The THESEUS Space Mission and the Infrared Telescope Calibration Unit

ABSTRACT The Transient High‐Energy Sky and Early Universe Surveyor (THESEUS) is an ESA M7 mission concept currently in Phase A, designed to exploit gamma‐ray bursts to probe the early Universe while advancing multi‐messenger and time‐domain astrophysics. To achieve its ambitious goals, THESEUS will combine wide‐band x‐ray and gamma‐ray monitors with an onboard InfraRed Telescope (IRT), providing autonomous arcsecond‐scale localisation and high‐precision redshift determination of near‐infrared counterparts to high‐energy transients. Achieving this precision requires a well‐characterised detector response enabled by an internal Calibration Unit. The unit will ensure accurate calibration of the detector throughout the mission's 4‐year nominal lifetime—and potentially beyond—using near‐infrared LEDs as internal illumination sources. We highlight THESEUS science cases relevant for the Calibration Unit performance, discuss key challenges in the LED selection process and present the current status of the optical and mechanical design concept enabling a spatially uniform and temporally stable detector illumination within a structure that ensures long‐term durability.