Experimental study of a low-order wavefront sensor for a high-contrast coronagraphic imager at 1.2 λ/D

High-contrast imaging will be a challenge for future ELTs, because their vibrations create low-order aberrations - mostly tip/tilt - that reduce coronagraphic performances at 1.2 λ/D and above. A Low-Order WaveFront Sensor (LOWFS) is essential to measure and control those aberrations. An experiment simulating a starlight suppression system is currently developed at NASA Ames Research Center, and includes a LOWFS controlling tip/tilt modes in real-time at 500 Hz. The LOWFS allowed us to reduce the tip/tilt disturbances to 10^-3 λ/D rms, enhancing the previous contrast by a decade, to 8×10^-7 between 1.2 and 2 λ/D. A Linear Quadratic Gaussian (LQG) controller is currently implemented to improve even more that result by reducing residual vibrations. This testbed is developed for the mission EXCEDE (EXoplanetary Circumstellar Environments and Disk Explorer), selected by NASA for technology development, and designed to study the formation, evolution and architectures of exoplanetary systems and characterize circumstellar environments into stellar habitable zones. It is composed of a 0.7 m telescope equipped with a Phase-Induced Amplitude Apodization Coronagraph (PIAA-C) and a 2000-element MEMS deformable mirror, capable of raw contrasts of 10^-6 at 1.2 λ/D and 10^-7 above 2 λ/D. Although the testbed simulates space conditions, its LOWFS has the same design than on the SCExAO instrument at Subaru telescope, with whom it shares the same kind of problematic. Experimental results show that a good knowledge of the low-order disturbances is a key asset for high contrast imaging, whether for real-time control or for post processing, both in space and on ground telescopes.