Mikhail Belenkii (Trex Enterprises Corporation), Edward Cuellar (Trex Enterprises Corporation), Kevin A. Hughes (Trex Enterprises Corporation), Vincent A. Rye (Trex Enterprises Corporation)
Keywords: Adaptive Optics
Abstract:
We investigated the spatial structure of atmospheric turbulence at Maui Space Surveillance Site (MSSS) using a 3.6 m telescope and a spatial filtering receiver and imaging selected stars simultaneously through four pupil masks representing aperture diameters of 0.1 m, 0.5m, 1.5 m, and 3.6 m. In our optical setup, four star images were recorded simultaneously on one camera frame. Multiple data sets of short-exposure star images were collected during six nights at frame rates ranging from 100 Hz to 285 Hz. The camera orientation was determined for each data set by moving the telescope at a given angle in azimuth and elevation. The horizontal and vertical components of the image centroid were calculated. The spatial and temporal statistics of the horizontal and vertical wavefront tilt, as well as parameters of the long-exposure star images, were determined as a function of the aperture diameter, azimuth, and zenith angle. In addition, the Fried parameter, the outer scale of turbulence, and the turbulence anisotropy coefficient, defined as a ratio of the variance of horizontal wavefront tilt to variance of the vertical tilt, were retrieved from the star imagery data.
We found that the atmospheric wavefront tilt is anisotropic. On four nights we observed that the on-axis horizontal tilt variance exceeded the vertical tilt variance by a factor of 1.3 to 3.5. We believe that this is due to anisotropy of large-scale turbulence, where the horizontal scale of the turbulent inhomogeneities exceeds their vertical scale. The estimates of the horizontal and vertical turbulence outer scale confirmed this conclusion. In addition, in several data sets, the horizontal image spot diameter in the long-exposure star image exceeded the vertical image spot diameter. We also found that the anisotropy coefficient of the wavefront tilt increased linearly with the aperture diameter. This is because large-scale turbulent inhomogeneities are anisotropic, whereas small-scale inhomogeneities are isotropic. When the telescope diameter increases, the contribution of small-scale isotropic turbulence to the image centroid reduces, whereas the contribution of large-scale anisotropic turbulence increases. Also, in the case of isotropic turbulence, we found that the power spectral densities (PSDs) of the wavefront tilt are consistent with theoretical models. In the case of anisotropic turbulence, the PSDs of the horizontal and vertical tilt components exhibit different behavior. The telescope vibration modes were observed at 15 Hz and 20 Hz. The anisotropy of turbulence and wavefront tilt should be considered in design and performance analysis of optical trackers.
Date of Conference: September 10-14, 2006
Track: Adaptive Optics