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

TL;DR

This paper advances optical turbulence profiling techniques for the SHIMM instrument, validating methods through simulations and demonstrating high accuracy in measuring atmospheric turbulence parameters.

Contribution

It introduces new methods for Z-tilt weighting, exposure correction, and coherence time estimation, validated via Monte Carlo simulations for the SHIMM instrument.

Findings

Measurements closely match simulation inputs with high correlation.

High accuracy achieved for a four-layer turbulence model.

Sensitivity limit of Cn^2 around 2x10^-15 m^(1/3) identified.

Abstract

Atmospheric optical turbulence (OT) monitoring is crucial for site characterisation 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 the 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. All measurements of integrated OT parameters were found to be in strong agreement with the simulation inputs evidenced by correlation coefficients close to one, small RMS error and bias. The accuracy of a four-layer model was also investigated, which showed high correlation with simulation inputs for all layers even in daytime OT conditions. This study suggests a Cn^2 sensitivity limit in the region of 2x10^-15 m^(1/3) and displays evidence of a cross-talk effect between the strong ground layer and first atmospheric layer.