Light from a star travels for millions of years, yet in a telescope it still appears as a point: it's just too far away. To see a star's surface, you need an instrument the size of a city. In Chile, such a giant is already under construction: ELT with a 39-meter mirror made of 798 hexagonal segments.

The new ELTI project proposes turning this telescope into a kind of radio dish that catches not radio waves but visible light. At the focus, a matrix of millions of SPAD detectors — avalanche photodiodes, each triggered by a single photon and recording time with picosecond precision (a millionth of a microsecond)—will be installed. Light from different mirror segments will converge on these detectors, and from tiny signal delays, a computer will reconstruct the image just as the brain builds a 3D picture from two eyes.

This technique is called intensity interferometry. It doesn't require a perfectly smooth mirror, just precise knowledge of when each photon arrives. Thanks to this, ELTI could resolve details the size of a city on a star's surface, even if the star is hundreds of light-years away. It could also peer into the vicinity of black holes, where space and time are warped, and spot an exoplanet crossing the stellar disk, slightly dimming its light.

By the 2040s, ELTI could become the premier tool for imaging stellar surfaces. Instead of points in the sky, we'd see disks with spots, flares, and swirls, like images of the Sun, but for other stars.