Search

We measure the wavefront coherence and the irradiance fluctuations of stellar sources to obtain integrated refractive-turbulence profiles of two regions of the atmosphere. Comparison of experimental data during our measurement program shows an equal contribution of upper and lower layers to the limitation of the optical seeing. We also note the great variability of turbulence located above 3 km up to the stratosphere, from night to night. When simultaneously operated, these two methods are suitable for astronomical site testing.

Two-color single-star scintillation cross-correlation functions are estimated involving the use of two wavelength filters centered on 409 and 800 nm. From these estimations and using the atmospheric dispersion theory already proposed by Hudgin, turbulent layer heights are measured. Then, vertical Cn2Δh profiles are obtained every 2 min 45 sec using the slowly decreasing elevation of Sirius. The coherence with another method proves its interest. Furthermore, when the zenith angle of the observed star exceeds ~70°, turbulence heights smaller than 800 m (otherwise unattainable with stellar scintillation measurements) can be detected with a vertical resolution of 300 m.

The spatiotemporal cross-correlation function of single-star scintillation is estimated. From this estimation and using a priori knowledge of the theoretical shape of the correlation peaks, a method for simultaneously measuring horizontal velocity, altitude, and integrated CN2 value for each atmospheric turbulent layer between 2 and 20 km is described. Taylor’s hypothesis is tested for one particular layer and the lifetime of certain turbulent eddies is estimated. Results are in good agreement with two other methods.

Up to now only a few numerical or experimental simulations of atmospheric turbulent layers have been performed in the laboratory. These are devoted mainly to show the validity of Kolmogorov behavior but are not suitable to implement in an optical bench to test light propagation. Here we present a small size experimental simulation of an optical turbulent layer. With optical and in situ measurements, we managed to determine its characteristics: the mean variance of the refractive-index fluctuations integrated over the thickness of the turbulent flow and longitudinal and transverse structure functions of angles of arrival. From these measurements we found that the power spectrum of the refractive index is well fitted by a Von Karman function with an outer scale of 91 mm and an inner scale of 4.7 mm. Moreover, the temporal stationarity of these parameters indicates the reproducibility of this simulated turbulent flow.

Data from balloon soundings taken at sites in the Canary Islands, France, and Chile are used to show that hydrodynamic instability, perhaps engendered by the propagation of buoyancy (gravity) or other waves, leads to the formation of thin, turbulent laminae, or “seeing layers.” These seeing layers occur almost invariably in pairs and exhibit large values for the temperature-structure coefficient CT^2 because they form where the gradient of temperature is particularly steep. The refractive-index-structure coefficient is correspondingly large, and so these layers adversely affect the quality of optical propagation. The mechanism proposed here is already known to create clear air turbulence in the stratosphere, and we show how it is consistent with the formation of thin turbulent seeing layers in the troposphere and the stratosphere at night, when the atmosphere is generally stably stratified.

The small value found for the outer scale of turbulence (namely, ≤5 m) implies a narrow range of validity for application of Kolmogorov’s law to calculate the atmospheric limitations to the performance of telescopes and interferometers. Optical and radio seeing measurements are analyzed in support of a proposed turbulence spectrum which exhibits a spectral gap for scales (and hence interferometer baselines) between ∼10 and 1500 m but which obeys a 5/3 power law between 1500 and 20,000 m. The implications for forecasting the performance of optical and radio telescopes and interferometers are important.

We propose an optical technique that allows us to estimate the inner scale by measuring the variance of angle of arrival fluctuations of collimated laser beams of different sections w_i passing through a turbulent layer. To test the potential efficiency of the system, we made measurements on a turbulent air flow generated in the laboratory, the statistical properties of which are known and controlled, unlike atmospheric turbulence. We deduced a Kolmogorov behavior with a 6-mm inner scale and a 90-mm outer scale in accordance with measurements by a more complicated technique using the same turbulent channel. Our proposed method is especially sensitive to inner-scale measurement and can be adapted easily to atmospheric turbulence analysis. We propose an outdoor experimental setup that should work in less controlled conditions that can affect astronomical observations. The inner-scale assessment might be important when phase retrieval with Laplacian methods is used for adaptive optics purposes.

Statistical analysis of stellar scintillation on the pupil of a telescope, known as the scidar (scintillation, detection, and ranging) technique, is sensitive only to atmospheric turbulence at altitudes higher than a few kilometers. With the generalized scidar technique, recently proposed and tested under laboratory conditions, one can overcome this limitation by analyzing the scintillation on a plane away from the pupil. We report the first experimental implementation of this technique, to our knowledge, under real atmospheric conditions as a vertical profiler of the refractive-index structure constant C_N^2(h). The instrument was adapted to the Nordic Optical Telescope and the William Hershel Telescope at La Palma, Canary Islands. We measure the spatial autocorrelation function of double-star scintillation for different positions of the analysis plane, finding good agreement with theoretical expectations.

Two-color single-star scintillation cross-correlation functions are estimated involving the use of two wavelength filters centered on 409 and 800 nm. From these estimations and using the atmospheric dispersion theory already proposed by Hudgin, turbulent layer heights are measured. Then, vertical C^2_nΔh profiles are obtained every 2 min 45 sec using the slowly decreasing elevation of Sirius. The coherence with another method proves its interest. Furthermore, when the zenith angle of the observed star exceeds ~70°, turbulence heights smaller than 800 m (otherwise unattainable with stellar scintillation measurements) can be detected with a vertical resolution of 300 m.

The outer scale of turbulence L_0 has been calculated from values of the refractive-index structure coefficient CN2 obtained from spatio-angular correlation measurements of stellar scintillation. It is found that L_0 ≤ 5 m for a large range of observations in France, U.S.A., and Chile and that its dependence on altitude Z follows the same general form at all these sites. The prediction of CN2(Z) profiles is shown to be feasible utilizing standard meteorological radiosonde data and this L_0(Z) curve. A simple model based on dimensional analysis and a more complicated stochastic model are compared, but the latter appears to have no advantage.

The spatiotemporal cross-correlation function of single-star scintillation is estimated. From this estimation and using a priori knowledge of the theoretical shape of the correlation peaks, a method for simultaneously measuring horizontal velocity, altitude, and integrated C^2_N value for each atmospheric turbulent layer between 2 and 20 km is described. Taylor’s hypothesis is tested for one particular layer and the lifetime of certain turbulent eddies is estimated. Results are in good agreement with two other methods.

In this paper we describe a two-part experiment: measuring atmospheric turbulence both optically and thermally from 2 km up to the stratosphere. The consistency of the results shows that the optical technique may be a powerful tool for studying atmospheric turbulence from ground-based equipment.

Data obtained by two optical ground-based experiments, a scidar (scintillation detection and ranging) and a lidar, during a coordinated campaign conducted on the same site, are analyzed in terms of atmospheric dynamics. It is shown that the conjunction of the two measurements allows to characterize the parameters of a tropospheric gravity wave and thus could be of importance for the study of such phenomena.