Real-time optical path difference compensation at the Plateau de Calern 12T interferometer

The fringe tracker system of the ASSI (Active Stabilization in Stellar Inte rferometry) beam combining table at the I2T interferometer is described and its performance evaluated. A new real-time algorithm for the optical path difference (OPD) measurement is derived and validated. It is based on a sin usoidal phase modulation whose amplitude is optimized. It also allows autom atic fringe detection at the beginning of an observation when scanning the OPD. The fringe tracker servo-loop bandwidth is adjusted by a numerical gai n and ranges between 20 and 50 Hz in the reported experiments. On stars, fr inge-locked sequences are limited to 20 s due to fringe jumps. However, the fringe tracker is able to recover the coherence area after a few seconds. Such a fringe tracker operation can last more than one hour. A fringe track ing accuracy of 85 nm is achieved for visibility ranging between 7 and 24%, a turbulence coherence time of approximately 9 ms at 0.85 mum, a Fried par ameter of around 14 cm at 0.5 mum and an average light level of 100 000 pho toevents/s, (typically visual magnitude 2 in the conditions of the experime nt). Visibility losses are evaluated and are found to be mainly due to turb ulent wavefront fluctuations on the tao telescopes and to the static aberra tions of the optical train. The measurements of OPD and angle of arrival ar e reduced to derive turbulence parameters: the coherence time, the average wind speed, the Fried parameter and the outer scale. Our estimations for th e outer scale range between 20 and 120 m: with an average value of the orde r of 40 m. Both OPD and angle of arrival data, obtained on 15 m baseline an d a 26 cm telescope diameter respectively, are fully compatible with the sa me modified Kolmogorov spectrum of the turbulence, taking into account a fi nite outer scale.