Vertical Path Atmospheric Turbulence: Temporal Spectra and Differential Image Motion Measurements

The familiar "twinkling" of starlight is caused by atmospheric turbulence, which is characterized by variations in the index of refraction of the atmosphere from one turbulent cell to the next. As emerging fields such as adaptive optics and high resolution digital imaging continue to progress, there is a strong interest in quantifying the turbulent effects. Lacking the ability to measure fundamental quantities such as the index of refraction in situ over vertical propagation paths, we turn to a parameter known as the transverse coherence length, or r_ {rm o}, to quantify the turbulent effects. A means of determining r_{ rm o}, (which in a rough sense is a measure of the quality of the atmosphere as an optical transmission medium) by measuring differential stellar image motion is presented. Particular attention is paid to the effects of varying the sampling parameters of the detection system used to acquire the data (particularly the frame rate and exposure time) in order to establish suitable bounds for these data acquisition parameters. In addition, the measurements are presented in the form of power spectral density plots, which provide crucial temporal frequency content information for adaptive optics system designers. The power spectral density analysis also suggests that numerous layers within the atmosphere are contributing to the overall image degradation encountered with ground based imaging systems.