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The mission EXCEDE (EXoplanetary Circumstellar Environments and DiskExplorer), selected by NASA for technology development, is designed to studythe formation, evolution and architectures of exoplanetary systems andcharacterize circumstellar environments into stellar habitable zones. It iscomposed of a 0.7 m telescope equipped with a Phase-Induced AmplitudeApodization Coronagraph (PIAA-C) and a 2000-element MEMS deformable mirror,capable of raw contrasts of 1e-6 at 1.2 lambda/D and 1e-7 above 2 lambda/D. Oneof the key challenges to achieve those contrasts is to remove low-orderaberrations, using a Low-Order WaveFront Sensor (LOWFS). An experimentsimulating the starlight suppression system is currently developed at NASA AmesResearch Center, and includes a LOWFS controlling tip/tilt modes in real timeat 500 Hz. The LOWFS allowed us to reduce the tip/tilt disturbances to 1e-3lambda/D rms, enhancing the previous contrast by a decade, to 8e-7 between 1.2and 2 lambda/D. A Linear Quadratic Gaussian (LQG) controller is currentlyimplemented to improve even more that result by reducing residual vibrations.This testbed shows that a good knowledge of the low-order disturbances is a keyasset for high contrast imaging, whether for real-time control or for postprocessing.

Experimental study of a low-order wavefront sensor for the high-contrast coronagraphic imager EXCEDE