The CAT is a three-element interferometer which can operate at any frequency between 13 and 17 GHz with an observing bandwidth of 500 MHz and a system temperature of 50 K. The baselines are variable from 1 to 5 m and for cosmic microwave background radiation (CMBR) observations a synthesised beam of approximately 1/2 degree is used. The antennas have a diameter of 70 cm and the primary beam has a FWHM of 2 degrees at 15 GHz. All three antennas are mounted on a single turntable which tracks in azimuth, and have a separate elevation drive. The CAT simultaneously records data from orthogonal linear polarisations. Its alt-az mount means that the plane of polarisation rotates on the sky as the telescope tracks a given field. The telescope is surrounded by a 5-m high earth bank lined with aluminium to form a ground shield. This shielding reduces the effect of spillover and terrestrial radio interference, but also limits observations to elevations greater than 30 degrees.
Temperature anisotropies in the CMBR are expected to result from inhomogeneities in the distribution of matter at the surface of last scattering. Measurements of these primordial anisotropies provide powerful constraints on theories of structure formation in the early Universe. Observations on different angular scales probe the amplitude of structures on different mass scales. On angular scales greater than 3 degrees, the Sachs-Wolfe effect (resulting in a scale-invariant spectrum) is expected to be the principal cause of anisotropy. The COBE satellite has already detected anisotropies at a level of deltaT/T of about one in a hundred thousand on an angular scale of about 10 degrees. Many current experiments are sensitive to degree scales, with both detections and upper limits similar to that of COBE. There have also been attempts to detect anisotropies on arcminute scales. On angular scales in the range 10-60 arcmin. Doppler effects at the surface of last scattering are expected to result in large peaks in the amplitude of the fluctuation spectrum. The Cosmic Anisotropy Telescope (CAT) has been designed to detect primordial anisotropies in this angular-scale range.