Euclid: Early Release Observations – High-precision imaging astrometry and photometry. I.⋆ Internal kinematics of NGC6397 by combining Euclid and Gaia data

The cameras at the focus of the Euclid space observatory offer superb, diffraction-limited imaging over an unprecedented (from space) wide field of view of ∼0.57 square degree. This exquisite image quality has the potential to produce high-precision astrometry for point sources once the severe undersampling of the Euclid’s cameras is opportunely taken into account for with accurate effective point spread function (ePSF) modelling. We present a complex, detailed workflow to simultaneously solve for the geometric distortion (GD) and model the undersampled ePSFs of the Euclid detectors. Our procedure was successfully developed and tested with data from the Early Release Observations (ERO) programme focused on the nearby globular cluster NGC 6397. Our final one-dimensional astrometric precision for a well-measured star just below saturation is 0.8 mas (0.008 pixel) for VIS and 3 mas (0.01 pixel) for NISP. Finally, we present a demonstrative and challenging scientific application: the combination of Euclid and Gaia data to compute proper motions and study the internal motions of NGC 6397. Future work, when more data will be available, will allow for a better characterization of the ePSFs and GD corrections that are derived here, along with assessing their temporal stability, and their dependencies on the spectral energy distribution of the sources as seen through the wide-band filters of Euclid. Key words. astrometry – photometry – proper motions – stellar clusters – globular clusters