Euclid reference frames. Left panel: Euclid spacecraft reference frame. X_SC points towards the Sun disk centre. The edge and centre of the joint FoV are offset by and , respectively. The longer side of the FoV is typically aligned with ecliptic meridians during observations. Right panel: Euclid optical reference frame. We note that the X_optY_opt plane is defined as looking onto the sky with towards the spacecraft. There are four additional chips used as fine guidance sensors that are placed on each side of the VIS FoV.