Abstract. The measurement of the strength and velocity of atmospheric optical turbulence using a generalised SCIDAR technique is outlined and demonstrated. This method allows the full turbulent profile to be characterised from the telescope pupil up to any desired altitude. A number of example profiles from various astronomical observing sites are presented.
Experimental results indicate that the statistics of phase measured across a telescope aperture do not always obey the power laws associated with the Kolmogorov model of atmospheric turbulence. We show that the statistical relations between a wave front and its aperture-averaged f irst derivative previously derived for a Kolmogorov spectrum can be easily generalized for any power law. We also show that a Shack-Hartmann sensor can be used to measure the form of the structure function of phase f luctuations, and experimental data are presented.
The structure function of atmospheric phase fluctuations is defined as follows: A generalised model for the structure function of phase is: ℛ
0
is related to the size of the long-exposure image formed by a large-aperture telescope, while γ
ß
is a parameter which depends on ß and on the precise definition of ℛ
0
The widely-used Kolmogorov model of refractive index fluctuations [1] predicts a value of 11/3 for ß; in this case, with the appropriate definition, ℛ0 is equivalent to r0, the Fried parameter [2], Under the Kolmogorov assumption, the parameter r0 entirely describes the time-averaged statistics of the phase fluctuations. A knowledge of r0 can be used, for example, to predict how the energy in the phase fluctuations is distributed between the Zernike modes.
Atmospheric turbulence strength profile measurements are presented for a number of astronomical observation sites. These results were obtained using the generalised SCIDAR (SCIntillation, Distance And Ranging) technique which allows both the turbulence strength profile and the velocity of the turbulent layers to be determined as a function of height.
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