iSHELL is a facility-class infrared cross-dispersed spectrograph for the IRTF using an immersion grating. This instrument will provide a resolving power of up to 80,000 at 1.2–2.5 μm and 67,000 at 3–5 μm. No other spectrograph in the Northern Hemisphere presently provides such high resolving power at near infrared wavelengths.
A silicon immersion grating is used in order to reduce the size of the instrument. The immersion grating design also has the advantage of allowing high spectral resolving power without requiring an extremely narrow slit or large collimated beam diameter. If built this will be the first facility instrument at 1–5 μm to employ an immersion grating, and therefore it will be an important demonstration of this technology for future instrumentation.