The spectrometer optics of GIANO-TNG

GIANO is a cryogenic cross-dispersed spectrometer operating at near IR wavelengths (0.9-2.5 microns). The aim of the optical design is to obtain wide spectral coverage, high resolution, large throughput and high spectral stability in a sufficiently compact instrument which can be built and cooled using relatively simple and inexpensive technologies. This ambitious goal is achieved using a 3-mirrors anastigmat in double-pass which acts both as collimator and camera. The collimated beam has a diameter of 100 mm and feeds a commercial 23.2 lines/mm echelle grating. Cross-dispersion is performed by prisms which also operate in double pass. By inserting a flat mirror before the grating, the instrument changes its face (hence the name "GIANO") and transforms into a low resolution spectrometer with a unique combination of spectral coverage, resolution and efficiency.