Demonstration of on sky contrast improvement using the modified Gerchberg-Saxton algorithm at the Palomar Observatory

We have successfully demonstrated significant improvements in the high contrast detection limit of the Well-Corrected Subaperture (WCS) using a number of steps aimed at reducing non-common path (NCP) wavefront errors, including the Autonomous Phase Retrieval Calibration (APRC)¹ software package developed at the Jet Propulsion Laboratory (JPL) for the Palomar adaptive optics instrument (PALAO). APRC utilizes the Modified Gerchberg-Saxton (MGS) wavefront sensing algorithm, also developed at JPL². The WCS delivers such excellent correction of the atmosphere that NCP wavefront errors not sensed by PALAO but present at the coronagraphic image plane begin to factor heavily as a limit to contrast. The APRC program was implemented to reduce these NCP wavefront errors from 110 nm to 35 nm (rms) in the lab, and now these exceptional results have been extended to targets on the sky for the first time, leading to a significant suppression of speckle noise. Consequently we now report a contrast level of very nearly 1×10^-4 at separations of 2λ/D before the data is post processed, and 1×10^-5 after post processing. We describe here the major components of our instrument, the work done to improve the NCP wavefront errors, and the ensuing excellent on sky results, including the detection of the three exoplanets orbiting the star HR8799.