<em>Euclid</em>: High-precision imaging astrometry and photometry from Early Release Observations I. Internal kinematics of NGC 6397 by combining <em>Euclid </em>and <em>Gaia </em>data

\ua9 The Authors 2024.The instruments 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 deg2. This exquisite image quality has the potential to produce high-precision astrometry for point sources once the undersampling of Euclid’s cameras is taken into account by means of 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 0.7 mas (0.007 pixel) for the Visible Instrument (VIS) and 3 mas (0.01 pixel) for the Near-Infrared Spectrometer and Photometer (NISP). Finally, we present a specific scientific application of this high-precision astrometry: the combination of Euclid and Gaia data to compute proper motions and study the internal kinematics of NGC 6397. Future work, when more data become available, will allow for a better characterisation of the ePSFs and GD corrections that are derived here, along with assessment of their temporal stability, and their dependencies on the spectral energy distribution of the sources as seen through the wide-band filters of Euclid

<em>Euclid</em>: the potential of slitless infrared spectroscopy: a z = 5.4 quasar and new ultracool dwarfs

\ua9 2025 The Author(s). We demonstrate the potential of Euclid \u27s slitless spectroscopy to discover high-redshift (5$]]&gt;) quasars and their main photometric contaminant, ultracool dwarfs. Sensitive infrared spectroscopy from space is able to efficiently identify both populations, as demonstrated by Euclid Near-Infrared Spectrometer and Photometer Red Grism (NISP) spectra of the newly discovered quasar EUCL J181530.01652054.0, as well as several ultracool dwarfs in the Euclid Deep Field North and the Euclid Early Release Observation field Abell 2764. The ultracool dwarfs were identified by cross-correlating their spectra with templates. The quasar was identified by its strong and broad and emission lines in the NISP 1206-1892 nm spectrum, and confirmed through optical spectroscopy from the Large Binocular Telescope. The NISP Blue Grism (NISP) 926-1366 nm spectrum confirms and emission. NISP can find bright quasars at and, redshift ranges that are challenging for photometric selection due to contamination from ultracool dwarfs. EUCL J181530.01652054.0 is a high-excitation, broad absorption line quasar detected at 144 MHz by the LOw-Frequency Array (W Hz). The quasar has a bolometric luminosity of and is powered by a black hole. The discovery of this bright quasar is noteworthy as fewer than one such object was expected in the 20 deg surveyed. This finding highlights the potential and effectiveness of NISP spectroscopy in identifying rare, luminous high-redshift quasars, previewing the census of these sources that Euclid\u27s slitless spectroscopy will deliver over about deg of the sky