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Spectral Overlap in 2-Slit Mode

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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).


Grating/Filter combination Clean wavelength range
or comment
nm/pixel Slit Separation [pix]
B1200 + g + OG515 520nm - 552nm 0.023 2700
R831 + g + GG455 460nm - 552nm 0.034 3090
R831 + r + RG610 615nm - 698nm 0.034 2870
R831 + i + CaT 780nm - 850nm 0.034 2640
R831 + z + CaT 848nm - 933nm 0.034 2530
B600 + g, r, i, z Some spectral overlap
0.045 3330(g), 3165(r), 3010(i)
R600 + g, r, i, z Some spectral overlap 0.047 3165(r), 3010(i)
R400 + r 562nm - 698nm 0.067 3390
R400 + i 706nm - 850nm 0.067 3280
R400 + CaT 780nm - 933nm 0.067 3225
R400 + z >=848nm, some spectral overlap 0.067 3180
R150 + GG455
>=460nm, some spectral overlap
0.174 3665

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.

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