7.7.3.1 Slit Selection

There are four IRS slits, or six ``slits'' if we count the two spectral orders available in each of the two low resolution modules. In addition, the IRS Spectral Mapping Mode AOT offers two additional slit choices, ``Low Short Both'' and ``Low Long Both,'' which select both orders of the indicated module (however, see below). Because of their two-order configuration, the use of the low resolution slits deserves further description.

If only one order of one or both low resolution slit(s) is selected, then the IRS will also simultaneously obtain a spectrum in the unselected order of the slit(s); however, the latter spectrum will not be centered on the target. If both orders of one or both low resolution slit(s) are selected, then spectra will be obtained in both orders for all pointings, and each pointing will be repeated with each order subslit centered on the target. Finally, if either of the ``Both'' options is selected, then spectra from both orders of the selected slit(s) will be obtained with the entire slit centered on the target position (i.e., neither order subslit will be centered exactly on the target coordinates). The ``Both'' options should be used with caution since they can result in neither order obtaining a spectrum of the target. The various slit configurations are illustrated in Figure 7.5.

Figure 7.5: Comparison of order selection using the SL module. The left panel shows the result of selecting only the 1st order - one spectrum spanning both orders is obtained, but only the selected order subslit is centered on the target (the bright star at the center of the image). The middle panel shows the result of selecting both 1st and 2nd orders - two spectra spanning both order subslits are obtained, with the target centered in the 1st order subslit for one spectrum and in the 2nd order subslit for the other spectrum. The right panel shows the result of selecting the ``Low Short Both'' option - one spectrum spanning both orders is obtained, with the full slit centered on the target (in this case, the target ends up in neither order subslit). In all panels, the 1st order subslit is at the top (red) and the 2nd order subslit is at the bottom (blue). In the middle panel, the ``slit'' in the middle (purple) is the overlapping 1st and 2nd order subslits from successive exposures.