0 Assigned ID: 1565 ----- Attempting Submission 1 (Wed Apr 07 19:49:08 GMT 2021) ----- We will probe astrometric acceleration - one of the most challenging measurement with HST. Assuming that the main source of astrometric acceleration are binaries containing the star and a dim or invisible component, there are exciting prospects as to what such a component could be. The 12-year-long and regularly-scheduled WFC3/UVIS calibration programs provide a unique opportunity to do that now. To make it even more compelling, the target of this program is the core of NGC 5139, the most enigmatic Galactic globular cluster. This project might have potential synergy with the ongoing Gaia observations, if there are common objects revealing signs of acceleration. We will also calculate new relative proper motions which are expected to have superior accuracy and helping to better characterize the internal velocity dispersion. Imants Platais is an expert in HST WFC3/UVIS and ACS/WFC astrometry. He has a vast knowledge and expertise on proper motions in stellar clusters. Vera Kozhurina-Platais is part of the HST ACS and WFC3 teams with experience on HST/WFC3 & ACS/WFC astrometric and photometric calibrations. She also is an extensive user of the effective PSF fitting techniques. Johannes Sahlmann is an expert in the detection of Keplerian motion of stars orbiting by brown dwarfs and extra-solar planets. He is also an expert in HST astrometric calibrations. Dimitri Pourbaix is the manager of Gaia DPAC/CU4 "Object processing" and his expertise is a full astrometric characterization of all Gaia targets. Stellar Physics and Stellar Types Stellar Physics and Stellar Types 0 Regular false true false false false SUPPORTED