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arXiv 2001.06844astro-ph.IM
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用laser导星测量视线钠密度结构

Measuring line-of-sight sodium density structure using laser guide stars

Joschua A. Hellemeier, Domenico Bonaccini Calia, Paul Hickson, Angel Otarola, Thomas Pfrommer

摘要:

采用钠激光导引星的自适应光学系统,通过沿视线方向连续监测中间层钠的垂直密度结构,可以提高系统的性能。我们证明了钠密度分布可以恢复连续波(CW)激光的振幅调制。在大天顶望远镜(LZT)进行的一项实验中,ESO的Wendelstein拉曼光纤激光器用伪随机二进制序列进行幅度调制,并通过调制模式与观察到的激光引导星返回信号的互相关获得轮廓。为了进行比较,同时使用LZT的激光雷达系统获得了高分辨率的剖面。通过这两种技术获得的配置文件显示噪声污染,但被发现同意的测量误差内。作为进一步的核对,还将几个激光雷达剖面图与使用远程望远镜从侧面对激光羽流成像同时进行观测所获得的剖面图进行了比较。调制连续波激光雷达技术可以通过将一小部分返回的激光转移到光子计数探测器来实现。理论分析和数值模拟表明,对于50%的调制强度,钠质心高度可以检索每5秒从一个单一的激光引导星,与精度,这将导致相应的波前误差为50 nm的ELT和小于30 nm的TMT和GMT。如果采用多个激光引导星,则所需的调制幅度将较小。

英文摘要:

The performance of adaptive optics systems employing sodium laser guide stars can be improved by continuously monitoring the vertical density structure of mesospheric sodium along the line of sight. We demonstrate that sodium density profiles can be retrieved by amplitude modulation of continuous wave (CW) lasers. In an experiment conducted at the Large Zenith Telescope (LZT), ESO's Wendelstein Raman-fibre laser was amplitude-modulated with a pseudo-random binary sequence and profiles were obtained by cross-correlation of the modulation pattern with the observed return signal from the laser guide star. For comparison, high-resolution profiles were obtained simultaneously using the lidar system of the LZT. The profiles obtained by the two techniques show noise contamination, but were found to agree to within the measurement error. As a further check, a comparison was also made between several lidar profiles and those obtained by simultaneous observations using a remote telescope to image the laser plume from the side. The modulated CW lidar technique could be implemented by diverting a small fraction of the returned laser light to a photon counting detector. Theoretical analysis and numerical simulations indicate that, for 50 per cent modulation strength, the sodium centroid altitude could be retrieved every 5 s from a single laser guide star, with an accuracy which would induce a corresponding wavefront error of 50 nm for the ELT and less than 30 nm for the TMT and GMT. If multiple laser guide stars are employed, the required modulation amplitude will be smaller.

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