Interferometry has been intensively done at long wavelengths, starting with the radio interferometers in the years 50 since it was easier to guide radio wavelengths in cable while keeping the phase information or using a local oscillator and a correlator to recombine "a posteriori" the beams over intercontinental distances. In the optical a lot of work as been done at IR and near-IR wavelengths since it was technically easier, or we must say, less difficult to recombine directly the optical beams since the coherence length is larger and the turbulence slower than at shorter wavelengths.
Therefore, the visible domain of the electromagnetic spectrum is not covered at the same level than near or mid infrared.
Some very nice and important results have been however obtained with the GI2T interferometer in south of France, the
Mark III interferometer on the Mount Wilson, USA, the NPOI array in Flagstaff, USA or the SUSI interferometer in Australia. We will present in this paper the science cases of a new but already existing and tested instrument: the
REGAIN focal instrument which was designed and built for the GI2T. This instrument, in his CHARA adaptation, called VEGA will open new fields in a wide range of Astrophysical topics only addressable in the visible domain. It will provide a spectral resolution up to 30000 within the spectral range 0.4-0.9 micron and a spatial resolution of less than 1mas for up to 4 telescopes in its X-lambda special configuration. A polarimetric device (SPIN) measuring simultaneously the polarization in 2 directions either circular or linear is also implemented in this instrument. Since VEGA was already tested on the sky on 1.5 m telescopes it is also very well suited for the 1m CHARA array and will only need minor adaptations for the injection of the CHARA beams. This paper will focus on some of the most promising science drivers only possible with this visible instrument.