One day in 2024, the world’s largest optical telescope will see first light in Chile’s Atacama Desert. Already, as the pieces of this remarkable instrument are constructed, the telescope is a feat of engineering, destined to make its mark on the field of science.
When completed, the Extremely Large Telescope, or ELT, will have a primary mirror spanning 39 meters in diameter, making it nearly four times bigger than its nearest competitors in the 10-meter class of lenses. It will gather more light than all of the world’s 8-meter to 10-meter-class telescopes combined, and more than 100 million times more light than the human eye.
One giant leap
The ELT is so powerful that it is expected to capture images of rocky exoplanets in other solar systems, and be able to characterize their atmospheres. That’s a huge step forward, because currently, the world’s largest telescopes can only obtain images of planets outside of our solar system in rare instances.
Without images, scientists infer the existence of exoplanets by watching the subtle shifts of gravity or light of faraway stars. By observing how a star moves, scientists predict that an exoplanet is nearby, assuming the movement is caused by the gravity of a small planet on its larger star. Likewise, scientists are often on the lookout for a very slight dimming of a star’s brightness, an indication that a star is passing between it and Earth.
Many compare the ELT to the Hubble Space Telescope, which brought us amazing images because it was free from the distortion of Earth’s atmosphere. But the ELT is even more powerful than Hubble, and it features an adaptive optics system that will make ultra-fine adjustments to correct for atmospheric distortion. The atmosphere, by the way, will be pretty thin. ELT will sit 10,000 feet above sea level.
Putting it all together
The European Southern Observatory (ESO), which is building the telescope, has so far called on SCHOTT four times to build mirror substrates – the glass-ceramic component of the mirror – for the telescope.
The first mirror substrates for the segmented 2.4-meter mirror (M4) of the adaptive optical system left SCHOTT facilities in 2016. That was followed by contracts to make two others: the 3.8-meter concave substrate (M3), and the 4.2-meter secondary mirror substrate (M2), which will be the largest convex mirror ever produced, and is bigger than the biggest primary mirrors in reflective telescopes.
In May 2017, SCHOTT announced that it would build the largest segment of mirror substrate, a 39-meter section that will be made from 798 hexagonal mirror tiles. This represents an interesting technical problem. Not only must each tile be machined to exacting standards, but each must have the exact same optical properties to ensure the clearest picture.
The perfect material
For the physicists and engineers that design the world’s largest telescopes, ZERODUR glass-ceramic is the clear choice of material. That’s because the degree to which it changes size in hot or cold weather is near zero. That’s a clear benefit for telescopes, because when the mirrors change size, images become less focused. In fact, ZERODUR was specifically developed for use in telescopes, and many of the instruments with ZERODUR mirrors are still in use decades later.
SCHOTT made significant upgrades to its own facility to produce ELT’s large mirrors. The company built a second melting tank in Mainz, Germany to be used exclusively for ZERODUR. The company also installed new CNC machines, capable of grinding 4 meter class (13 feet) mirror substrates to a tolerance of less than 100 micrometers (0,004 inches).
Casting and shaping the hexagonal tiles – SCHOTT will make 949 of them – will begin in 2020. Polishing and coating the tiles will be handled by the French company Safran-Reosc, which has years of experience in working with ZERODUR (e.g. polishing and coating the 8-meter mirror substrates for the Very Large Telescope). The substrates for the primary mirror are scheduled to be gradually delivered through to March 2024.
In the grand scheme of things, the universe is billions of years old. Seven years isn’t so long to wait for this amazing instrument to start producing amazing results.