The cross dispersed grating for the IRS consists of a prismatic cross disperser bonded to a grating. The cross disperser provides 5 orders of continuous spectra from 8500Å to 2.4 microns in a single setting. Equally important, the spectra of the various orders are obtained simultaneously through the same slit. Thus, the pieces of the spectra fit together seamlessly, providing accurate line ratios between bands. For reasons which are not understood, the throughput of the Cross-Dispersed Grating is reduced by about a factor of two in the K band relative to a standard grating. In other orders it is as good or better as a standard grating. For programs requiring multi-band coverage the loss of efficiency at K is usually offset by the gain in multiplex advantage. For programs involving observations in the K band only, a standard grating will usually be preferable.
It should be noted that the slit length has to be reduced significantly for the cross-dispersed grating, to avoid order-overlap. The usable slit length is about 34 pixels = 11 arcsec. The full slit length, available with a standard grating is about 57 pixels = 18 arcsec.
| Band | 
Central |
Resolution | Dispersion |
 m |
2 Pixels | m pixel-1 |
|
| K | 2.17 | 558 | 0.00194 |
| H | 1.63 | 561 | 0.00145 |
| J | 1.30 | 559 | 0.00116 |
| I | 1.08 | 558 | 0.00097 |
| O | 0.93 | 559 | 0.00083 |
To use the cross dispersed grating, simply set the grating drive to the (normally unique) position (in motor steps) that corresponds to the setup of the spectra that you like e.g.:
These values will be close to correct but you will want to check the
wavelength coverage carefully. (Recall, 1 motor
step 
2 pixels.)
The cross disperser is used with a thermal IR blocker made of PK50 glass which should be selected in the filter wheel with the command filt to pk50. This filter position has been masked down slightly so as to shorten the length of the slit and thereby avoid reduce overlap in the cross-dispersed mode of operation. Some further adjustment is usually required, depending somewhat on which orders are desired. This is accomplished by adopting a filter position rotated somewhat further from the nominal pk50 position, to further vignette the ends of the slit. Thus, to set the filter instead of using filt to pk50 you will type filt to 14300. The folding flat in the camera will have to be adjusted for optimal placement of the cross-dispersed spectra on the array in the ``spatial" direction. This is accomplished by manually turning the folding-flat tilt-adjustment knob on the front face of the IRS dewar. Turning the adjustment knob clockwise will translate the spectra to the left on the array. This adjuster has a number of peculiarities which make it vulnerable to damage. Adjustment should not be undertaken without consultation with the instrument support scientist.