Expected performance of the Pyramid wavefront sensor with a laser guide star for 40 m class telescopes,Astronomy & Astrophysics

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Expected performance of the Pyramid wavefront sensor with a laser guide star for 40 m class telescopes
Astronomy & Astrophysics ( IF 5.4 ) Pub Date : 2024-05-24 , DOI: 10.1051/0004-6361/202348691
F. Oyarzún , V. Chambouleyron , B. Neichel , T. Fusco , A. Guesalaga

Context. The use of artificial laser guide stars (LGS) is planned for the new generation of giant segmented mirror telescopes in order to extend the sky coverage of their adaptive optics systems. The LGS, being a 3D object at a finite distance, will have a large elongation that will affect its use with the Shack–Hartmann (SH) wavefront sensor.Aims. In this paper, we compute the expected performance for a Pyramid WaveFront Sensor (PWFS) using an LGS for a 40 m telescope affected by photon noise, and also extend the analysis to a flat 2D object as reference.Methods. We developed a new way to discretize the LGS, and a new, faster method of propagating the light for any Fourier filtering wavefront sensors (FFWFS) when using extended objects. We present the use of a sensitivity model to predict the performance of a closed-loop adaptive optic system. We optimized a point-source-calibrated interaction matrix to accommodate the signal of an extended object by computing optical gains using a convolutional model.Results. We find that the sensitivity drop, given the size of the extended laser source, is large enough to make the system operate in a low-performance regime given the expected return flux of the LGS. The width of the laser beam is identified as the limiting factor, rather than the thickness of the sodium layer. Even an ideal, flat LGS will have a drop in performance due to the flux of the LGS, and small variations in the return flux will result in large variations in performance.Conclusions. We conclude that knife-edge-like wavefront sensors, such as the PWFS, are not recommended for use with LGS for a 40 m telescope, as they will operate in a low-performance regime, given the size of the extended object.

中文翻译：

用于 40 m 级望远镜的带有激光导星的金字塔波前传感器的预期性能

语境。新一代巨型分段镜面望远镜计划使用人造激光导星（LGS），以扩大其自适应光学系统的天空覆盖范围。 LGS 是有限距离内的 3D 物体，将具有较大的伸长率，这将影响其与 Shack–Hartmann (SH) 波前传感器的使用。在本文中，我们使用 LGS 计算受光子噪声影响的 40 m 望远镜的金字塔波前传感器 (PWFS) 的预期性能，并将分析扩展到平面 2D 物体作为参考。我们开发了一种离散化 LGS 的新方法，以及一种在使用扩展对象时为任何傅立叶滤波波前传感器 (FFWFS) 传播光的更快的新方法。我们提出使用灵敏度模型来预测闭环自适应光学系统的性能。我们通过使用卷积模型计算光学增益来优化点源校准交互矩阵，以适应扩展物体的信号。结果。我们发现，考虑到扩展激光源的尺寸，灵敏度下降足够大，足以使系统在 LGS 预期返回通量的低性能状态下运行。激光束的宽度被认为是限制因素，而不是钠层的厚度。即使是理想的平坦 LGS，也会因 LGS 的通量而导致性能下降，并且返回通量的微小变化将导致性能的较大变化。结论。我们得出的结论是，不建议将刀刃状波前传感器（例如 PWFS）与 LGS 一起用于 40 m 望远镜，因为考虑到扩展物体的尺寸，它们将在低性能状态下运行。