and Cosmology

1.3 The Tools of Extragalactic Astronomy
ing of four telescopes each with a diameter of 8.2m.
ESO already operates the La Silla Observatory in Chile
(see Fig. 1.26), but a better location was found for the
VLT, the Cerro Paranal (at an altitude of 2600 m). This
mountain is located in the Atacama desert, one of the
driest regions on Earth. To build the telescopes on the
mountain a substantial part of the mountain top first had
to be cut off (Fig. 1.29).
In contrast to the Keck telescopes, which have
a primary mirror that is segmented into 36 hexagonal
elements, the mirrors of the VLT are monolithic,
i.e., they consist of a single piece. However, they are
very thin compared to the 5-m mirror on Mt. Palomar,
far too thin to be intrinsically stable against gravity
and other effects such as thermal deformations. Therefore,
as for the Kecks, the shape of the mirrors has to
be controlled electronically (see Fig. 1.30, right). The
monolithic structure of the VLT mirrors results in better
image quality than that of the Keck telescopes, resulting
in an appreciably simpler point-spread function.
Each of the four telescopes has three accessible foci;
this way, 12 different instruments can be installed at
the VLT at any time. Switching between the three
instruments is done with a deflection mirror. The perma-
nent installation of the instruments allows their stable
operation.
The VLT (Fig. 1.31) also marks the beginning of
a new form of ground-based observation with large optical
telescopes. Whereas until recently an astronomer
proposing an observation was assigned a certain number
and dates of nights in which she could observe with the
telescope, the VLT is mainly operated in the so-called
service mode. The observations are performed by local
astronomers according to detailed specifications that
must be provided by the principal investigator of the observing
program and the data are then transmitted to the
astronomer at her home institution. A significant advantage
of this procedure is that one can better account for
special requirements for observing conditions. For example,
observations that require very good seeing can
be carried out during the appropriate atmospheric conditions.
With service observing the chances of getting
a useful data set are increased. At present about half
of the observations with the VLT are performed in service
mode. Another aspect of service observing is that
the astronomer does not have to make the long journey
(see Fig. 1.30), at the expense of also missing out on the
adventure and experience of observing.
29
Fig. 1.30. Left: Transport of one of the VLT mirrors from
Antofagasta to Paranal. The route passes through an extremely
dry desert, and large parts of the road are not paved. The
VLT thus clearly demonstrates that astronomers search for
ever more remote locations to get the best possible observing
conditions. Right: The active optics system at the VLT. Each
mirror is supported at 150 points; at these points, the mirror
is adjusted to correct for deformations. The primary mirror is
always shaped such that the light is focused in an optimal way,
with its form being corrected for the changing gravitational
forces when the telescope changes the pointing direction. In
adaptive optics, in contrast to active optics, the wavefront is
controlled: the mirrors are deformed with high frequencies
in such a way that the wavefront is as planar as possible
after passing through the optical system. In this way one can
correct for the permanently changing atmospheric conditions
and achieve images at diffraction-limited resolution, though
only across a fairly small region of the focal plane