“Smoothing” starlight

Adaptive Optics: How extremely sharp images are generated by an ultra-thin telescope mirror

Astronomers want to look as far back as possible into the universe’s past. Telescopes with gigantic mirrors can make that happen. The Extremely Large Telescope (ELT) has a 39-meter-diameter primary mirror and will lead to new discoveries from the year 2024 onward. Scientists hope to gain new insights into the Big Bang over 13 billion years ago, for example, and to be able to spot Earth-like planets.

In order to ensure that observations are largely unaffected by disturbances in the atmosphere, a region with the best possible weather conditions was chosen as the building site: the 3,046-meter Cerro Armazones mountain in the Chilean Atacama desert. Even though the sky is clear and starry for most of the year, astronomical observations from the ground cannot completely escape air turbulence. The engineers’ solution to this challenge is “adaptive optics.” Modern technology almost completely compensates for vibrating air on the incident light waves, approaching theoretical resolution.

The 5-mirror design of the ELT is a mobile construct equipped with a mirror substrate of only 1.95-millimeter thick ZERODUR® glass-ceramic. This construction is able to perfectly “smooth” starlight. The mirror substrate is composed of six individual glass-ceramic segments arranged like petals. The 2.4-meter adaptive mirror area is supported by approx. 5,600 actuators and is the largest of its kind. During operation, the actuators distort the mirror surface, mounted on magnetic fields, with virtually no contact. Thousands of times per second in real time. This perfectly compensates for the influence of air disruptions. An artificial star, simulated with six lasers in the atmosphere, is used to calibrate the apparatus. Images taken by the earth-bound mega-telescope will be 15 times sharper than those taken by the Hubble Space Telescope.

How the Mirror 4 works

The “contactless” adaptive mirror is based on a 1.95-millimeter-thin mirror surface made of ZERODUR® glass-ceramic. The position and shape of the surface is continuously distorted by a large number of electromagnetic actuators to compensate for disturbances from air turbulence. During operation, the mirror levitates in a controlled float over magnets attached to each actuator. There is no mechanical contact between the actuators and the mirror. Each actuator is made of a fixed coil and a magnet, with a counter-magnet glued to the rear of the thin adaptive mirror.