Integration of the Stellar Intensity Interferometry Instrument on the ASTRI Mini-Array

Abstract

The ASTRI Mini-Array is an International collaboration, led by the Italian National Institute for Astrophysics, that is constructing and operating an array of nine Imaging Atmospheric Cherenkov Telescopes. The ASTRI Stellar Intensity Interferometry Instrument (SI3) is a fast single-photon-counting instrument for performing intensity interferometric observations of bright stars with the ASTRI Mini-Array. ASTRI SI3 is the first implementation of very narrow-band, multi-channel intensity interferometry in photon counting on Cherenkov telescopes with sub-nanosecond sampling. The basic concept of SI3 is to inject light into a multimode optical fiber bundle at the telescope focal plane, handle the output beam to efficiently insert narrow-band filters, and finally focus light into modern single-photon Silicon detectors, fully exploiting their ∼100ps resolution. ASTRI SI3 will benefit the simultaneous baselines over distances between 100m and 700m of the ASTRI Mini-Array to achieve angular resolutions below 100 microarcsec in the visible band and a dense coverage of the U-V plane. It is expected to significantly improve the boundaries and capabilities of stellar intensity interferometry with Cherenkov telescope arrays. In November 2025 the first ASTRI SI3 unit was successfully integrated on one of the ASTRI telescopes in Tenerife. In April 2026, two additional units were mounted on two other telescopes. Here we present the activities carried out during these technical shifts and the plans for the full integration and the commissioning of the instrument.