COSMOS_M25H mask designed with the lms tool. The white square shows the LUCI field of view (4 × 4 arcmin²), the long-dashed vertical lines bracket the optimal field of view for spectroscopy (2.8 arcmin wide). The green circle at the center of the field allows the mask to be moved. The cyan circles identify the alignment stars. The yellow rectangles show the slits; slit number 25 is positioned on an acquisition star to monitor seeing and transparency during the observations. The square slit number 26, also positioned on an acquisition star, allows verification of the centering of the mask after alignment. At the lower end of the mask, a series of six small holes is present for engineering purposes. Below these, the red rectangle is the area occupied by the identification number cut into the mask.