In the field of ground-based telescopes, laser guide stars (LGS) are artificial stars formed in the sky to serve as a reference for the adaptive optics. The artificial star should have a small lateral extent as this is an important factor for how well the adaptive optics can compensate for atmospheric turbulence. The laser launch telescope (LLT) is a key component of the LGS facility. It uses an afocal system to increase the waist of the laser, therefore reducing the beam divergence and limiting the size of the star. We describe the design process of LLTs and show how a combination of two afocals with controlled defocus can be used to optimize the LGS. First, the impact of defocusing a single afocal and tuning the position of the input beam waist is presented. We then demonstrate how an intermediary afocal system can be used to vary the properties of the beam at the input of the second afocal. With such a configuration, a controlled defocus of both afocals can be performed to tune the artificial star size. Moreover, the two afocals configuration can be used to adapt the system to the amount of atmospheric perturbations affecting the beam during the upward propagation.

中文翻译：

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激光发射望远镜的无焦点组合和聚焦控制

在地基望远镜领域，激光制导星（LGS）是在天空中形成的人造星，用作自适应光学的参考。人造星的横向范围应较小，因为这是自适应光学器件可以很好地补偿大气湍流的重要因素。激光发射望远镜（LLT）是LGS设施的关键组成部分。它使用共焦系统来增加激光的腰围，因此减少了光束发散并限制了恒星的大小。我们描述了LLT的设计过程，并展示了如何使用两个散焦与可控散焦的组合来优化LGS。首先，介绍了散焦单个焦点和调整输入光束腰部位置的影响。然后，我们演示如何使用中间无焦点系统来改变第二无焦点输入处光束的特性。利用这种配置，可以执行两个焦点的受控散焦以调整人造星的大小。此外，两个无焦点配置可用于使系统适应向上传播期间影响光束的大气扰动量。