iSHELL: a 1-5 micron cross-dispersed R=70,000 immersion grating spectrograph for IRTF

iSHELL is 1.15-5.4 μm high spectral resolution spectrograph being built for the NASA Infrared Telescope Facility on Mauna Kea, Hawaii. Dispersion is accomplished with silicon immersion gratings in order to keep the instrument small enough to be mounted at the Cassegrain focus of the telescope. The white pupil spectrograph is designed to produce resolving powers of up to R=70,000. Cross-dispersing gratings mounted in a tilt-able mechanism at the second pupil allow observers to select different wavelength ranges and, in combination with a slit wheel and dekker mechanism, slit lengths ranging from 5″ to 25″. One Teledyne 2048x2048 Hawaii 2RG array is used in the spectrograph, and one Raytheon 512x512 Aladdin 2 array is used in a slit viewer for object acquisition, guiding, and imaging. About $4 million in funding has been provided by NSF, NASA and the University of Hawaii. First light is expected in about 2015. In this paper we discuss the science drivers, instrument design and expected performance.