Tests with a Carlina-type diluted telescope: Primary coherencing

Studies are under way to propose a new generation of post-VLTI interferometers. The Carlina concept studied at the Haute-Provence Observatory is one of the proposed solutions. Getting stellar fringes using delay lines is the main difficulty for astronomical interferometers. Carlina does not use delay lines, but the primary segments have to be positioned on a sphere ie coherencing the primary mirrors. As described in this paper, the authors used a supercontinuum laser source to coherence the primary segments. They characterize the Carlina's performances by testing its whole optical train: 1. servo loop, 2. metrology, and 3. the focal gondola. The servo loop stabilizes the mirror of metrology under the helium balloon with an accuracy better than 5 mm while it moves horizontally by 30 cm in open loop by 10.20 km/h of wind. They have obtained the white fringes of metrology; ie, the three mirrors are aligned with an accuracy of approximately 1 mu m.