  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.
