Atmospheric turbulence measurements with the Palomar Testbed Interferometer

Data from the Palomar Testbed Interferometer, with a baseline length of 110 m and an observing wavelength of 2.2 mum, were used to derive information on atmospheric turbulence on 64 nights in 1999. The measured two-aperture v ariance coherence times at 2.2 mum ranged from 25 to 415 ms (the lower valu e was set by instrumental limitations-the interferometer could not operate when the coherence time was lower than this). On all nights, the spectrum o f the short-timescale (less than 600 ms) delay fluctuations had a shallower spectrum than the theoretical Kolmogorov value of 5/3. On most nights, the mean value of the power-law slope was between 1.40 and 1.50. Such a sub-Ko lmogorov slope will result in the seeing improving as the approximate to0.4 power of wavelength, rather than the slower 0.2 power predicted by Kolmogo rov theory. On four nights, the combination of delay and angle-tracking mea surements allowed a derivation of the (multiple) wind velocities of the tur bulent layers, for a frozen-flow model. The derived wind velocities were al l less than or equal to4 m s(-1), except for a small 10 m s(-1) component o n one night. The combination of measured coherence time, turbulence spectra l slope, and wind velocity for the turbulent layer(s) allowed a robust solu tion for the outer scale size (beyond which the fluctuations do not increas e). On the four nights with angle-tracking data, the outer scale varied fro m 6 to 54 m, with most values in the 10-25 m range. Such small outer scale values cause some components of visibility and astrometric errors to averag e down rapidly.