The Cosmic Background Imager 2
  We describe an upgrade to the Cosmic Background Imager instrument to increase
its surface brightness sensitivity at small angular scales. The upgrade
consisted of replacing the thirteen 0.9-m antennas with 1.4-m antennas
incorporating a novel combination of design features, which provided excellent
sidelobe and spillover performance for low manufacturing cost. Off-the-shelf
spun primaries were used, and the secondary mirrors were oversized and shaped
relative to a standard Cassegrain in order to provide an optimum compromise
between aperture efficiency and low spillover lobes. Low-order distortions in
the primary mirrors were compensated for by custom machining of the secondary
mirrors. The secondaries were supported on a transparent dielectric foam cone
to minimize scattering. The antennas were tested in the complete instrument,
and the beam shape and spillover noise contributions were as expected. We
demonstrate the performance of the telescope and the inter-calibration with the
previous system using observations of the Sunyaev-Zel'dovich effect in the
cluster Abell 1689. The enhanced instrument has been used to study the cosmic
microwave background, the Sunyaev-Zel'dovich effect and diffuse Galactic
emission.
