Tertiary Mirror Allows World’s Biggest Eye on the Sky to Image With Unprecedented Quality

ESO’s Extremely Large Telescope (ELT), the world’s biggest eye on the sky, will have a pioneering five-mirror optical system that will allow it to unveil the Universe in unprecedented detail. The tertiary mirror in this system, M3, has completed a key production stage and has now been delivered to French company Safran Reosc for final polishing.

Rendering of ELT’s tertiary mirror, the so-called M3. Credit: ESO (L. Calçada)/SENER

Each of the mirrors on the ELT presents a significant technological challenge, with extreme precision required at each production stage to ensure flawless optical quality. The German company SCHOTT produced the mirror blank for M3 — a cast block of a glass-ceramic material known as Zerodur^Ⓡ measuring more than four meters from edge to edge and weighing in at over three tonnes. After casting and machining the M3 blank to its approximate shape, SCHOTT delivered the mirror to Safran Reosc, who will now grind and polish it to a precision of 15 nanometers across the entire optical surface.

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This animation beautifully visualizes the passage that light takes through the ELT telescope’s novel five-mirror design. Light first reflects from the innovatively designed main mirror (M1), a 39-meter array of 798 hexagonal pieces, and then onto the secondary mirror (M2) which at 4 meters dwarfs the primary mirrors of all of ESO’s instruments at the La Silla site. Two of the mirrors (M4 and M5) form part of the adaptive optics, and this unique collection of optics will produce images of incredible quality. Credit: ESO/L. Calçada/ACe Consortium

M3 is a notable feature of the ELT. Most large telescopes, including ESO’s Very Large Telescope (VLT) and the NASA/ESA Hubble Space Telescope, use just two curved mirrors to form an image, with a small, flat, tertiary mirror sometimes introduced to divert the light to a convenient focus. However, in the ELT the tertiary mirror also has a curved surface, as the use of three curved mirrors delivers a better image quality over a larger field of view than would be possible with a two-mirror design. This design will allow the ELT to image the night sky with unprecedented quality.

This diagram shows the novel 5-mirror optical system of ESO’s Extremely Large Telescope (ELT). Before reaching the science instruments the light is first reflected from the telescope’s giant concave 39-meter segmented primary mirror (M1), it then bounces off two further 4-meter-class mirrors, one convex (M2) and one concave (M3). The final two mirrors (M4 and M5) form a built-in adaptive optics system to allow extremely sharp images to be formed at the final focal plane.
Contracts for the casting of the M2 and M3 mirrors, their cells and sensors for the M1 segments were awarded at a ceremony at ESO’s Garching Headquarters in January 2017. Credit: ESO

The five mirrors on the ELT all have different shapes, sizes and roles. The primary, M1, is the most spectacular, a giant 39-meter concave mirror made up of 798 hexagonal segments, which will collect light from the night sky and reflect it to the secondary mirror, M2. Measuring 4.2 meters across and hanging above M1, M2 will be the largest secondary mirror ever employed on a telescope, as well as the largest convex mirror ever produced. It will reflect light back down to M3, which in turn will relay it to an adaptive flat mirror (M4) above it. This fourth mirror, which will be the largest adaptive mirror ever made, will adjust its shape a thousand times a second to correct for distortions caused by atmospheric turbulence. M5, a flat tiltable mirror, will then stabilize the image and send it to the instruments.