Atmospheric Turbulence Measurements with the Palomar Testbed Interferometer

Data from the Palomar Testbed Interferometer, with a baseline length of 110 mand an observing wavelength of $2.2 \mu{\rm m}$, were used to deriveinformation on atmospheric turbulence on 64 nights in 1999. The measuredtwo-aperture variance coherence times at $2.2 \mu{\rm m}$ ranged from 25 msecto 415 msec (the lower value was set by instrumental limitations---theinterferometer could not operate when the coherence time was lower than this).On all nights, the spectrum of the short time scale ($<600$ msec) delayfluctuations had a shallower spectrum than the theoretical Kolmogorov value of5/3. On most nights, the mean value of the power law slope was between 1.40 and1.50. Such a sub-Kolmogorov slope will result in the seeing improving as the$\approx 0.4$ power of wavelength, rather than the slower 0.2 power predictedby Kolmogorov theory. On four nights, the combination of delay and angle tracking measurementsallowed a derivation of the (multiple) wind velocities of the turbulent layers,for a frozen-flow model. The derived wind velocities were all $\le 4 {\rm m\s^{-1}}$, except for a small $10 {\rm m s^{-1}}$ component on one night. The combination of measured coherence time, turbulence spectral slope, andwind velocity for the turbulent layer(s) allowed a robust solution for theouter scale size (beyond which the fluctuations do not increase). On the fournights with angle tracking data, the outer scale varied from 6 to 54 m, withmost values in the 10-25 m range. Such small outer scale values cause somecomponents of visibility and astrometric errors to average down rapidly.Comment: 25 pages (including 9 figures), accepted by Ap.