Onboard Phase Retrieval for Habitable Worlds Observatory

A key area of technology development for the Habitable Worlds Observatory (HWO) is telescope wavefront control capable of meeting the picometer-level telescope stability requirement. We studied a dedicated image-based wavefront sensing instrument with onboard, closed-loop control. The proposed instrument would take direct optical measurements of the telescope wavefront using out-of-field stars to measure the primary and secondary mirror alignment and allow for control or calibration of environmental disturbances. We have conducted preliminary studies to show that phase retrieval is a viable option for picometer-level sensing. The number of photons collected from the out-of-field stars was the driving factor of phase retrieval performance. Bandwidth, wavelength, and defocus had minimal impact on sensing error. Given sufficient field of view or integration time, a single-image multi-star phase retrieval system can achieve the required picometer sensitivity. Through high-level design trade studies, we have outlined the performance of an out-of-field system using a hypothetical large field-of-view guider layout and realistic science targets to inform future mission studies. As a proof of concept for onboard operation, we have investigated the computational and memory requirements and compared parallel programming paradigms for phase retrieval.