee presented a report to the ESO Council concluding that Cerro Armazones, near Paranal, stood out as the clearly preferred site, because it has the best balance of sky quality across all aspects and it can be integrated with the existing ESO Paranal Observatory. Several possible sites in Argentina, Chile, Morocco and Spain were tested. 

Cerro Armazones is a mountain at an altitude of 3046 metres in the central part of Chile’s Atacama Desert, some 130 kilometres south of the town of Antofagasta and about 20 kilometres from Cerro Paranal, home of ESO’s Very Large Telescope.

On 13 October 2011, an agreement was signed between ESO and the Chilean government, including the donation of 189 km² of land around Cerro Armazones for the installation of the ELT, as well as a concession for 50 years relating to the surrounding area.

Source: ESO

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