JBO - Lovell Multi-beam System

The Multi-Beam Receiver

The Multi-Beam Receiver being hoisted to the Focus Box.

A new multi-beam receiver has recently been commissioned and installed on the 76-metre Lovell Radio Telescope at Jodrell Bank, which will be able to do the work of three additional telescopes - for a thousandth of the cost! The receiver will enable the Lovell Telescope to participate in two new whole-sky surveys, looking for a new population of very faint galaxies identified by the characteristic spectral line of hydrogen at 21cm and searching for new pulsars, neutron stars that emit very regular radio pulses.

A radio telescope usually receives radio waves from only a single small area of the sky. A `feed horn' collects these signals which are then amplified in a high-performance receiver. This is cooled to close to absolute zero to minimise its noise contribution and hence achieve the maximum possible sensitivity. If, however, we need to survey a broad region of the sky, the time taken can be greatly reduced by clustering several feed horns together in the focal region of the telescope so that they can simultaneously observe adjacent areas of the sky. Each feed horn is slightly offset from the axis of the telescope and thus receives radio waves from a different region of sky than the others By aligning them correctly by rotating the whole system, they can be made to scan several parallel strips of the sky for each scan of the telescope.

The system installed on the Lovell Telescope uses four feed horns. Their associated receivers are kept at 30 K within a large, cooled enclosure. The telescope had to be considerably modified to allow the equipment to be hoisted up and mounted at the focus.

The Multi-Beam Feeds and Cryostat in the Cryogenic Laboratory.

The multi-feed system was developed in collaboration with astronomers at Cardiff University and in Australia, where the 64-metre Parkes telescope has been similarly equipped. Because of the larger space available at the focus of the telescope, the Parkes set-up has 13 beams which will cover an even larger area of sky. Jodrell Bank has provided the state-of-the-art low-noise amplifiers for both the Parkes and Lovell telescope receivers, along with data acquisition systems for the pulsar survey, while Parkes has provided the spectral line observing systems for use in the hydrogen-line surveys.

The Lovell and Parkes telescopes will carry out the two surveys together. The pulsar search is already underway at Parkes and is proving spectacularly successful with, on average, one new pulsar being discovered for each hour of observing time. Some 800 pulsars have been discovered in the 30 years previous to the new survey, but this number should increase by 50 per cent, since it is expected to identify another 400 or more pulsars in the next two years. The number now known already exceeds 1000! Without the multi-beam systems at Jodrell Bank and Parkes this new survey would have taken well over 10 years. Meanwhile the multi-beam system on the Lovell telescope has been used to carry out a survey of the Ursa-Major cluster of galaxies.

Current Status

The Multi-Beam System was installed on the Lovell Telescope at the beginning of 1999 for a 6 week observing session to study the Ursa Major and Virgo Clusters. The Virgo observations are linked with a deep ccd optical survey of the Virgo Cluster being carried out by astronomers at Cardiff University and their collaborators. The system is also being used to carry out a "blind" survey of a region near to the North Celestial Pole which will act as a field comparison for the two cluster surveys.

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