GMTNIRS, the Giant Magellan Telescope near-infrared spectrograph, is a first-generation instrument for the GMT that
will provide detailed spectroscopic information about young stellar objects, exoplanets, and cool and/or obscured stars.
The optical and mechanical design GMTNIRS presented at a conceptual design review in October 2011 covered all
accessible parts of the spectrum from 1.12 to 5.3 microns at R=50,000 (1.12-2.5 microns) and R=100,000 (3-5.3
microns). GMTNIRS uses the GMT adaptive-optics system and has a single 85 milliarcsecond slit. The instrument
includes five separate spectrographs for the different atmospheric windows. By use of dichroics that divide the incident
light between five separate spectrographs, it observes its entire spectral grasp in a single exposure while having only one
cryogenic moving part, a rotating pupil stop.
Large, highly accurate silicon immersion gratings are critical to GMTNIRS, since they both permit a design within the
allowable instrument volume and enable continuous wavelength coverage on existing detectors. We describe the effort
during the preliminary design phase to refine the design of the spectrograph to meet the science goals while minimizing
the cost and risk involved in the grating production. We discuss different design options for the individual spectrographs
at R=50,000, 67,000, 75,000, and 100,000 and their impact on science return.