Atmospheric fluctuations - Empirical structure functions and projected performance of future instruments

Observations of path length fluctuations from a star to a two-telescope spatial interferometer at 11-micron wavelength, and also measurements of path length fluctuations 3 m above the ground by laser distance interferometers are presented. They show substantial deviations from theoretical models such as Kolmogorov turbulence combined with Taylor's 'frozen atmosphere' swept past the observer by winds. It is found that large-aperture telescopes or long baseline interferometry, particularly for IR wavelengths, will often provide better imaging than is expected on the basis of the common assumption that relative fluctuations in path lengths through the atmosphere increase with the 5/3 power of their separation. The results are also favorable for adaptive optics.