The Telescope That Will See the Surfaces of Distant Stars express
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.
Original abstract
as written by the authors arXiv:2603.05589
The ELTI concept capitalizes on recent breakthroughs in large-format SPAD (Single-Photon Avalanche Diode) imaging sensors, combining them with the unprecedented collecting area and segmented architecture of the ELT to deliver a fundamentally new observational capability for the 2040s: visible-light intensity interferometry with ELT-scale angular resolution and quantum-limited temporal sampling. By uniting three transformative innovations - segmented sub-pupil beam combination, megapixel SPAD arrays with picosecond time resolution, and high-dispersion spectroscopy - ELTI opens an entirely new region of observational parameter space in angular resolution, spectral bandwidth, and temporal precision. This capability enables true milliarcsecond imaging of stellar surfaces, direct probes of extreme-gravity and strong electromagnetic-field environments around compact objects, precision studies of accretion physics, and even the potential detection of Earth-sized exoplanets through minute visibility modulations. ELTI thus positions the ELT as the world-leading facility for ultra-high-resolution optical astronomy in the coming decades.
The SPAD detectors that will form the basis of ELTI are already used in automotive lidar to measure distances to obstacles, but there they catch reflected laser beams, while here they will catch light from distant stars.
In Stanisław Lem's novel 'Solaris', the planet's ocean observes humans; ELTI will let astronomers gaze just as intently at the surfaces of distant stars, resolving details the size of a city.