Interferometric seeing measurements on Mt Wilson: power spectra and outer scales

Abstract

We have measured power spectra of atmospheric phase fluctuations with the Mark III stellar interferometer on Mt. Wilson under a wide variety of seeing conditions. On almost all nights, the high-frequency portions of the temporal power spectra closely follow the form predicted by the standard Kolmogorov-Tatarski model. At lower frequencies, a variety of behavior is observed. On a few nights, the spectra clearly exhibit the low-frequency flattening characteristic of turbulence with an outer-scale length of the order of 30 m. On other nights, examination of individual spectra yields no strong evidence of an outer scale less than a few kilometers in size, but comparison of the spectra on different interferometer baselines shows a saturation of the spatial structure function on long baselines. This saturation is consistent with the assumption of an outer-scale length similar to that derived for the nights when low-frequency flattening of the spectra is clearly seen. We discuss possible explanations of this behavior and conclude that power spectra from a single interferometer baseline are a poor diagnostic for the effective outer scale compared with multiple-baseline spectra.

Optical Interferometric Polarimetry. II. Theory

“…V obs (t c ; p) is insensitive to scintillation (Buscher et al 1995), variable extinction, and variable detector efficiency because they affect I sig 0 (t f ; p) and I sig 12 (t f ; p) in the same manner. However, the variability of K(t c ), due to variable seeing and fringe/angletracking errors (Paper I), adversely affects V obs (t c ; p).…”

Real-time optical path difference compensation at the Plateau de Calern I2T interferometer

“…Considering the von-Karman model, it can be shown that the theoretical temporal power spectrum of the OPD fluctuations (Buscher et al 1995; can be divided into different cases as plotted in Fig. C.1: -At very low frequencies, f < f 0 = v/L 0 where v is the average wind speed, the spectrum flattens to a f 0 power law as the result of the finite outer scale; -At low frequency, f 0 < f < f 1 = 0.2v/B, the conventional Kolmogorov f −2/3 law holds.…”

“…Following Buscher (1995), the coherence time t 0 can be deduced from the vertical position of the f −8/3 asymptote of the power spectrum W Φ , since:…”

“…The performance evaluations are coupled to measurements of the spatial and temporal characteristics of the turbulence. Atmospheric measurements previously were made by other stellar interferometers (Mariotti & Di Benedetto 1984;Bester et al 1992;Buscher et al 1995;Davis et al 1995).…”

Adaptive optics performance model for optical interferometry

“…Optical interferometry is limited by the spatial and temporal dimensions of atmospheric turbulence, characterized by r 0 , the coherence length defined by Fried [4] and t 0 , the atmospheric coherence time (we adopt the definition advocated by Buscher [5]). Once the collecting aperture or the integration time exceeds a few times r 0 or t 0 , the interference fringe signal-to-noise ratio S begins to decrease.…”