Vertical Scales of Turbulence at the Mt. Wilson Observatory

The vertical scales of turbulence at the Mt. Wilson observatory are inferred from data from the University of Califcmnia at Berkeley Infrared Spatial Interferometer (1S1), by modeling path length fluctuations observed in the interferometric paths to celestial objects and those in instrumental ground-based paths. The correlations between the stellar and ground-based path length fluctuations and the temporal statistics of those fluctuations are modeled on various time scales to constrain the vertical scales. A Kolmogorov-Taylor turbulence model with a finite outer scale was used to simulate 1S1 data. The simulation also included the white thermal noise of the interferometer, aperture filtering effects, and the data analysis algorithms. The simulations suggest that the path delay fluctuations observed in the 1992-3 1S1 data are largely consistent with being generated by refractivity fluctuations at two characteristic vertical scales: one extending to a height of 45 m above the ground, with a wind speed of about 1 m/see, and another at a much higher altitude, with a wind speed of about 10 m/see. The height of the lower layer is of the order of the height of trees and other structures near the interferometer, which suggests that these objects, including elements of the interferometer, may be responsible for generating the lower layer of turbulence. The modeling suggests that the high-altitude component contributes primarily to short-period (< 10 seconds) fluctuations, while the lower component dominates the long-period (up to a few minutes) fluctuations. Simulations further show that there is the potential for improving the seeing or astrometric accuracy by about 30-50910 on average, if the path length fluctuations in the lower component are directly calibrated. Statistical and systematic effects induce an error of about 15 m in the estimate of the lower component turbulent altitude. Subject headings: turbulent atmospheric height, interferometry, astrometry