VLTI Unit Telescope coudé train vibration control upgrade for GRAVITY+

Fringe stability and tracking are a determining aspect for the performance of current interferometric observations. While the theory predicts that the aperture of large telescopes such as the VLTI UT should yield smoothed-out piston perturbations that could be compensated using a slow fringe tracker running at a few tens of Hz, this is far from the current experimental reality. In practice, the optical path variations observed with the GRAVITY fringe tracker still contain high frequency components that limit the fringe-tracking exposure time and therefore its precision and limiting magnitude. Most of these perturbations seem to come from mechanical vibrations in the train of mirrors, leading to the instrument, and in particular from the mirrors of the telescope. With this work, and as part of the GRAVITY+ efforts, accelerometers were added to all the mirrors of the coudé train, including the coming M8, to complement the existing instrumentation of M1, M2, and M3, and compensate in real-time the optical path using the main delay lines. We show how the existing architecture, while optimal for the first mirrors, is not suitable for the vibration content found in the new mirrors, and we opt instead for narrow-band filters based on phase-locked-loop filters (PLL). Thanks to this architecture, we were able to reclaim up to 50nm of OPD RMS from vibrations peaks between 40 and 200Hz. We also outline the avenues to push this approach further, through the upgrade of the deformable mirrors and the beam-compressor differential delay lines (BCDDL) as part of GRAVITY+, paving the way to obtaining better than 100nm RMS fringe tracking, even on faint targets.