VLTI system upgrades: recent engineering progress and future developments
Abstract
Over the past six years, the VLTI has undergone major upgrades, significantly improving its stability, sensitivity, and operational reliability. This has enabled the highest-angular-resolution images of stars orbiting our Galactic Center, high-precision interferometric astrometric measurements of exoplanet systems, and the resolution of quasars up to redshift 4. The GRAVITY near-infrared instrument, with its high-speed Fringe Tracker, provides real-time fringe stabilization (including for the mid-infrared instrument MATISSE with GRA4MAT) and acts as a reference for multiple VLTI configurations. At the telescope level, as part of the GRAVITY+ project, all 8.2m Unit Telescopes are now equipped with state-of-the-art adaptive optics systems (GPAO) using laser guide stars, achieving enhanced sensitivity (up to Klim~20mag, Llim~0.03Jy and Nlim~0.1Jy for one hour on target), wide sky coverage, and high Strehl ratios. The VLTI also incorporates BCDDL (Beam Compressor Differential Delay Lines), enabling dual-field interferometry up to 30” separation and significantly expanding operational flexibility. Looking forward, GRAVITY+ will complete its upgrade cycle with a new high-resolution grism and improved fiber coupler optics, while new instruments will extend VLTI capabilities toward shorter wavelengths. Future developments in vibration control, metrology, and operational automation will further enhance the performance, flexibility, and efficiency of VLTI operations.