The Cosmic Anisotropy Telescope (CAT) is a three element,
ground-based interferometer telescope, of novel design [85]. 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
, (baselines
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
, Dec.
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) [77] and Scott
et al.
(1996) [89].
Recently observations of a new blank field (called the CAT2
field), centred on RA
, Dec.
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) [33] 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
, in the surface of last scattering.
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
.
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The Cosmic Microwave Background
Aled W. Jones and Anthony N. Lasenby http://www.livingreviews.org/lrr-1998-11 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |