Our atmosphere makes UV spectroscopy from the ground (300-380 nm spectral range) extremely challenging, nevertheless an instrument customised for that range offers a large increase of efficiency with respect to existing ESO instruments (UVES and X-Shooter). In addition, this spectral range is complementary to the ELT and JWST. An efficient UV spectrograph can cover a broad science case and will be a world-leading instrument for many years to come. CUBES will provide access to a tremendous diversity of iron-peak and heavy elements in stellar spectra, as well as some lighter elements (notably Beryllium) and light-element molecules (CO, CN, OH). The near-UV range is also critical in extragalactic observations, such as studies of the circumgalactic medium (CGM) of distant galaxies and in measuring the contribution of different types of galaxies/AGN to the cosmic UV background. Located at the Cassegrain focus, CUBES will cover the 300 - 400 nm spectral range  with a  resolving power of ~20K.

The current baseline design includes:

• A foreoptics sub-system that includes an Atmospheric Dispersion Corrector and an Acquisition and Guiding system;
• Two image slicers, enabling different spectral resolutions: ~22 K and ~6.5 K, minimise slit losses mitigating the unavailability of AO in the UV.
• Two arms, both equipped with first-order transmission gratings produced by microlithographic techniques with average efficiencies of 85% over the whole wavelength range.
• An active flexure compensation system to improve spectral format stability
• An advanced calibration subsystem that includes fibre fed simultaneous wavelength calibration alongside science spectra.