ngVLTI: Cost and Feasibility of a Four-Telescope VLTI Expansion for Milliarcsecond-Scale Imaging
We assess the cost and technical feasibility of extending the Very Large Telescope Interferometer (VLTI) through the addition of four 8 m-class Unit Telescopes (UTs), an upgrade we refer to as the new generation VLTI (ngVLTI). Such an upgrade would provide a dense and homogeneous uv coverage with baselines up to 220 m, enabling true imaging at milliarcsecond angular resolution across science cases ranging from Solar System bodies to distant active galactic nuclei. Because each image is reconstructed at a single wavelength, repeating the reconstruction across the spectral channels of the instrument would deliver spectral-imaging cubes -- a qualitatively new capability for the VLTI, bringing it close to the imaging power of ALMA at near-infrared wavelengths. Motivated by this scientific case, we examine a compact telescope concept consisting of a fast ($f/0.64$), segmented, parabolic primary mirror feeding a subterranean coud\'e focus compatible with the existing VLTI infrastructure. We summarise the optical design, which achieves diffraction-limited performance over a 1 arcmin field of view and a well-matched reimaged pupil, discuss the mechanical trade-offs behind the choice of a 60-segment, 1.2 m primary -- a mirror mass of ~13.5 tons and an altitude moving mass of ~50 tons -- and quantify the gravitational flexure of the telescope structure and its resulting optical sensitivity as a function of pointing elevation. We then present a back-of-the-envelope cost estimate of order 80 MEuro per telescope (2026 prices), broken down into the segmented primary, adaptive secondary, coud\'e train, mount, enclosure, and ancillary instrumentation. The proposed upgrade appears both cost-competitive and technically achievable, offering a long-term perspective for Paranal Observatory in the ELT era.