Measuring parameters of ground-level atmospheric turbulence via Fourier telescopy

摘要

Scientists continually seek to improve atmospheric turbulence models. Employing Fourier telescopy techniques, we have assessed the effect of humidity, temperature, atmospheric pressure, and airflow velocity on horizontal-path, ground-level turbulence. The measurements were made at different times of day. Turbulence parameters investigated include C_n~2, the scintillation index σ_I~2, and the inner scale l_o. The results showed temporal movement patterns of the turbulence to be consistent with Taylor's frozen turbulence theory. We plan to compare the results of these outdoor measurements with measurements conducted indoors with an optical turbulence generator. Fourier telescopy measurements rely on the distant interference of two mutually-coherent and frequency-offset laser beams, the resulting moving interference fringe pattern "beating" against a fixed grating, such as a Ronchi ruling, and yielding a photo-detected signal that can be analyzed statistically to infer turbulence parameters. Preliminary study suggests that the Fourier telescopy-based measurements can provide more information on turbulence parameters than can measurements made with single laser beams. The basic experimental setup will be described, along with first results of the experiments.