Five hundred mirror pieces in the desert: the world's largest telescope is three-quarters built

High on Cerro Armazones in Chile's Atacama Desert, 3,046 metres up where the air is dry and the nights are dark, the Extremely Large Telescope is taking shape — and it just passed a milestone you can count. The European Southern Observatory says nearly 500 segments of its giant primary mirror have now arrived in Chile, and production of the final segment has begun in France. The mirror will be 39 metres across, made of 798 hexagonal pieces that must act as one surface, precise to tens of nanometres — a light-collecting area of about 978 square metres, far beyond any optical telescope ever built. The project is more than 75% complete. When it opens its eye, astronomers expect to study planets around other stars closely enough to read their air, and galaxies from the universe's first chapters. First test light is targeted for March 2029 — a date, not a promise — but the pieces, literally, are arriving.

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