Towards Unified Vibration Control for the VLTI: Multi-Sensor Data Fusion as a Path Beyond GRAVITY+

The identification and correction of vibrations remain among the key limiting factors of the Very Large Telescope Interferometer (VLTI). Despite active control at telescope level, recent GRAVITY+ results show median fringe tracker phase residuals of 100 nm on the Auxiliary Telescopes and ∼150 nm on the Unit Telescopes, well above the fundamental limits. We present new results from a data-fusion analysis that jointly exploits telemetry from adaptive optics, MANHATTAN-II accelerometers, and the GRAVITY fringe tracker. Analysis of pseudo-open-loop fringe tracker data confirms the presence of long-lived vibration modes that propagate through the optical train. Correlations with accelerometer and AO telemetry help separate their contributions, though a full quantification of their impact on the residual error budget is still in progress. Preliminary results nonetheless indicate that structural vibrations remain an important limitation. The multi-source analysis makes it possible to separate local from system-wide vibration modes, providing a basis to estimate how much a unified controller could improve over current schemes. Establishing the viability of such hybrid strategies is therefore a key step before considering an upgraded RTC that integrates multi-sensor telemetry. Such a system-wide approach, together with hardware upgrades, would represent the next logical step beyond GRAVITY+ and outline possible pathways and requirements for a future upgrade.