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 μm, were used to derive information on atmospheric turbulence on 64 nights in 1999. The measured two-aperture variance coherence times at 2.2 μm ranged from 25 to 415 ms (the lower value was set by instrumental limitations—the interferometer could not operate when the coherence time was lower than this). On all nights, the spectrum of 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-Kolmogorov slope will result in the seeing improving as the ≈0.4 power of wavelength, rather than the slower 0.2 power predicted by Kolmogorov theory. On four nights, the combination of delay and angle-tracking measurements allowed a derivation of the (multiple) wind velocities of the turbulent layers, for a frozen-flow model. The derived wind velocities were all ≤4 m s-1, except for a small 10 m s-1 component on one night. The combination of measured coherence time, turbulence spectral slope, and wind velocity for the turbulent layer(s) allowed a robust solution for the outer scale size (beyond which the fluctuations do not increase). On the four nights with angle-tracking data, the outer scale varied from 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 average down rapidly.