Optics

ESO's Extremely Large Telescope tertiary mirror (M3) ready for polishing

March 6, 2020

The ESO's ELT 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 is ready to be polished by Safran Reosc.

Gail Overton

This diagram shows the novel five-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 m segmented primary mirror (M1), it then bounces off two further 4 m 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.

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. SCHOTT produced the mirror blank for M3--a cast block of a glass-ceramic material known as Zerodurmeasuring more than 4 m from edge to edge and weighing in at over 3 tons. 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 nm across the entire optical surface.

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.

The five mirrors on the ELT all have different shapes, sizes and roles. The primary, M1, is the most spectacular, a giant 39 m 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 m 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.

SOURCE: ESO; https://www.eso.org/public/announcements/ann20003/

About the Author

Gail Overton | Senior Editor (2004-2020)

Gail has more than 30 years of engineering, marketing, product management, and editorial experience in the photonics and optical communications industry. Before joining the staff at Laser Focus World in 2004, she held many product management and product marketing roles in the fiber-optics industry, most notably at Hughes (El Segundo, CA), GTE Labs (Waltham, MA), Corning (Corning, NY), Photon Kinetics (Beaverton, OR), and Newport Corporation (Irvine, CA). During her marketing career, Gail published articles in WDM Solutions and Sensors magazine and traveled internationally to conduct product and sales training. Gail received her BS degree in physics, with an emphasis in optics, from San Diego State University in San Diego, CA in May 1986.