WINERED: Optical design of warm infrared echelle spectrograph

We are developing a short near-infrared (gimel = 0.9-1.35 mu m) high-resolution (R,,,,,, = 100,000) and high-sensitivity spectrograph "WINERED" by using several unique approaches. We adopt a classical cross-dispersed configuration for the optical system because it provides the best balance between the system throughput, alignment tolerances, and image quality (thereby producing high spectral resolution). Our design has four characteristics: (1) a ZnSe immersion echelle grating is used to realize a compact optical system even with R-max = 100,000; (2) a volume phase holographic (VPH) grating is used as a cross-disperser (in comparison with the classical reflective grating, the VPH grating decreases the camera optics and provides higher throughput); (3) a cooled refractive lens system is used for the camera optics; and (4) a reflective echelle grating can be replaced with the above-mentioned immersion grating to cover a wider wavelength range with R-max = 28,000. We have found a compact solution with high performance: all spots are well within 2 x 2 pixels throughout the entire detector array and the total throughput is more than 25% for all modes.