MIDI has recently been commissioned with the PRIMA Fringe Sensor Unit FSU-A as an external fringe tracker, enabling improved sensitivities. This mode has been offered from Period 91 for use with the Auxiliary Telescopes (ATs) in Visitor Mode. PRIMA+FSU-A enables increased visibilities, for feeble disc emission from young stars for example, with factors 5–10 times in sensitivity improvement (down to 50 mJy limit) over classical MIDI stand-alone operation modes.
A new VLTI instrument is under construction: MATISSE is a mid-infrared spectro-interferometer combining the beams of up to four UTs/ATs of VLTI. MATISSE has recently passed its Final Design Review and construction has already begun. Preliminary acceptance is planned for mid 2017. An early description of the instrument can be found here.
The VLTI GRAVITY instrument is a four-way, K-band, beam combiner. Its main operational mode makes use of all four 8-metre VLT Unit Telescopes to measure astrometric separations of objects located within the 2-arcsecond field of view of the VLTI. It will allow measurements of orbital motions near the Galactic Centre with an unprecedented precision and sufficient to test predictions of general relativity. GRAVITY is in its manufacturing and integration phase, aiming at a preliminary acceptance date in the second half of 2014.