Post-Coronagraph Wavefront Sensor For Gemini Planet Imager

The Gemini Planet Imager (GPI)(1) will employ an apodized-pupil coronagraph to make direct detections of faint companions of nearby stars to a contrast level of the 10(-7) within a few lambda/D of the parent star. Such high contrasts from the ground require exquisite wavefront sensing and control both for the AO system as well as for the coronagraph. Un-sensed non-common path phase and amplitude errors after the wavefront sensor dichroic but before the coronagraph lead to speckles which limit the contrase. The calibration wavefront system for GPI will measure the complex wavefront at the system pupil before the apodizer and provide slow phase corrections to the AO system to mitigate errors that would cause a loss in contrast. Here we describe the low-order and high-order wavefront sensors that compose the calibration wavefront sensor, how they operate, and how their information is combined to form the wavefront estimate before the coronagraph. Detailed Simulations that show the expected performance for this wavefront sensor will be described for typical observing scenarios. Finally, we will show preliminary lab results from our calibration testbed that demonstrate the operation of the key hardware.