  The Cosmic Background Imager (CBI) is an instrument designed to make images
of the cosmic microwave background radiation and to measure its statistical
properties on angular scales from about 3 arc minutes to one degree (spherical
harmonic scales from l ~ 4250 down to l ~ 400). The CBI is a 13-element
interferometer mounted on a 6 meter platform operating in ten 1-GHz frequency
bands from 26 GHz to 36 GHz. The instantaneous field of view of the instrument
is 45 arcmin (FWHM) and its resolution ranges from 3 to 10 arcmin; larger
fields can be imaged by mosaicing. At this frequency and resolution, the
primary foreground is due to discrete extragalactic sources, which are
monitored at the Owens Valley Radio Observatory and subtracted from the CBI
visibility measurements. The instrument has been making observations since late
1999 of both primordial CMB fluctuations and the Sunyaev-Zeldovich effect in
clusters of galaxies from its site at an altitude of 5080 meters near San Pedro
de Atacama, in northern Chile. Observations will continue until August 2001 or
later. We present preliminary results from the first few months of
observations.
