The implementation of the simultaneous combination of several telescopes (from four to eight) available at
Very Large Telescope Interferometer (VLTI) will allow the new generation interferometric instrumentation
to achieve interferometric image synthesis with unprecedented resolution and efficiency. The VLTI Spectro
Imager (VSI) is the proposed second-generation near-infrared
multi-beam instrument for the Very Large
Telescope Interferometer, featuring three band operations (J, H and K), high angular resolutions (down to
1.1 milliarcsecond) and high spectral resolutions. VSI will be equipped with its own internal Fringe Tracker
(FT), which will measure and compensate the atmospheric perturbations to the relative beam phase, and in
turn will provide stable and prolonged observing conditions down to the magnitude K=13 for the scientific
combiner. In its baseline configuration, VSI FT is designed to implement, from the very start, the minimum
redundancy combination in a nearest neighbor scheme of six telescopes over six baselines, thus offering better options for rejection of large intensity or phase fluctuations over each beam, due to the symmetric set-up.
The planar geometry solution of the FT beam combiner is devised to be easily scalable either to four or eight
telescopes, in accordance to the three phase development considered for VSI. The proposed design, based
on minimum redundancy combination and bulk optics solution, is described in terms of opto-mechanical
concept, performance and key operational aspects.