Towards an Extended VLTI: Turbulence Characterization for Kilometer-Scale Optical Links at Paranal
arXiv:2607.26540
The paper investigates the feasibility of extending the VLTI to kilometer-scale baselines by using free-space optical links with adaptive‑optics pre‑compensation, and proposes a dedicated experiment to measure horizontal atmospheric turbulence along the proposed 1.4 km path at Paranal.
Abstract
Future extensions of the VLTI aim to push infrared interferometry toward kilometer scale baselines, enabling angular resolutions of a few tens of micro-arcseconds. A first step would couple a new telescope on the VISTA platform to the existing VLTI infrastructure, creating a 1.4 km baseline at Paranal. Among the possible beam-transport solutions, direct free-space transmission with adaptive-optics (AO) pre-compensation, inspired by Free-Space Optical (FSO) communications, offers an attractive combination of spectral flexibility, moderate cost, and preservation of field information. We present a preliminary AO dimensioning study, including fitting error, photon noise, anisoplanatism, and scintillation, showing that moderate-order correction (around 10x10 actuators) may be sufficient, but that performance depends critically on the poorly known turbulence distribution along the horizontal path. We therefore propose a dedicated turbulence-monitoring experiment across the VISTA-VLTI line of sight, using two 30-50 cm telescopes equipped with calibrated light sources. The experiment combines a wide-field Shack-Hartmann sensor for tomographic reconstruction of the turbulence volume with an event-based (neuromorphic) camera capable of capturing fast, anisotropic turbulence at microsecond timescales. The resulting dataset will provide the first systematic characterization of kilometer-scale horizontal turbulence at Paranal, paving the way toward an extended kilometer-baseline VLTI.