ATMOSPHERIC TURBULENCE CHARACTERISTICS INFERRED FROM OPTICAL MEASUREMENTS AT LICK OBSERVATORY

Abstract

ABSTRACT This thesis used images of stars and wavefront sensor measurements taken at Lick Observatory on Mt. Hamilton, California by the Laser Guide Star Project of Lawrence Livermore National Laboratory over the period from February, 1993 to March, 1995. The observations were a part of the development of an adaptive optics system to be used on the Shane 120 inch telescope at Lick. Speckle images were made with a CCD camera operating with a fast shutter. For wavefront measurements, a tip-tilt tracking and control loop was used to correct for tip-tilt at a point in the optical train ahead of a Shack-Hartmann lenslet array. The latter measured slopes of the tip-tilt corrected wavefront at each subaperture. Output from the wavefront sensor was used for wavefront reconstruction and subsequent driving of a deformable mirror. Long exposure images were interleaved with wavefront measurements using the same CCD camera used for speckle imaging. Characterization of the atmospheric turbulence affecting image quality is an important aspect of the design of adaptive optics systems. This thesis looked at observed correlations across the telescope aperture, correlations between points in the field of view, and temporal correlations of signals. The measurements yielded functions that can be fit to model functions parameterized by a few variables that depend on the Kolmogorov model of turbulence and upon the altitude dependence of the strength of turbulence Cn2. The parameters of interest included the value of the spatial coherence length as given by Fried's r0, the isoplanatic angle theta0 and the atmospheric coherence time tau0. Analysis included testing of two models that permit calculation of a vertical distribution of turbulence from readily available data on temperatures, pressures and wind velocities, That data is routinely gathered by National Weather Service radiosondes. The models make some suppositions about how the vertical gradients of those quantities can be related to turbulence strength. The vertical profiles so generated have much greater resolution than any model constructed from an inversion of the parameters used in fitting the optical data. These detailed profiles were used to calculate r0, theta0 and tau0. Those values were then compared with experimental data. In general, agreement with experiment was poor, possibly due to the presence of low altitude turbulence not well accounted for by the models.