The Extremely Large Telescope (ELT) has been under construction on a mountaintop in Chile's Atacama Desert for several years now. Its dome is nearly complete, technical first light is scheduled for 2029, and the first scientific observations are planned for 2030. The telescope's segmented primary mirror will span roughly 39 meters across, making the ELT the largest optical telescope in the world once finished. Yet while one team is still finishing the telescope itself, another is already proposing what instrument to bolt onto it a decade after launch.
That concept is called ELTI — the ELT Interferometer. It is described in a paper submitted as a response to a European Southern Observatory (ESO) call for white papers on the science questions astronomy will need to answer in the 2040s. The authors are Francisco Prada, Enrique Perez, Sergio Fernandez-Acosta, Km Nitu Rai, and Joel Sanchez-Bermudez.
Three technologies, not one
ELTI does not simply add a single new sensor to the ELT — it combines three distinct approaches at once.
The first is visible-light intensity interferometry. Instead of directly combining light waves from different parts of the mirror, as classical interferometers do, this method correlates fluctuations in brightness (intensity) arriving from different segmented sub-pupils of the ELT's primary mirror.
The second element is a new generation of detectors based on SPADs (single-photon avalanche diodes). These are megapixel arrays capable of registering individual photon arrivals with picosecond timing precision — billionths of a millisecond.
The third component is high-dispersion spectroscopy, which splits incoming light into very narrow spectral bands. Together, the authors write, these three innovations open "an entirely new region" of observational parameter space in angular resolution, spectral bandwidth, and temporal precision.
Resolution beyond today's reach
The main practical outcome of combining these methods is angular resolution at the milliarcsecond level. The authors state that this is a level no current instrument can reach.
A short description of the project puts a figure on it: resolution roughly 50 times higher than the Hubble Space Telescope. Since Hubble already resolves details unattainable by ground-based telescopes without adaptive optics, a gap of that magnitude represents a shift into an entirely different observational regime.
That kind of precision is exactly what's needed for direct imaging of individual stellar surfaces — something currently possible only for the nearest, largest stars, and even then with limited detail.
Black holes, neutron stars, and Earth-sized worlds
Among ELTI's science targets, the authors list studies of extreme-gravity and strong electromagnetic-field environments around compact objects — black holes and neutron stars. This provides a direct probe of accretion physics: the process by which matter falling onto such an object heats up to enormous temperatures.
Another goal is the search for Earth-sized exoplanets. The idea is to detect minute visibility modulations — tiny changes in a star's apparent brightness caused by a planet's presence or gravitational influence. For now this remains a potential capability, one the authors describe as something ELTI could open up rather than a guaranteed outcome.
An instrument for a telescope that doesn't exist yet
The authors are explicit that ELTI is being designed as an instrument for the 2040s — a decade or more beyond the ELT's planned first light in 2029. This is typical practice for major observatories: while construction teams finish the primary structure, science teams are already designing second- or third-generation instruments.
The paper was submitted specifically as a contribution to the debate over which scientific questions astronomy should be asking in the 2040s — meaning it remains a concept for discussion rather than an approved project. Still, the scale of the claimed capabilities, from direct images of stellar surfaces to the search for Earth-like exoplanets, shows how far ahead teams are already looking, well beyond the ELT's imminent launch.
