Study of optimal wavefront sensing with elongated laser guide stars
Study of optimal wavefront sensing with elongated laser guide stars
复制标题
细长激光导星的最佳波前传感研究
DOI：
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发表时间：
2008
期刊：
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通讯作者：
V. Michau
中科院分区：
文献类型：
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作者：
Sandrine Thomas;S. Adkins;D. Gavel;T. Fusco;V. Michau
在过去的十年中，自适应光学（AO）已经成为克服大气湍流对天文成像和光谱观测影响的一种成熟方法。这些系统现在开始广泛使用激光导星（LGS）技术来提高性能并增加天空覆盖。钠LGS AO利用一个或多个波长为589 nm的激光，通过激发中间层（海拔90 km）的钠原子产生人工导星。由于依赖于中间层中钠原子的丰度和分布，这种方法有其独特的困难，这在天然恒星中是看不到的。钠层的密度和高度随时间变化，由于它在有限范围内，由于层的厚度和激光投影点与Shack - Hartmann波前传感器（SHWFS）的子孔径之间的偏移，LGS图像会被拉长。伸长率导致LGS图像被分散，导致信噪比降低，进而导致SHWFS测量误差增加，从而导致波前相位重建误差增加。为了解决伸长率问题，并提供更高水平的读出性能和降低读出噪声，目前正在开发一种新型的电荷耦合器件（CCD），用于Shack - Hartmann波前传感，称为极坐标CCD。在该装置中，在每个SHWFS子孔径中设置离散成像阵列，并调整每个离散成像阵列的大小、形状和方向，以优化LGS图像的采样。该装置被称为极坐标CCD，因为每个成像仪的位置是由以激光导星投影点为中心的极坐标系统定义的。这个概念特别适用于超大望远镜（elt），其中视角延伸的影响是一个重要因素。在本文中，我们通过评估质心误差方差以及与LGS图像采样和截断相关的线性问题来评估基于该CCD几何形状的质心仪的性能。我们还描述了我们将如何扩展这项工作，以解决钠层的时间变异性所带来的问题，以及这将如何影响LGS AO系统中的SHWFS性能。
Over the past decade, adaptive optics (AO) has become an established method for overcoming the effects of atmospheric turbulence on both astronomical imaging and spectroscopic observations. These systems are now beginning to make extensive use of laser guide star (LGS) techniques to improve performance and provide increased sky coverage. Sodium LGS AO employs one or more lasers at 589-nm wavelength to produce an artificial guide star through excitation of sodium atoms in the mesosphere (90 km altitude). Because of its dependence on the abundance and distribution of sodium atoms in the mesosphere, this approach has its own unique set of difficulties not seen with natural stars. The sodium layer exhibits time-dependent variations in density and altitude, and since it is at a finite range, the LGS images become elongated due to the thickness of the layer and the offset between the laser projection point and the subapertures of a Shack‐Hartmann wavefront sensor (SHWFS). Elongation causes the LGS image to be spread out resulting in a decrease in the signal-to-noise ratio which, in turn, leads to an increase in SHWFS measurement error and therefore an increased error in wavefront phase reconstruction. To address the problem of elongation, and also to provide a higher level of readout performance and reduced readout noise, a new type of charge-coupled device (CCD) is now under development for Shack‐Hartmann wavefront sensing called the polar coordinate CCD. In this device, discrete imaging arrays are provided in each SHWFS subaperture and the size, shape and orientation of each discrete imaging array are adjusted to optimally sample the LGS image. The device is referred to as the polar coordinate CCD because the location of each imager is defined by a polar coordinate system centred on the laser guide star projection point. This concept is especially suited to Extremely Large Telescopes (ELTs) where the effect of perspective elongation is a significant factor. In this paper, we evaluate the performance of centroiders based on this CCD geometry by evaluating the centroid error variance and also the linearity issues associated with LGS image sampling and truncation. We also describe how we will extend this work to address the problems presented by the time variability of the sodium layer and how this will impact SHWFS performance in LGS AO systems.