  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 lambda/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 1e-3 lambda/D rms, enhancing the previous contrast by a decade, to 8e-7
between 1.2 and 2 lambda/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 1e-6 at 1.2 lambda/D and
1e-7 above 2 lambda/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.
