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). The back of grating is a plane mirror so that, when the grating is rotated of 180 degrees, the instrument works as a camera, with a field of wiev of about 1.5 arcmin of diameter, in the band defined by the filters. This facility can be useful for tests, maintenance and in centering weak sources on the slit.
The filters wheel (positioned before the focal plane of telescope) can hold a total of eight one-inch filters that are used to sorting order of grating. In table 1 is reported the set presently mounted. The silits wheel (put on focal plane of telescope) has also eight positions each one holding a field stop (set is reported in table 2) and a field lens. All the mirrors are gold coated to have a good efficiency over a wide spectral range, the optics is usable at least as far as 5 µm.
The optical components are cooled at about 80 K through thermal contact with a liquid nitrogen vessel at atmospheric pressure as described in the cryo-mechanics section, The mounting of optical elements is designed to take account of the dimensional changes between mirrors (in pyrex) and supports (in aluminium) generated by temperature differences.
The analysis of the optical system was obtained using the package Zemax (of Focus Software Inc., Tucson - AZ). The image quality is very good compared with the pixel size; fig.2 shows, for example, the spot diagrams of point-like sources put in three points of slits (upper boundary, centre and bottom boundary) and for three wavelengths in H band: 1.575, 1.650 and 1.725 micron (that are the central and the extreme wavelengths falling on the used area of detector), with grating working in first order; the squares represent the pixel size (40 micron).