Fundamental Astronomy

72
3. Observations and Instruments
Fig. 3.26. The 15 metre
Maxwell submillimetre
telescope on Mauna Kea,
Hawaii, is located in a dry
climate at an altitude
of 4100 m. Observations
can be made down to
wavelengths of 0.5mm.
(Photo Royal Observatory,
Edinburgh)
tion (Table C.24). Among them are the 40 m Nobeyama
telescope in Japan, which can be used down to 3 mm,
the 30 m IRAM telescope at Pico Veleta in Spain, which
is usable down to 1 mm, and the 15 m UK James Clerk
Maxwell Telescope on Mauna Kea, Hawaii, operating
down to 0.5 mm (Fig. 3.26). The largest project in the
first decade of the 21st century is ALMA (Atacama
Large Millimetre Array), which comprises of 50 telescopes
with a diameter of 12 m (Fig. 3.27). It will be
built as an international project by the United States,
Europe and Japan.
As already mentioned, the resolving power of a radio
telescope is far poorer than that of an optical telescope.
The biggest radio telescopes can at present reach a resolution
of 5 arc seconds, and that only at the very highest
frequencies. To improve the resolution by increasing
the size is difficult, because the present telescopes are
already close to the practical upper limit. However,
by combining radio telescopes and interferometers, it
is possible to achieve even better resolution than with
optical telescopes.
As early as 1891 Michelson used an interferometer
for astronomical purposes. While the use of interferometers
has proved to be quite difficult in the optical
wavelength regime, interferometers are extremely useful
in the radio region. To form an interferometer, one
needs at least two antennas coupled together. The spacing
between the antennas, D, is called the baseline. Let
us first assume that the baseline is perpendicular to the
line of sight (Fig. 3.28). Then the radiation arrives at