Following the entrance mirror the light forms a focus at the aperture wheel of the IRS. This wheel allows one to select the slit size and thus the resolution. A .35mm slit width provides for a two pixel sampling of the slit. Narrower slits may be useful for lowering the background in the thermal infrared but will provide undersampled spectra. Wider slits may be useful on nights of poor seeing or for extended sources to improve the throughput at the expense of resolution. A list of the currently available slits is given below. To command the aperture wheel to a position one types a command of the form ``ap to <name>'' (i.e. ap to .35mm) in the instrument control window.
ap to <name>
| Name | Durant | Motor Step | Comment |
| open_hole | 000 | 1000 | Imaging hole |
| .15mm | 125 | 4000 | 1 pixel slit |
| .35mm | 250 | 7000 | 2 pixel slit |
| .5c | 375 | 10000 | 3 pixel w. coron. spot |
| .5mm | 500 | 13000 | 3 pixel slit |
| .8mm | 625 | 16000 | 5.3 pixels slit |
| 1mm | 750 | 19000 | 6.7 pixels slit |
| 2mm | 875 | 22000 | 13.3 pixel slit |
| pinhole | 187.5 | 2500 | 3 pixel pinhole |
The slit image should be aligned precisely along the rows of the array in order to optimize both sky subtraction and wavelength calibration. This can be achieved by moving the aperture wheel a small amount by using the ajog command. An ajog of about 100 steps changes the angle of the slit by about 1 pixel over the length of the slit. Once the slit image is aligned with the rows of the array, the position of the slit on the array can be set by changing the tilt of the folding flat. This is a manual process which requires turning the small knob on the front face plate of the IRS dewar. Turning the knob clockwise moves the spectra toward the left edge of the detector. At the current time the best array dark current performance is achieved along the right edge of the array and this is generally where we place the spectra.