5.7 Future experiments5 Current and future CMB 5.5 Python

5.6 The Cosmic Anisotropy Telescope 

[Project collaborators: J. Baker, P.J. Duffett-Smith, M. Hobson, M. Jones, A. Lasenby, C. O'Sullivan, G. Pooley, R. Saunders and P. Scott.]

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 tex2html_wrap_inline1861, (baselines tex2html_wrap_inline1863 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 tex2html_wrap_inline1865 tex2html_wrap_inline1867, Dec. tex2html_wrap_inline1869 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 tex2html_wrap_inline1873 tex2html_wrap_inline1875, Dec. tex2html_wrap_inline1877 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 tex2html_wrap_inline1881, in the surface of last scattering.

  

Click on thumbnail to view image

Figure 14: 16.5 GHz CAT image of tex2html_wrap_inline1883 area centred on the CAT2 field, after discrete sources have been subtracted. Excess power can be seen in the central tex2html_wrap_inline1885 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 tex2html_wrap_inline1549  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 tex2html_wrap_inline1617 discussed earlier, and the data may be used to place constraints directly on the CMB power spectrum in two independent bins in tex2html_wrap_inline1617 . These constraints, along with those from the other experiments, are shown in Figure  15 .

  

Click on thumbnail to view image

Figure 15: Recent results from various CMB experiments. The solid line is the prediction (normalised to COBE) for standard CDM with tex2html_wrap_inline1551, tex2html_wrap_inline1553 and tex2html_wrap_inline1555 km s tex2html_wrap_inline1557 Mpc tex2html_wrap_inline1557 . The Saskatoon points have a 14% calibration error.


5.7 Future experiments5 Current and future CMB 5.5 Python

image 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