This capability does not exist on the 1.5-m telescope. Acquisition of objects can also be done using the instrument in imaging mode, either using zero order of the grating or a mirror mounted on the grating table. Although only two gratings can be installed in the IRS at any given time, CTIO has a larger number of gratings available. Installation and removal of gratings is a process that requires warming up and cooling down the instrument. Finally there is a Ks filter, which is used for acquisition in imaging mode.
Several slits are available.
The usable slit length is almost exactly 50 pixels. The second table lists measured sensitivities for gratings. The figures are for the 1.5-m telescope, using a wide slit. Sensitivity on the 4-m telescope should be roughly 6 times higher, without allowance for light losses at the slit. Background levels are given in the following table, for wavelengths in the K, L, and M bands.