STELLIM: Fast Interferometric Imaging of Stellar Surfaces

Current interferometric imaging of stellar surfaces faces significant temporal limitations, requiring weeks of data collection to produce reliable images. STELLIM represents a revolutionary advancement that will enable interferometric imaging of bright stellar objects within a few hours of observation. The system combines light from 13 telescopes in the visible, leveraging the existing VLTI infrastructure to achieve milli-arcsecond resolution imaging capabilities. The interferometer employs single-mode optical fibers to transport light from telescopes to VLTI delay line tunnels. A mirror switchyard system enables sequential selection of various 7-telescope combinations. STELLIM's primary scientific objectives focus on imaging stellar surfaces to diagnose stellar dynamics and mass loss phenomena resolved on hourly timescales, including detection of oscillation modes, large-scale photospheric structures, convection patterns and outflows. The system will provide unprecedented spatio-temporal characterization of surface and cir- cumstellar structures in cool evolved stars, particularly red supergiants whose complex dynamics and photometric variability remain poorly understood. Beyond its immediate scientific applications, STELLIM serves as a critical technology demonstrator for opening the visible observing window at VLTI and for future interferometric facilities, testing key operational concepts for next-generation astronomical interferometry.