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.
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.
There are a wide variety of images, particularly quasars and jets, and
a
database may be found on the JIVE system.
Contact: www@jb.man.ac.uk
VLBI at Jodrell Bank
Space VLBI
VLBI Astronomy and Images
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: