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Data taken with the Infrared Spatial Interferometer (ISI) at Mt. Wilson have been analyzed to characterize atmospheric pathlength fluctuations, in preparation for astrometric work. In this paper we will discuss two features observed in recent astrometric phase time series data: 1) The presence of non-random fluctuations, and 2) the correlation of pathlength variations near the ground with those along the path to the stars.
The data analysis indicates that fluctuations in pathlength through the entire atmosphere at 11 $\mu$m as well as fluctuations near the ground show substantial deviations from the Kolmogorov-Taylor model with the commonly assumed large ($\gg$1 km) outer scale. Under excellent seeing conditions, the ISI astrometric phase structure functions are consistent with an outer scale in the range of 5 -- 20 m. Generally the results indicate that large-aperture telescopes and long baseline interferometers, particularly at IR wavelengths, will likely perform better than is expected on the basis of the Kolmogorov-Taylor model.
Under certain conditions, spikes are observed in the phase time series which are consistent with brief excursions of the index of refraction of air toward smaller values. The presence of such spikes could partially account for discrepancies in slopes between structure functions and power spectra of phase fluctuations as observed with the ISI.
A high correlation ($\approx$0.6) between the atmospheric fluctuations within the telescope optics and those along the path to the star has been observed for some of the ISI data. A statistical model of the fluctuations has been developed to optimally utilize this correlation in the application of ground-based calibrations to the stellar data. A byproduct of this model is a calculation of the height of the dominant atmospheric turbulence above Mt. Wilson, based on the observed ground-based/stellar correlation. This height is in the range of 10 -- 20 meters, consistent with the small outer scales inferred from structure-function analyses.
