The C-Band All Sky Survey (C-BASS) is a project to image the whole sky at a wavelength of six centimetres (a frequency of 5 GHz), measuring both the brightness and the polarization of the sky at a resolution of just under 1 degree. The C-BASS maps will be used to help better remove the low-frequency diffuse Galactic emission from maps of the CMB, in both temperature and polarization.

C-BASS employs very sensitive microwave amplifiers, cooled to within a few degrees of absolute zero, and configured to measure tiny differences in temperature and polarization. They are mounted on two separate telescopes — one at the Owens Valley Observatory (OVRO) in California, the other in South Africa. This allows C-BASS to observe both in the northern and southern hemispheres and hence map the whole sky.

C-BASS is a collaborative project between the Universities of Oxford and Manchester in the UK, the California Institute of Technology (supported by the National Science Foundation) in the USA, the Hartebeesthoek Radio Astronomy Observatory (supported by the Square Kilometre Array project) in South Africa, and the King Abdulaziz City for Science and Technology (KACST) in Saudi Arabia. The southern telescope is a 7.6-m dish donated to the project by Telkom. The northern telescope is a 6.1-m dish donated by the Jet Propulsion Laboratory.