Measuring the orbits of directly imaged exoplanets requires precise astrometry at the milliarcsec level over long periods of time due to their wide separation to the stars (≳10 au) and long orbital period (≳20 yr). To reach this challenging goal, a specific strategy was implemented for the instrument Spectro-Polarimetric High-contrast Exoplanet Research (SPHERE), the first dedicated exoplanet imaging instrument at the Very Large Telescope of the European Southern Observatory (ESO). A key part of this strategy relies on the astrometric stability of the instrument over time. We monitored for five years the evolution of the optical distortion, pixel scale, and orientation to the True North of SPHERE images using the near-infrared instrument IRDIS. We show that the instrument calibration achieves a positional stability of ∼1 mas over 2″ field of views. We also discuss the SPHERE astrometric strategy, issues encountered in the course of the on-sky operations, and lessons learned for the next generation of exoplanet imaging instruments on the Extremely Large Telescope being built by ESO.

中文翻译：

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SPHERE对下一代系外行星成像仪器的天体测量策略的经验教训

测量直接成像的系外行星的轨道需要长时间在毫弧秒级进行精确的天体测量，因为它们与恒星之间的距离很远（≳10 au），并且轨道周期很长（≳ 20 年）。为了实现这一具有挑战性的目标，光谱偏振高对比度系外行星研究 (SPHERE) 仪器实施了一项特定策略，这是欧洲南方天文台 (ESO) 甚大望远镜中的第一台专用系外行星成像仪器。该策略的一个关键部分依赖于仪器随时间推移的天体测量稳定性。五年来，我们使用近红外仪器 IRDIS 监测了光学畸变、像素尺度和 SPHERE 图像真北方向的演变。我们表明，仪器校准在 2 英寸视场内实现了 ∼1 mas 的位置稳定性。