ATMOSPHERIC PATH VARIATIONS FOR BASE-LINES UP TO 80 M MEASURED WITH THE SYDNEY-UNIVERSITY STELLAR INTERFEROMETER

Atmospheric turbulence introduces variations in optical path length for light from astronomical sources reaching optical instruments on the ground. These have serious implications for an optical/infrared interferometer, since they will cause variable losses in fringe visibility if not compensated for within the instrument. Theory predicts that the standard deviation of optical path length variations will increase with interferometer baseline to the 5/6 power as long as the turbulence obeys Kolmogorov statistics, but should increase less rapidly for baselines longer than the outer scale of the turbulence. Estimates of the outer scale range from a few metres to the order of kilometres. Observations made with the Sydney University Stellar Interferometer (SUSI) at baselines of 5, 20 and 80 m are reported which show a dramatic departure from the 5/6 power law even for the 5-m baseline, and which suggest an asymptotic limit to the standard deviation of optical path variations of <20 mu m for very long baselines at the SUSI site at the time of the observations.