First on-sky interference fringes at 810 nm with the CHARA array using servo controlled hectometric outdoor fibre links

In the framework of the ALOHA (Astronomical Light Optical Hybrid Analysis) project, we have implemented a fibre-linked interferometer connecting two telescopes of the CHARA (Center for High Angular Resolution Astronomy) array to the recombination beam facility using servo controlled hectometric outdoor fibres (240 m). During two consecutive nights, on-sky fringes at 810 nm were recorded on the star Vega (mag 0), with servo control of the fibre lengths. The optical path difference was set close to zero using internal fringes found before the on-sky observations. The repeatability of the delay line position offset between internal and on-sky fringes was less than 0.2 mm. The efficiency of the servo control systems has been demonstrated, leading to an enhancement of the signal-to-noise ratio from 68.9 with the servo off to 91.6 with the servo on. This result is a cornerstone for the ALOHA project goal of interferometry at 3.5 μm and a seminal step for the future kilometric infrared fibre-linked interferometer at CHARA.