5.6 The Cosmic Anisotropy Telescope

The Cosmic Anisotropy Telescope (CAT) is a three element, ground-based interferometer telescope, of novel design [77]. Horn-reflector antennas mounted on a rotating turntable, track the sky, providing maps at four (non-simultaneous) frequencies of 13.5, 14.5, 15.5 and 16.5 GHz. The interferometric technique ensures high sensitivity to CMB fluctuations on scales of 0.5°, (baselines  1 m) whilst providing an excellent level of rejection to atmospheric fluctuations. Despite being located at a relatively poor observing site in Cambridge, the data is receiver noise limited for about 60% of the time, proving the effectiveness of the interferometer strategy. The first observations were concentrated on a blank field (called the CAT1 field), centred on RA 08^h 20^m, Dec. +68° 59’, selected from the Green Bank 5 GHz surveys under the constraints of minimal discrete source contamination and low Galactic foreground. The data from the CAT1 field were presented in O’Sullivan et al. (1995) [65] and Scott et al. (1996) [82].

Recently observations of a new blank field (called the CAT2 field), centred on RA 17^h 00^m, Dec. +64° 30’, have been taken. Accurate information on the point source contribution to the CAT2 field maps, which contain sources at much lower levels, has been obtained by surveying the fields with the Ryle Telescope at Cambridge, and the multi-frequency nature of the CAT data can be used to separate the remaining CMB and Galactic components. Some preliminary results from CAT2 have been presented in Baker (1997) [4] and the 16.5 GHz map is shown in Figure 14. Clear structure is visible in the central region of this map, and is thought to be actual structure, on scales of about 1/4°, in the surface of last scattering.

Figure 14: 16.5 GHz CAT image of 6° × 6° area centred on the CAT2 field, after discrete sources have been subtracted. Excess power can be seen in the central 2° × 2° primary beam (because the sensitivity drops sharply outside this area, the outer regions are a good indicator of the noise level on the map). The flux density range scale spans ±40 mJy per beam.

When interpreting this map, however, it should remembered that for an interferometer with just three horns, the ‘synthesised’ beam of the telescope has large sidelobes, and it is these sidelobes that cause the regular features seen in the map. In the full analysis of the data, these sidelobes must be carefully taken into account.

For an interferometer, ‘visibility space’ correlates directly with the space of spherical harmonic coefficients discussed earlier, and the data may be used to place constraints directly on the CMB power spectrum in two independent bins in . These constraints, along with those from the other experiments, are shown in Figure 15.

Figure 15: Recent results from various CMB experiments. The solid line is the prediction (normalised to COBE) for standard CDM with , and  = 45 km s^–1 Mpc^–1. The Saskatoon points have a 14% calibration error.