NIRI contains eight KRS5 direct-ruled grisms (prisms with transmission gratings on one surface) and slits of various widths and offsets from the center of the field of view. Their combinations permit low to moderate resolution spectroscopy through most of the 1-5um region. Five of the grisms are for use with the f/6 camera optics and cover the five 1.0-5.5μm windows (J,H,K, L-L', M). The other three are for use at f/32 with the adaptive optics module ALTAIR, and cover 1.05-2.41μm (the J, H, and K windows) only. There are no grisms for the f/14 configuration. The first table below gives approximate spectral resolving powers and useful wavelength coverages. The second table gives the key physical properties of the f/6 grisms.

The accessible wavelength ranges given in the first table are limited either by the array size, the order-sorting (blocking) filters,or the atmospheric window. In general, full atmospheric windows are spectrally imaged onto the detector at f/6. Where this is not quite the case with f/6 center slits (slits centered in the focal plane), blue slits for the f/6 optics that are offset in the focal plane are available to provide the missing short wavelength region (however, note that the long wavelength part of the window may then not be covered; this is especially true for the L band). The blue slits are only needed for the J and L bands (and possibly for the H band).

See the slit section for the details (e.g., dimensions on the sky) of the slits.

The grism data are based on laboratory and telescope tests. Note that the resolving powers of many of the grisms do not match those expected for ideal grisms. For example, at f/6, except for the H band the resolutions using the 2 pixel-wide slits are not 2 pixels and, apart from H, these slits do not give twice the resolving powers of the 4 pixel wide slits.

Order-sorting filter transmission curves and data were measured at normal incidence and warm (~300 K) unless noted otherwise. The H-band order sorting filter has a red leak and is crossed with the PK50 long-wave blocker.