Demonstrating the combination of AT-UT baselines: VLTI as a true coherent array

In this talk, we present the experimental advances to recombine the Unitary Telescope (UTs) and Auxiliary Telescope (ATs) of the Paranal Observatory. The on-site tests were carried on VLTI in 2025, using GRAVITY for the beam-combination. The functional tests validate the possibility to use VLTI/GRAVITY in a hybrid mode. Combined with the on-going upgrade of the delay-lines to have automated relocation of M12, the automatic recombination and AT-UT combination would open up instantaneous, reconfigurable geometry of the VLTI among the 28 baselines of the array, a true leap in imaging capability at VLTI. Instantaneous, snapshot imaging capability is of fundamental importance, in particular for science involving short-term variability, such as the imaging of protoplanetary disks at terrestrial orbits (keplerian timescales). On a fundamental level, AT-UT demonstration is a crucial step to validate the scaling of sensitivity and field-of-view with different telescope diameters into large interferometry array in the future.