ULTIMATE-START: results of on-sky engineering observations of a prototype Shack-Hartmann Wavefront sensor with the Subaru telescope

ULTIMATE-Subaru Tomography Adaptive optics Research experimenT (ULTIMATE-START) is a laser tomography AO project on the Subaru telescope. The AO system utilizes 4 laser guide stars (LGSs) and 4 Shack-Hartmann wavefront sensors (SH-WFSs) to realize the tomographic turbulence measurements. The LTAO WFS unit is under assembling in the Subaru telescope, and the first light is planned in mid 2025. In the course of the development, we have assembled a prototype 32×32 SH-WFS and conducted on-sky engineering observations with bright natural stars and upgraded LGS of the Subaru telescope. We develop a realtime SH-WFS image analysis software dealing with the shift and rotation of the pupil image on the detector in real time. In the final LTAO system, the measured shift will be corrected by a pupil alignment mirror, and rotation will be considered in the tomographic reconstruction. We apply machine learning analysis on the measured time series of spot movements, and the results indicate predictive measurements is effective, especially for faint guide stars with relatively large wavefront measurement error.