First sky validation of an optical polarimetric interferometer

Aims. We present the first lab and sky validation of spectro-polarimetric equipment put at the combined focus of an optical long-baseline interferometer. We tested the polarimetric mode designed for the visible GI2T Interferometer to offer spectropolarimetric diagnosis at the milliarcsecond scale. Methods. We first checked the whole instrumental polarization in the lab with a fringe simulator, and then we observed alpha Cep and alpha Lyr as stellar calibrators of different declinations to tabulate the polarization effects throughout the GI2T declination range. Results. The difference between both linear polarizations is within the error bars and the visibilities recorded in natural light (i.e. without the polarimeter) for calibration purposes are the same order of magnitude as the polarized ones. We followed the a Cep visibility for 2 h after the transit and a Lyr for 1.5 h and detected no decrease with hour angle due to the fringe pattern smearing by instrumental polarization. Conclusions. Differential celestial rotation due to the dissymetric Coude trains of the GI2T is well-compensated by the field rotators, so the instrumental polarization is controlled over a relatively wide hour angle range (+/- 2 h around the transit at least). Such a polarimetric mode opens new opportunities especially for studies of circumstellar environments and significantly enhances both the potential of an optical array and its ability for accurate calibration.