Throughout the
universe, the common astronomical objects - stars, stellar
clusters, molecular clouds, galaxies, AGNs, QSOs, clusters of
galaxies - have typical linear dimensions, and only seldom span a
factor of 10 to 100 in scale. The more distant the objects, the
smaller therefore their apparent angular size, and hence a higher
angular resolution and larger collecting area of a telescope is
required to distinguish significant structural detail at a
significant level of detection. According to a fundamental optics
principle, the angular resolution of a full aperture
telescope (optical or radio) of diameter
, or of an
interferometer consisting of several connected or disconnected
telescopes of longest baseline separation
, observing at
the wavelength
, is
If, therefore, the telescope or the baseline has a diameter/length
of
= n
, the resolution is
In words, the larger the
number (n) of wavelengths spanned by the diameter/baseline the
higher is the angular resolution. From these relations it is
evident that a high angular resolution is obtained by using short
wavelengths (for instance millimeters instead of centimeters),
and/or large telescopes, and/or long baselines ([inter]continental
distances instead of kilometers). In order to obtain a resolution
of = 1
[the seeing limit at optical wavelengths set
by the turbulence of the Earth's atmosphere], the size of the
telescope or interferometer baseline must be
which is [1
] =
[1
] = 600m at
=
3mm (100GHz). To be comparable in resolution with the HST of
, a
telescope/baseline of
= 60km at
= 3mm
is required. To obtain however a resolution of
=
10
10
1
= 0.0001
= 0.1 mas at mm-wavelengths it is evident that the telescope must have
Earth dimensions. Such a 'telescope' can only be an interferometer
of some sort, of which the telescopes are disconnected and located
across a continent, or on different continents, or on different
continents and in Space. cm The image quality
of a mm-VLBI array depends on the available uv-coverage.
However, mm-VLBI telescopes cannot be displaced, they are
arranged in the given configuration of the observatory sites (see
Fig. 3.1), and uv-coverage is only obtained by Earth
rotation. The sensitivity of a mm-VLBI array depends on the
collecting area and the precision (aperture efficiency) of the
participating telescopes. cm Very long
baseline interferometry, on baseline dimensions of the Earth's
diameter and satellite orbits, requires special techniques to
record in-phase the different segments of a wavefront emitted by
a source and being received by the individual telescopes of the
array. This in-phase recording is achieved by locking the
oscillators of the receiver and tape recorder unit to a very
precise observatory time standard (Hydrogen-maser), which in turn
is synchronized to an 'outside clock', available at all stations.
This outside clock is provided by time signals of the Global
Positioning System (GPS).