Astrophysics Group

Cavendish Laboratory


Mullard Radio Astronomy Observatory
The Cosmic Anisotropy Telescope (CAT)

The Cosmic Anisotropy Telescope (CAT), built in the mid 1990s, was the first interferometer to measure fluctuations in the cosmic microwave background (CMB). Its first results, published in 1996, were the highest resolution CMB detection at that time, and showed that the rise in fluctuation power towards scales of ~1 degree (l ~ 200) measured by the Saskatoon experiment were matched by a decline in power at smaller angles (l = 500-700), thus showing the existence of the long-predicted acoustic peak in the CMB power spectrum. Further results were published in 1999.

The CAT was the forerunner to the VSA, establishing and testing some of the principal design features for that more ambitious project. It was a three-element interferometer which operated frequencies between 13 and 17 GHz with an observing bandwidth of 500 MHz and a system temperature of 50 K. The baselines were variable from 1 to 5 m and for cosmic microwave background radiation (CMBR) observations a synthesised beam of approximately 1/2 degree was used. The antennas had a diameter of 70 cm and the primary beam had a FWHM of 2 degrees at 15 GHz. All three antennas were mounted on a single turntable which tracked in azimuth, and had a separate elevation drive. The CAT simultaneously recorded data from orthogonal linear polarisations. Its alt-az mount meant that the plane of polarisation rotated on the sky as the telescope tracked a given field. The telescope is surrounded by a 5-m high earth bank lined with aluminium to form a ground shield. This shielding reduced the effect of spillover and terrestrial radio interference, but also limited observations to elevations greater than 30 degrees. The telescope was turned off and partly dismantled in 2000.

he Cosmic Anisotropy Telescope at the Lord's Bridge Observatory, Cambridge.

he Cosmic Anisotropy Telescope at the Lord’s Bridge Observatory, Cambridge. (© Mullard Radio Astronomy Observatory, 1996.)

The three horn-reflector antennas track in elevation while the turntable rotates in azimuth, allowing the telescope to point anywhere in the sky. The turntable sits inside a circular earth bank lined with metal sheeting. This protects the telescope from radio interference and prevents it from seeing the ground (which is a relatively strong source of radio waves.

For information on other MRAO telescopes, follow the links on the left.