Single-star optical turbulence profiling techniques for SHIMM and other Shack–Hartmann instruments

Atmospheric optical turbulence (OT) monitoring is crucial for site characterization at astronomical observatories and optical communications ground stations. The Shack–Hartmann image motion monitor (SHIMM) instrument implements a fast, infrared Shack–Hartmann sensor to measure a low-resolution OT profile continuously throughout the day and night. This work presents advances made in Shack–Hartman optical turbulence profiling techniques implemented on the SHIMM, including a derivation and validation of Z-tilt weighting functions, implementation of methods for correcting for non-zero exposure times, and for estimating the coherence time of optical turbulence using the profile coupled with the fast defocus method. These techniques were tested via end-to-end Monte Carlo simulations of the SHIMM instrument using real turbulence profiles from the Paranal stereo-SCIDAR instrument with an augmented ground layer. All measurements of integrated OT parameters were in strong agreement with the simulation inputs, evidenced by correlation coefficients close to one, small RMS error, and bias. The accuracy of the four-layer SHIMM model was also investigated, which showed high correlation with simulation inputs for all layers even in daytime OT conditions. This study suggests that, for the turbulence database used, and under realistic daytime noise conditions, a C n 2 ( h )d h sensitivity limit in the region of 2×10 −15 m 1/3 was encountered in the highest altitude layer. There was also evidence of a cross-talk effect between the strong ground layer and the first atmospheric layer.