Nulling interferometers require a very accurate control of the optical paths. For DARWIN, requirements are real-time piston/tip/tilt sub-nanometric correction and measurement of higher order modes with a 10 nm accuracy.
A study was performed to identify the best cophasing sensor for DARWIN, taking into account the large number of beams (6 initially) and Zernike modes (1 to 11). The selected solution is based on focal-plane sensing, allowing the combination of all the beams and the measurement of all the modes of interest in a single frame with a simple opto-mechanical device. For piston/tip/tilt, wavefront estimation is based on "Phase Retrieval", using the sole focal-plane image, whereas "Phase Diversity", based on the joint analysis of a focal and an extra-focal images, allows the measurement of higer order modes. A breadboard "DWARF" (DarWin AstRonomical Fringe sensor) was manufactured by Kayser-Threde and used at ONERA to validate experimentally this concept with three beams, using the ONERA laboratory test bench BRISE (cf companion paper: BRISE: a multipupose bench for cophasing sensors). New algorithms were also developped and gathered in the stand-alone tool MASTIC (Multiple-Aperture Software for Telescope Imaging and Cophasing).
We present here the selected concept and algorithms. Experimental results, which confirm the correct behaviour of the algorithms, are reported. Sub-nanometric repeatability is demonstrated for piston/tip/tilt up to magnitude 12 and a 10 nanometer error is obtained for high-order modes. These results confirms the validity of focal-plane sensors for the cophasing of multiple-aperture telescopes.