VLBI at Jodrell Bank

This Page Last Updated 25/Feb/98.

VLBI (Very Long Baseline Interferometry) is a method of utilising two or more radio telescopes at the same time to emulate a single, very large dish. The resolution of such an instrument is effectively that of a dish of diameter equivalent to the separation, or baseline, defined by the widest-separated telescopes in the array.

A network such as this is capable of producing images with resolution better than one milli-arc second. A maximum of 17 telescopes have participated in simultaneous observations, although a typical number would be between 4 and 10.

The Jodrell VLBI Group operates the Lovell Telescope and two other MERLIN dishes in conjunction with other European telescopes as part of EVN (The European VLBI Network), now further supported by JIVE: The Joint Institute for VLBI in Europe.

European observatories frequently work with U.S. colleagues such as the Very Long Baseline Array, or VLBA, and the VLA (Very Large Array). There are also VLBI stations in Australia, South Africa, Eastern Europe, India and Japan, with whom we work.

It is not yet economical to link telescopes in different countries in 'real time', so high-density, wideband tape recording systems are used. The systems used at Jodrell are specially developed at the MIT Haystack Observatory in the U.S. and comprise:

In order to recover data from the tapes they must be resynchronised during playback. To allow this, a time code is written on the tapes as they are recorded. This code must be synchronised to Universal Time (UT1) to better than one microsecond. The recording process also requires extremely precise frequency stability: at high observing frequencies our clocks must be stable to about one part in 10E15.

The necessary time stability is obtained from a Hydrogen Maser Frequency Reference. This device is sufficiently accurate such that if it had been started at the Big Bang, it would still be less than one second wrong. The actual time-of-day information is now obtained exclusively from GPS (Global Positioning System) satellite receivers - Jodrell Bank maintains four of these.

Space VLBI

More information on Space VLBI may be found on other sites. A brief summary is given below.

In order to obtain base lines wider than the diameter of the earth,it is proposed to make use of an orbiting satellite observatory. In September 1996 the Japanese VSOP vehicle was launched. The mission is coordinated in America by a team at the Jet Propulsion Laboratory (JPL). Many observatories will take part in experiments with this satellite,including Jodell Bank, where the large Lovell Telescope (76m) will make a major contribution at 1.6 Gigaherz.

VLBI Astronomy and Images

There are a wide variety of images, particularly quasars and jets, and a database may be found on the JIVE system.

The Jodrell Bank VLBI Team

Jodrell Bank takes part three or four times a year in intensive, round-the-clock observing sessions lasting up to three weeks. The resulting recorded tapes are then shipped to a central processor located at the Max Planck Institute for Radioastronomy in Bonn, Germany. The Jodrell VLBI Team currently comprises the following people:
  • Astronomers: Ralph Spencer, Alastair Gunn, Derek McKay
  • Engineering Support: Paul Burgess and Les Parry. (pb@jb.man.ac.uk and lrp@jb.man.ac.uk).

Contact: www@jb.man.ac.uk