This award supports measurement and analysis of the nature of fluctuations in optical paths through the atmosphere. The results should be of importance for optimizing designs for adaptive optics, for large telescopes, and for spatial interferometers operating from the ground. Measurements will be made with the Infrared Spatial Interferometer (ISI) now installed on Mount Wilson, which has already been useful in initiating such measurements. The initial measurements appear to show that for conditions encountered in ground-based astronomy, pathlength fluctuation often differ significantly from those expected on the basis of the standard model of Kolmogorov turbulence and Taylor's approximation. Furthermore, these deviations can have a significant bearing on the design of future large aperture instruments and their extension into the infrared region. The program will measure fluctuations in pathlength differences from a stellar object to the two telescopes and also fluctuations near the ground at the position of the two telescopes. These will be correlated with such parameters as wind speed, time of day, astronomical seeing, and cloud cover. The measurements will yield the structure function of time, Allan variances, and power spectra. In addition, examination of correlations between the laser interferometers and the IR interference fringe signals with various telescope separations will provide direct information on the structure function of distance. Observations over a period of time and under various conditions applicable to astronomical observations from the ground are needed in order to obtain an adequate statistical sample.
