High-Contrast imaging and wavefront control with a PIAA coronagraph: Laboratory system validation

The Phase-Induced Amplitude Apodization (PIAA) coronagraph is a high-performance corona-graph concept able to work at small angular separation with little loss in throughput. We present results obtained with a laboratory PIAA system including active wavefront control. The system has a 94.3% throughput (excluding coating losses) and operates in air with monochromatic light. Our testbed achieved a 2.27×10^-7 raw contrast between 1.65λ/D (inner working angle of the coronagraph configuration tested) and 4.4λ/.D (outer working angle). Through careful calibration, we were able to separate this residual light into a dynamic coherent component (turbulence, vibrations) at 4.5 × 10^-8 contrast and a static incoherent component (ghosts and/or polarization mismatch) at 1.6 × 10^-7 contrast. Pointing errors are controlled at the 10^-3 λ/D level using a dedicated low-order wavefront sensor. While not sufficient for direct imaging of Earthlike planets from space, the 2.27 × 10^-7 raw contrast achieved already exceeds requirements for a ground-based extreme adaptive optics system aimed at direct detection of more massive exoplanets. We show that over a 4 hr period, averaged wavefront errors have been controlled to the 3.5 × 10^-9 contrast level. This result is particularly encouraging for ground-based extreme-AO systems relying on long-term stability and absence of static wavefront errors to recover planets much fainter than the fast boiling speckle halo.