Exploring atmospheric optical turbulence: observations across zenith angles

We present measurements of the atmospheric optical turbulence as a function of zenith angle using two identical instruments, Shack-Hartmann Image Motion Monitors (SHIMMs), to measure atmospheric parameters concurrently. One instrument was pointed near zenith, while the other collected data by tracking a single star until it set and thus sampling zenith angles continuously to the horizon. By comparing these measurements, we can attribute changes in the atmospheric parameters to the changing zenith angle rather than variations in local turbulence conditions. The primary purpose of this experiment is to make comparisons between the measurements of the scintillation index, 𝜎²_𝐼, and Fried parameter, 𝑟₀, with current theories. In this demonstration, we find that there is a strong agreement between the models and the instrument up until zenith angles of 70∘, above which model and measurements begin to deviate. We discuss various ways in which limitations in models and our instrument may cause these deviations.