The optical scheme of the instrument is sketched in Figure 1 , it is designed to match the F/20 focal ratio of the TIRGO telescope. We have, following the optical path, the window of the dewar, the order sorting filter (not in figure), a field lens, in Calcium Floride, and the slit, in the focal plane of the telescope. ... The resolving power is (for first order of grating) of about 600 in the center of J band and 950 in the center of the K band, using a slit of two pixel (3.46 arcsec). ...