Euclid Data Release 1 (DR1). NIR astrometric calibration

This paper presents the methodology and results of the astrometric calibration applied to the Euclid Near-Infrared (NIR) images in the Euclid Data Release 1, focusing on the techniques used to model and correct instrumental distortions, as well as the assessment of calibration quality. This work details the calibration pipeline, including quality control tests, and the evaluation of astrometric residuals against the Gaia DR3 catalogue and internal consistency metrics based on data encompassing more than one year of Euclid operation: the achieved accuracy is 30 mas for 97% of the total area, thus exceeding the requirements for Data Release 1. For the remaining 3%, accuracy declines due to a persistence correction issue: a method to correct the issue is presented. Furthermore, we discuss the stability of the solution over time, its dependence on source properties and instrument configuration, and outline the improvements planned for future Euclid data releases to enhance the astrometric calibration accuracy and efficiency.