and Cosmology

1.3 The Tools of Extragalactic Astronomy
21
Fig. 1.20. The world’s two largest fully steerable radio telescopes.
Left: The 100-m telescope in Effelsberg. It was
commissioned in 1972 and is used in the wavelength range
from 3.5 mm to 35 cm. Eighteen different detector systems
are necessary for this. Right: The Green Bank Telescope. It
does not have a rotationally symmetric mirror; one axis has a
diameter of 100 m and the other 110 m
astronomers soon began utilizing interferometric methods,
where the signals obtained by several telescopes are
correlated to get an interference pattern. One can then
reconstruct the structure of the source from this pattern
using Fourier transformation. With this method one gets
the same resolution as one would achieve with a single
telescope of a diameter corresponding to the maximum
pair separation of the individual telescopes used.
Following the first interferometric measurements in
England (around 1960) and the construction of the large
Westerbork Synthesis Radio Telescope in the Netherlands
(around 1970), at the end of the 1970s the Very
Large Array (VLA) in New Mexico (see Fig. 1.21) began
operating. With the VLA one achieved an angular
resolution in the radio domain comparable to that of
optical telescopes at that time. For the first time, this allowed
the combination of radio and optical images with
the same resolution and thus the study of cosmic sources
over a range of several clearly separated wavelength
regimes. With the advent of the VLA radio astronomy
experienced an enormous breakthrough, particularly in
the study of AGNs. It became possible to examine the
large extended jets of quasars and radio galaxies in detail
(see Sect. 5.1.2). Other radio interferometers must
also be mentioned here, such as the British MERLIN,
where seven telescopes with a maximum separation of
230 km are combined.
In the radio domain it is also possible to interconnect
completely independent and diverse antennae to form
an interferometer. For example, in Very Long Baseline
Interferometry (VLBI) radio telescopes on different
continents are used simultaneously. These frequently
also include Effelsberg and the VLA. In 1995 a system
of ten identical 25-m antennae was set up in the
USA, exclusively to be used in VLBI, the Very Long
Baseline Array (VLBA). Angular resolutions of better
than a milliarcsecond (mas) can be achieved with
VLBI. Therefore, in extragalactic astronomy VLBI is