DARWIN fringe sensor : Experimental results on the BRISE bench

ABSTRACT Interferometer performances are lin ked to the measurement and the correct ion of telescope aberrations. Forcophasing the large number of beams required by the DARWIN mission with the speci“ed requirements (real-time piston/tip/tilt correctionandmeasurementof higherordersup tosphericalaberration), focal-planeapproachhas been selected due to its simple opto-mechanical device. Several focal-plane algorithms, developed at ONERAand gathered in the stand-alone MASTIC tool, were validated by experiment with a dedicated breadboard onthe laboratory test bench BRISE. Our study shows the correct behaviour of the algorithms for linearity andrepeatability; speci“c requirements are reached for piston/tip/tilt and higher order aberrations. These resultscon“rm the validity of focal-plane sensors for the cophasing of multiple-aperture telescopes.Keywords: interferometry, wave-front sensing, cophasing, phase retrieval, phase diversity 1. INTRODUCTION - CONTEXT OF THE STUDY For optical imaging instruments such as telescopes, the resolution is de“ned by the aperture diameter. Dueto current technology and mass/volume considerations, only interferometry allows signi“cant resolutions to bereach; the equivalent of larger diameter telescope, ca lled MAOT for Multiple Aperture Optical Telescope, isobtained by recombination of several apertures of small diameter. In ord er to have the best image quality andstability at the common recombination plane, aberrations on each aperture must be corrected by a Wave-FrontSensor (WFS) with a very accurate control of the optical paths, particularly for nulling interferometers.For DARWIN,