CUBES

The Cassegrain U-Band Efficient Spectrograph (CUBES) is ESO’s Very Large Telescope (VLT)’s upcoming ultraviolet eye on the sky. It is anticipated to see first light in 2029, at a similar time to ESO’s upcoming Extremely Large Telescope (ELT), and will complement the optical observations taken by the ELT by observing at wavelengths in the ultraviolet ground-based range (300-400 nm) of the electromagnetic spectrum. The consortium assembled to design, build, and operate CUBES is led by INAF, the National Institute of Astrophysics in Italy.  

Observations in the ultraviolet (UV) are very difficult from the ground, because our atmosphere blocks most UV light particles or photons, while in space the relatively small telescope apertures mean that faint sources are difficult to observe at these wavelengths. Neither the ELT nor the James Webb Space Telescope are able to detect photons bluer than 350nm. The VLT can detect photons down to the atmospheric cut-off at 300nm but, until now no 8-m-class telescope has been equipped with a spectrograph optimised to detect photons in the 300-400 nm range. CUBES will achieve this thanks to its location on the telescope, its efficient optical path (just one dispersive element), a clever image slicer that enables high spectral resolution without large slit losses, and the use of UV optimised optical elements throughout. 

This means CUBES will be uniquely suited to target a range of scientific cases, both within the Milky Way galaxy — especially in Solar System and planetary sciences — and extragalactic. Within the Solar System, CUBES will be a powerful tool for better understanding comets, including very faint ones. The rocky cores of comets are often surrounded by a cloud of gas, called a coma, which releases ultraviolet emissions that CUBES will be able to detect. Studying this gas will reveal properties of the comets themselves, including the composition of the rocky core and whether they contain ice. Since these comets are leftovers from the earliest days of our Solar System, these results will allow researchers to trace back the historical environment in which our planets formed. 

Reaching out further into the Milky Way galaxy, CUBES will be able to investigate a whole new range of stars that are too faint to be studied in detail with current facilities. In the hot cores of stars, atoms of light elements are fused together to form new, heavier chemical elements in a process called nucleosynthesis. There are still many questions about nucleosynthesis — such as how the environment in which a star forms may influence the amount of heavy elements it produces — which CUBES will be poised to tackle. 

Beyond the Milky Way, in the extragalactic cosmos, CUBES is designed to aid investigations into missing amounts of baryonic (normal) matter. At very far distances, much less normal matter is seen in the gas surrounding galaxies than expected, and CUBES will be used to help understand why. 

Massive stars and supermassive black holes in active galaxies also give out immense amounts of ultraviolet radiation, which can leak into the surrounding space between galaxies, the intergalactic medium. The small amounts of matter in the intergalactic medium will be stripped of electrons by this ultraviolet radiation, which is called the cosmic ultraviolet background. CUBES will be able to constrain measurements of the cosmic UV background, and help astronomers better understand how it influences star formation in galaxies.