Asgard/NOTT (previously Hi-5) is a European Research Council (ERC)-funded project hosted at KU Leuven and a new visitor instrument for the Very Large Telescope Interferometer (VLTI). Its primary goal is to image the snow line region around young stars using nulling interferometry in the L′-band (3.5 to 4.0) μm, where the contrast between exoplanets and their host stars is advantageous. The breakthrough is the use of a photonic beam combiner, which only recently allowed the required theoretical raw contrast of 10−3 in this spectral range. Nulling interferometry observations of exoplanets also require a high degree of balancing between the four pupils of the VLTI in terms of intensity, phase, and polarization. The injection into the beam combiner and the requirements of nulling interferometry are driving the design of the warm optics and the injection system. The optical design up to the beam combiner is presented. It offers a technical solution to efficiently couple the light from the VLTI into the beam combiner. During the coupling, the objective is to limit throughput losses to 5% of the best expected efficiency for the injection. To achieve this, a list of different loss sources is considered with their respective impact on the injection efficiency. Solutions are also proposed to meet the requirements of beam balancing for intensity, phase, and polarization. The different properties of the design are listed, including the optics used, their alignment and tolerances, and their impact on the instrumental performances in terms of throughput and null depth. The performance evaluation gives an expected throughput loss <6.4% of the best efficiency for the injection and a null depth of ∼2.10−3, mainly from optical path delay errors outside the scope of this work.

中文翻译：

---

Asgard/NOTT：VLTI 的 L 波段调零干涉测量。二.温光设计及注入系统

Asgard/NOTT（以前称为 Hi-5）是欧洲研究委员会 (ERC) 资助的项目，在鲁汶大学主办，是甚大望远镜干涉仪 (VLTI) 的新型访客仪器。其主要目标是使用 L' 波段（3.5 至 4.0）μm 的归零干涉仪对年轻恒星周围的雪线区域进行成像，其中系外行星与其宿主恒星之间的对比度是有利的。突破在于使用光子合束器，直到最近才允许在此光谱范围内实现所需的 10−3 的理论原始对比度。系外行星的归零干涉测量还需要 VLTI 的四个光瞳在强度、相位和偏振方面实现高度平衡。光束组合器的注入和归零干涉测量的要求正在推动热光学器件和注入系统的设计。介绍了光束组合器的光学设计。它提供了一种技术解决方案，可将来自 VLTI 的光有效耦合到合束器中。在耦合过程中，目标是将吞吐量损失限制在最佳预期注射效率的 5% 以内。为了实现这一目标，需要考虑一系列不同的损耗源及其各自对注入效率的影响。还提出了满足光束平衡强度、相位和偏振要求的解决方案。列出了设计的不同属性，包括所使用的光学器件、它们的对准和公差，以及它们对吞吐量和零点深度方面的仪器性能的影响。性能评估给出了预期的吞吐量损失 < 注入最佳效率的 6.4% 和 ∼2.10−3 的零深度，主要来自本工作范围之外的光路延迟误差。