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 LTAOWFS 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.
Atmospheric turbulence profile plays an important role in designing and operating adaptive optics (AO) systems with multiple laser guide stars. To obtain representative free atmospheric profiles and resolved ground layer profiles for future AO systems at the Subaru Telescope, we are conducting the SHARPEST (Shack-Hartmann Atmospheric tuRbulence Profiling Experiment at the Subaru Telescope) project. In this project, we develop a turbulence profiler comprising two Shack-Hartmann (SH) sensors to observe a pair of bright stars through the Subaru Telescope with high spatial sampling by 2cm subapertures. We perform two main analyses on the SH spot data: variance analysis on the spot scintillation for free atmospheric profiles, and on the spot slope for ground layer profiles. Through the three previous engineering observations, we establish a method to constrain the turbulence profile as well as the total seeing and the wind profile by analyzing data from the SH sensors. The free atmospheric profiles reconstructed by the two independent SH sensors show good agreement and are also consistent with simultaneous measurements by another profiler except for turbulence strength at ∼1km, which might be explained by an overestimation problem of scintillation-based profilers. The wind profiles show good consistency with the direct measurements by a rawinsonde. The ground layer profile results, obtained by combining data from the two SH sensors, suggest that turbulence is concentrated in the range up to ∼50m above the primary mirror. As a comparative measurement for the dome seeing, we are currently planning to install AIRFLOW, a small local turbulence sensor inside the dome.
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