Multi-conjugated adaptive optics (MCAO) is essential for performing astrometry with the Extremely Large Telescope (ELT). Unlike most of the 8-m class telescopes, the ELT will be a fully adaptive telescope, and a significant portion of the adaptive optics (AO) dynamic range will be depleted by the correction and stabilization of the telescope aberrations and instabilities. MCAO systems are of particular interest for ground-based astrometry since they stabilize the low-order field distortions and transient plate scale instabilities, which originate from the telescope and in the instrument. All instruments have several optical elements relatively far away from the pupil that can potentially challenge the astrometric precision of the observations with their residual mid-spatial frequencies errors. Using a combined simulation of ray tracing and AO numerical codes, we assess the impact of these systematic errors at different field-of-view (FoV) scales and fitting scenarios. The distortions have been assessed at different sky position angles (PA) and indicate that over large FoVs only small PA ranges (±1 deg to 3 deg) are accessible with astrometric residuals ≤50 μas. A full compliance with the astrometric requirement, at any PA, is achievable for 2 arc sec2 FoV patches already with a third-order polynomial. The natural partition of the optical system into three segments, i.e., the ELT, the MAORY MCAO module, and the MICADO instrument, resembles a splitting of the astrometric problem into the three subsystems that are characterized by different distortion amplitudes and calibration strategies. The result is a family portrait of the different optical segments with their specifications, dynamic motions, conjugation height, and AO correctability, leading to tracing their role in the bigger puzzle of the 50-μas as astrometric endeavor.

中文翻译：

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使用 ELT 和 MCAO 进行 50 μas 天体测量的性能和局限性

多共轭自适应光学 (MCAO) 对于使用超大望远镜 (ELT) 进行天体测量至关重要。与大多数 8 米级望远镜不同，ELT 将是一个完全自适应的望远镜，自适应光学 (AO) 动态范围的很大一部分将因望远镜像差和不稳定性的校正和稳定而耗尽。MCAO 系统对地基天体测量特别感兴趣，因为它们可以稳定源自望远镜和仪器的低阶场失真和瞬态板块尺度不稳定性。所有仪器都有几个距离瞳孔相对较远的光学元件，它们的残余中空间频率误差可能会挑战观测的天体测量精度。使用光线追踪和 AO 数值代码的组合模拟，我们评估了这些系统误差在不同视场 (FoV) 尺度和拟合场景下的影响。已经在不同的天空位置角 (PA) 评估了失真，并表明在大视场上，只有小 PA 范围（±1 度到 3 度）可访问，天体测量残差≤50 μas。对于已经使用三阶多项式的 2 arc sec2 FoV 贴片，可以在任何 PA 上完全符合天体测量要求。光学系统自然划分为三个部分，即 ELT、MAORY MCAO 模块和 MICADO 仪器，类似于将天体测量问题拆分为三个子系统，这些子系统的特征在于不同的失真幅度和校准策略。