We investigate the astrometric performance of the FORS1 and FORS2 cameras of the VLT at long time scales with emphasis on systematic errors which normally prevent attainning a precision better than 1mas. The study is based on multi- epoch time series of observations of a single sky region imaged with a time spacing of 2-6 years at FORS1 and 1-5 months at FORS2. We performed a detailed analysis of a random error of positions that was shown to be dominated by the uncertainty of the star photocenter determination. The component of the random error corresponding to image motion was found to be caused primarily by optical aberrations and variations of atmospheric PSF size but not by the effect of atmospheric image motion. Comparison of observed and model annual/monthly epoch average positions yielded estimates of systematic errors for which temporal properties and distribution in the CCD plane are given. At frame center, the systematic component is about 25 mu-as. Systematic errors are shown to be caused mainly by a combined effect of the image asymmetry and seeing variations which therefore should be strongly limited to avoid generating random and systematic errors. For a series of 30 images, we demonstrated presicion of about 50 mu-as stable on daily, monthly, and annual time scales. Relative proper motion and trigonometric parallaxes of stars in the center of the test field were derived with a precision of 20 mu-as/yr and 40 mu-as for 17-19 mag stars.
Constraining the Orbit and Mass of epsilon Eridani b with Radial Velocities, Hipparcos IAD-Gaia DR2 Astrometry, and Multi-epoch Vortex Coronagraphy Upper Limits
Bayesian analysis of Gaia epoch astrometry and radial velocities with kima
Tsung-Dao Lee Institute, Shanghai Jiao Tong University（上海交大李政道研究所）；School of Physics and Astronomy, University of Birmingham（伯明翰大学物理与天文学院）；Observatoire Astronomique de l’Université de Genève（日内瓦大学天文台）
K-Stacker, an algorithm to hack the orbital parameters of planets hidden in high-contrast imaging. First applications to VLT SPHERE multi-epoch observations
A pilot study on the CSST astrometric capability: Detecting astrometric binaries with Gaia synergy via simulated data
Shanghai Astronomical Observatory, Chinese Academy of Sciences（中国科学院上海天文台）；School of Astronomy and Space Sciences, University of Chinese Academy of Sciences（中国科学院大学天文与空间科学学院）
Exomoon search with VLTI/GRAVITY around the substellar companion HD 206893 B
Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS（巴黎天文台，PSL大学，索邦大学，巴黎西岱大学，塞吉-蓬图瓦兹大学，法国国家科学研究中心）；Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University（西北大学天体物理学跨学科探索与研究中心及物理与天文学系）；European Southern Observatory（欧洲南方天文台）；Department of Physics & Astronomy, Johns Hopkins University（约翰斯·霍普金斯大学物理与天文学系）；Space Telescope Science Institute（太空望远镜科学研究所）
Parallaxes, Proper Motions, and Near-Infrared Photometry for 173 L and T Dwarfs From The US Naval Observatory Infrared Astrometry Program
United States Naval Observatory（美国海军天文台）；Flagstaff Dark Skies Coalition（弗拉格斯塔夫黑暗天空联盟）；L-3 Communications/Brashear（L-3通信/布莱尔）；Gemini Observatory/NSF’s NOIRLab（吉米尼天文台/美国国家科学基金会NOIR实验室）；California Institute of Technology (IPAC)（加州理工学院（IPAC））；Center for Astrophysics and Space Science, University of California San Diego（加州大学圣地亚哥分校天体物理与空间科学中心）
High-Precision Radio and Infrared Astrometry of LSPM J1314+1320AB - I: Parallax, Proper Motions, and Limits on Planets
利用甚长基线射电天文台阵（VLBA）对年轻的超冷矮双星LSPM J1314+ 1320 AB进行了多历元射电天体测量观测。射电辐射来自第二颗恒星。结合VLBA数据和凯克近红外自适应光学观测的两个组成部分，视差（$π_{\rm abs}=57.975\pm0.045$ mas，对应于$d=17.249\pm0.013$ pc的距离），自行（$μ_{\rm αcos δ}=-247.99\pm0.10$ mas yr $^{-1}$，$μ_δ=-183.58\pm0.22$ mas yr $^{-1}$），和轨道运动的完整的天体测量拟合得到。尽管这两颗恒星的质量在$\pm2$ %内几乎相同，但次级恒星的射电辐射超过了初级恒星的$\gtrsim$ 30倍，这表明恒星自转历史的差异，这可能导致不同的磁场配置。或者，发射可以是各向异性的，并向我们发射次级辐射，而不是初级辐射。仅使用反射运动，我们排除了质量为0.7至10 $M_{\rm jup}$、轨道周期分别为600至10天的行星。此外，我们使用完整的轨道解决方案的二进制导出的上限为0.23 AU的半长轴稳定的行星轨道在这个系统中。这些限制涵盖了一个参数空间，这是无法访问的，并补充，近红外径向速度调查的超冷矮星。我们的绝对天体测量将构成盖亚天体测量校准的重要测试。
