Cosmic Background Imager, Radio interferometer at Llano de Chajnantor Observatory, Chile

The Cosmic Background Imager was a 13-element radio interferometer on the Llano de Chajnantor plateau in the Chilean Andes, at 5,080 metres. Its dishes, 0.9 m across until 2006 and 1.4 m afterwards, combined as one instrument observing between 26 and 36 GHz in ten 1 GHz bands, with resolution better than 0.1 degree. It was built at the California Institute of Technology.

The instrument began operating in 1999 and collected data for nine years before shutting down in 2008. Over that period, it produced measurements that helped refine the understanding of the structure of the early universe.

The name of this instrument refers directly to the cosmic microwave background, the faint glow left over from the early universe. Scientists from several countries shared the data it collected, making it a point of reference in radio astronomy research worldwide.

The Cosmic Background Imager was dismantled in June 2008, and its mount was reused for the QUIET telescope in August 2008. The site at Llano de Chajnantor is not open to casual visitors; access is controlled by the observatory. The 5,080-metre elevation means low oxygen and cold, dry conditions, so acclimatisation is necessary for anyone working there.

This instrument was the first to map in detail a specific type of polarized radiation left over from the early universe. The radio amplifiers used in its receivers were developed with technology that was, at the time, at the frontier of what was possible for that frequency range.