GRAVITY, a VLTI second generation instrument, requires a fringe tracker combining four beams. Its design is driven by the observation of the Galactic Center and implies stringent fringe sensor specifications. We present the simulations of the fringe tracking closed-loop performance with an optical path difference (OPD) turbulence spectrum using a Kolmogorov model of the atmosphere for typical seeing conditions at VLTI (r₀ = 0.95 m, t₀ = 47 ms at 2.2 μm). We show that the total residual OPD standard deviation can be as low as λ/12 at a sampling frequency of 350 Hz on a guide star with a magnitude of mK = 10. To obtain this performance, we compared several 4-beam pairwise co-axial combination conceptual architectures and show that the optimal 4-beam combination is the one measuring the OPD on the six baselines.

Date Published: 28 July 2008
PDF: 12 pages
Proc. SPIE 7013, Optical and Infrared Interferometry, 70131B (28 July 2008); doi: 10.1117/12.790967

Published in SPIE Proceedings Vol. 7013:
Optical and Infrared Interferometry
Markus Schöller; William C. Danchi; Françoise Delplancke, Editor(s)