Low-order wavefront sensing and control for WFIRST-AFTA coronagraph

To maintain the required Wide-Field Infrared Survey Telescope (WFIRST) coronagraph performance in a realistic space environment, a low-order wavefront sensing and control (LOWFS/C) subsystem is necessary. The LOWFS/C uses the rejected stellar light from the coronagraph to sense and suppress the telescope pointing errors as well as low-order wavefront errors (WFEs) due to changes in thermal loading of the telescope and the rest of the observatory. We will present a conceptual design of a LOWFS/C subsystem for the WFIRST-AFTA coronagraph. This LOWFS/C uses a Zernike phase contrast wavefront sensor (ZWFS) with a phase shifting disk combined with the stellar light rejecting occulting masks, a key concept to minimize the noncommon path error. We will present our analysis of the sensor performance and evaluate the performance of the line-of-sight jitter suppression loop, as well as the low-order WFE correction loop with a deformable mirror on the coronagraph. We will also report the LOWFS/C testbed design and the preliminary in-air test results, which show a very promising performance of the ZWFS.