We present an investigation of the stability of the two chips in the FORS2 camera CCD mosaic on the basis of astrometric observations of stars in 20 sky fields, some of which were monitored for four to seven years. We detected a smooth relative shear motion of the chips along their dividing line that is well approximated by a cubic function of time with an amplitude that reaches ~0.3 pixels (px) or ~38 mas over seven years. In a single case, we detected a step change of ~0.06 px that occurred within four days. In the orthogonal direction that corresponds to the separation between the chips, the motion is a factor of 5-10 smaller. This chip instability in the camera significantly reduces the astrometric precision when the reduction uses reference stars located in both chips, and the effect is not accounted for explicitly. We found that the instability introduces a bias in stellar positions with an amplitude that increases with the observation time span. When our reduction methods and FORS2 images are used, it affects stellar positions like an excess random noise with an RMS of ~0.5 mas for a time span of three to seven years when left uncorrected. We demonstrate that an additional calibration step can adequately mitigate this and restore an astrometric accuracy of 0.12 mas, which is essential to achieve the goals of our planet-search program. These results indicate that similar instabilities could critically affect the astrometric performance of other large ground-based telescopes and extremely large telescopes that are equipped with large-format multi-chip detectors if no precautions are taken.
Discovering and Characterising Exoplanets and Ultracool Dwarfs with the Square Kilometre Array
University of Amsterdam（阿姆斯特丹大学）；ASTRON, The Netherlands Institute for Radio Astronomy（荷兰射电天文研究所）；Observatoire de Paris（巴黎天文台）；National Astronomical Observatory of Japan（日本国立天文台）；University of Tokyo（东京大学）；California Institute of Technology（加州理工学院）；University of Colorado Boulder（科罗拉多大学博尔德分校）；University of Groningen（格罗宁根大学）
High precision astrometry mission for the detection and characterization of nearby habitable planetary systems with the Nearby Earth Astrometric Telescope (NEAT)
Astrometric positions for 18 irregular satellites of giant planets from 23 years of observations
巨行星的不规则卫星被认为是在太阳系演化过程中被捕获的。了解它们的物理参数，如大小、密度和密度，对于限制它们来自何处以及如何捕获它们非常重要。获得这些参数的最佳方法是由航天器进行现场观测或由天体进行掩星观测。这两种技术都要求轨道是已知的。我们的目标是获得良好的天体测量位置的不规则卫星，以改善他们的轨道和星历。我们从三个数据库中识别并减少了几颗不规则卫星的观测结果，这些数据库包含1992年至2014年在三个站点（Observatório do Pico dos Dias，Observatoire de Haute-Provence和European Southern Observatory - La Silla）获得的8000多张图像。我们使用PRAIA（Platform for Reduction of Astronomical Images Automatically）软件对CCD图像进行了天体测量归算。UCAC 4星表代表国际天体参考系。帧中卫星的识别是通过SPICE/NAIF内核确定的星历完成的。为了克服信息（坐标、日期）缺失或不完整的问题，采取了一些程序，主要是针对较旧的图像。我们设法获得了18颗不规则卫星的6000多个位置：木星的12个，土星的4个，天王星的1个（Sycorax）和海王星的1个（Nereid）。对于某些卫星，获得的位置数超过早期轨道数值积分中使用的位置数的5…
On-sky performance and prospects of near-infrared CMOS detectors in exoplanet detection: first year of SPIRIT
Cavendish Laboratory（卡文迪许实验室）；ETH Zurich（苏黎世联邦理工学院）；Instituto de Astrofísica de Canarias (IAC)（加那利群岛天体物理研究所）；Astrobiology Research Unit, Université de Liège（列日大学天体生物学研究单元）；Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology（麻省理工学院地球、大气与行星科学系）；School of Physics & Astronomy, University of Birmingham（伯明翰大学物理与天文学院）
