The spectrometer optical design is based on a three mirrors anagstigmat (TMA) combination used in double-pass, which acts both as collimator and camera. The collimated beam is 100mm and the system focal length is 420mm. The dispersing element is a commercial 23.2 ll/mm R2 echelle working at a fixed position in a quasi-Littrow configuration with an off-axis angle along the slit of a few deg.which re-images the slit at a convenient place far from any critical element. A system of prisms from high dispersing IR optical materials acts as cross-disperser. As the entrance image of this system lies close to the detector, we also included a 2-mirrors system which re-images the slit at a convenient place, far from any critical element.

The entrance slit is the first optical element at cryogenic temperatures. It is illuminated by the beam from the pre-slit optics, which feeds two conic mirrors. These re-image the focal plane at a focal aperture of F/4.2 on a side of the detector. The light then enters into the TMA which creates a 100mm collimated beam feeding the cross-dispersing prisms and the grating. The rays reflected from the grating pass again through the prisms and are focused onto the array by the same three mirrors. Two flat mirrors are added to conveniently position the array, the input slit and the slit re-imaging mirrors. The optical performances of the system are excellent, with > 80% en-squared energy within one pixel over most of the array area.

The main mechanical components of GIANO are the spectrometer cryostat, the tank for liquid nitrogen and the optical bench,  which are both machined out of two blocks of Al-6082, whose thermal expansion coefficient was accurately measured and found to be equal to that of Al-6061, the alloy used to manufacture the mirrors. The bench is fixed in one point and elastically pressed onto the tank. Thin Pb foils are inserted to improve the thermal contact.The aluminum liquid nitrogen tank is isostatically mounted on a hexapod system. The hexapod arms and joints are made of stainless steel to achieve a reasonably low heat conduction, while maintaining a rigid and easy to manufacture system.

The spectrometer vessel and all the cryogenic systems are permanently mounted onto a structure (``cart'') . Four load cells are included to provide a direct measurement of the cryostat weight, which in turn is used to infer the amount of liquid nitrogen present in the BeTank. The cart, which also hosts the racks with the array and motor controllers, can be easily displaced and transported using retractable wheels.

GIANO is positioned on the floor of the Nasmyth A room, close to its dedicated electronics rack and LN2 plant.  It is fed through a bundle of 2 ZBLAN fibers (manufactured by IR-photonics) connected to the TNG straight focus at the Nasmyth-A . These fibers are standard off-the-shelf products with a core of 85 micron which corresponds to a sky-projected angle of 1”, and a cladding of 125 micron. The two fibers are aligned and mounted inside a custom connector. The cores are at a distance of 0.25mm, equivalent to a sky projected angle of 3”. The bundle is inside a steel jacket, in order to protect these fragile fibers from bending and breaking.

The fibers entrance is mounted at the straight Nasmyth-A focus. The F/11 beam from the telescope is focused onto the fibers at F/5 by a singlet CaF2 lens positioned 6 cm before the TNG focus.A pellicle beam-splitter, positioned just before the lens, deviates 8% of the light to the guider channel working in the Z-band. The autoguide channel consists ofa 4-lenses camera feeding a commercial CCD 512x512 camera through a  RG850 filter. Since the relative positions of the fiber and the image on the guiding camera is fixed, acquisition and guiding simply consists of centering and maintaining the image at a given (fixed) X,Y position on the guiding camera.

The pre-slit system also includes a calibration channel which re-images, onto the science fibers, the light from a 0.6mm calibration fiber. This is connected to the calibration unit which includes a U-Ne lamp for wavelengths calibration and an halogen lamp for flat-fielding.

To avoid mechanical stress on the fiber and simplify telescope operations when GIANO is not observing, the fiber pre-slit module is mounted on a bearing structure which maintains a fixed orientation relative to gravity, independently of the position of the Nasmyth de-rotator.