The first generation of ELT instruments includes an optical-infrared High Resolution Spectrograph, ANDES (ArmazoNes high Dispersion Echelle Spectrograph). The optical design and architecture of ANDES is primarily dictated by its high spectral resolving power (R=100'000), the area of the spectrograph slit projected onto the sky (> 1 arcsec2), its broad wavelength coverage and the large primary mirror of the ELT, and must foresee several huge fiber-fed spectrograph units. One of them is the RIZ spectrograph, covering wavelengths from 620 to 960 nm. It deals with a recomposed ~40-mm-long entrance slit and a pupil anamorphic magnification to overcome the limitation size of a mosaic 1.6-meter R4 Echelle grating. It requires two fast cameras with F/# close to the unity. This paper describes the preliminary optical design of the RIZ spectrograph instrument, its challenges, and its nominal and expected performances.
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