Pfrommer Thomas, Hickson Paul
Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, British Columbia V6T1Z1, Canada.
J Opt Soc Am A Opt Image Sci Vis. 2010 Nov 1;27(11):A97-105. doi: 10.1364/JOSAA.27.000A97.
Observations of sodium density variability in the upper mesosphere/lower thermosphere, obtained using a high-resolution lidar system, show rapid fluctuations in the sodium centroid altitude. The temporal power spectrum extends above 1 Hz and is well-fit by a power law having a slope that is -1.95±0.12. These fluctuations produce focus errors in adaptive optics systems employing continuous-wave sodium laser guide stars, which can be significant for large-aperture telescopes. For a 30 m aperture diameter, the associated rms wavefront error is approximately 4 nm per meter of altitude change and increases as the square of the aperture diameter. The vertical velocity of the sodium centroid altitude is found to be 23 ms(-1) on a 1 s time scale. If these high-frequency fluctuations arise primarily from advection of horizontal structure by the mesospheric wind, our data imply that variations in the sodium centroid altitude on the order of tens of meters occur over the horizontal scales spanned by proposed laser guide star asterisms. This leads to substantial differential focus errors (107 nm over a 1 arc min separation with a 30 m aperture diameter) that may impact the performance of wide-field adaptive optics systems. Short-lasting and narrow sodium density enhancements, more than 1 order of magnitude above the local sodium density, occur due to advection of meteor trails. These have the ability to change the sodium centroid altitude by as much as 1 km in less than 1 s, which could result in temporary disruption of adaptive optics systems.
利用高分辨率激光雷达系统对中层顶/低热层钠密度变化的观测结果表明，钠质心高度存在快速波动。时间功率谱延伸至1 Hz以上，并且能很好地拟合为斜率为-1.95±0.12的幂律。这些波动在采用连续波钠激光导星的自适应光学系统中产生聚焦误差，对于大口径望远镜而言，这种误差可能会很显著。对于直径30 m的孔径，每米高度变化相关的均方根波前误差约为4 nm，并且随孔径直径的平方增加。钠质心高度的垂直速度在1 s时间尺度上约为23 m·s⁻¹。如果这些高频波动主要由中层风对水平结构的平流作用引起，我们的数据表明，在拟议的激光导星星群所跨越的水平尺度上，钠质心高度会出现几十米量级的变化。这会导致显著的差分聚焦误差（对于直径30 m的孔径，在1分弧度的间距上约为107 nm），可能会影响宽视场自适应光学系统的性能。由于流星余迹的平流作用，会出现持续时间短且狭窄的钠密度增强，其强度比当地钠密度高出1个多量级。这些增强能够在不到1 s的时间内使钠质心高度变化多达1 km，这可能会导致自适应光学系统暂时中断。
