Fundamental Astronomy

Fig. 3.30. The VLA at Socorro, New Mexico, is a synthesis telescope consisting of 27 movable antennas
and the locations of the telescopes are chosen to give
optimal spacings for each configuration. In the largest
configuration each arm is about 21 km long, thereby
resulting in an antenna with an effective diameter of
35 km. If the VLA is used in its largest configuration and
at its highest frequency, 23 GHz (1.3 cm), the resolution
achievedis0.1 arc second, clearly superior to any optical
telescope. Similar resolution can also be obtained with
the British MERLIN telescope, where already existing
telescopes have been coupled together by radio links.
Other well-known synthesis telescopes are the Cambridge
5 km array in Britain and the Westerbork array in
the Netherlands, both located on east–west tracks.
Even higher resolution can be obtained with an extension
of the aperture synthesis technique, called VLBI
(Very Long Baseline Interferometry). With the VLBI
technique the spacing between the antennas is restricted
only by the size of the Earth. VLBI uses existing antennas
(often on different continents), which are all
pointed towards the same source. In this case the signal
is recorded together with accurate timing signals from
atomic clocks. The data files are correlated against each
3.4 Radio Telescopes
other, resulting in maps similar to those obtained with
a normal aperture synthesis telescope. With VLBI techniques
it is possible to achieve resolutions of 0.0001 ′′ .
Because interferometry is very sensitive to the distance
between the telescopes, the VLBI technique also provides
one of the most accurate methods to measure
distances. Currently one can measure distances with
an accuracy of a few centimetres on intercontinental
baselines. This is utilized in geodetic VLBI experiments,
which study continental drift and polar motion
as a function of time.
In radio astronomy the maximum size of single antennas
has also been reached. The trend is to build
synthesis antennas, similar to the VLA in New Mexico.
In the 1990’s The United States built a chain of
antennas extending across the whole continent, and the
Australians have constructed a similar, but north–south
antenna chain across their country.
More and more observations are being made in the
submillimetre region. The disturbing effect of atmospheric
water vapour becomes more serious at shorter
wavelengths; thus, submillimetre telescopes must be lo-
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