In one-slit mode the grating and filter combinations may be selected as for long slit spectroscopy.

In two-slit mode the broad-band filters can be used to prevent the two banks of spectra from overlapping on the detector. The useful combinations of gratings and filters that allow the use of two slits with no or minimal overlap are given in the table below. The calculated separations between the slits in unbinned pixels for the various grating/filter combinations are also given. Differences in the slit separations are due to anamorphic magnification. For most of the configurations below, the complete filter passband fits on the detector for both slits (typically with a few nm lost due to overlap at the center). With the B600 (or R600) grating + g filter at 475nm, 11.5nm of the g passband is lost at either end of the detector, so the blue slit has slightly less blue coverage and the red slit slightly less red coverage. In general, one should set the grating central wavelength to that of the filter for two-slit mode (+/- 11.5nm for B600+g), to avoid losing spectral range at the ends of the detector unnecessarily. Small adjustments may be needed to account for the CCD gaps. For the R150 + GG455, a central wavelength of ~680nm will cover roughly 460-900nm at both slits (truncating the very red end of the red slit; longer settings will reach >=1000nm at the expense of the blue end of the blue slit).

Note that with the R150 grating in 2-slit mode, there is a small amount of second order contamination from the blue slit overlapping the red slit (eg. for a central wavelength of 630nm, this peaks at the level of ~4%, dying away in the red). See the gratings and GMOS-N and GMOS-S filters page for more information on GMOS gratings and filters.