and Cosmology

1.3 The Tools of Extragalactic Astronomy
27
Fig. 1.27. Left: The HST mounted on the manipulator arm of
the Space Shuttle during one of the repair missions. Right: The
Hubble Deep Field (North) was taken in December 1995 and
the data released one month later. To compile this multicolor
image, which at that time was the deepest image of the sky,
images from four different filters were combined
Scientists managed to convince Robert Williams, then
director of the Space Telescope Science Institute, to
use the HST to take a very deep image in an empty
region of the sky, a field with (nearly) no foreground
stars and without any known clusters of galaxies. At
that time it was not clear whether anything interesting
at all would come from these observations. Using the
observing time that is allocated to the Director, the “director’s
discretionary time”, in December 1995 HST
was pointed at such a field in the Big Dipper, taking
data for 10 days. The outcome was the Hubble Deep
Field North (HDFN), one of the most important astronomical
data sets, displayed in Fig. 1.27. From the
HDFN and its southern counterpart, the HDFS, one
obtains information about the early states of galaxies
and their evolution. One of the first conclusions was
that most of the early galaxies are classified as irregulars.
In 2002, the Hubble Ultra-Deep Field (HUDF)
was observed with the then newly installed ACS camera.
Not only did it cover about twice the area of the
HDFN but it was even deeper, by about one magnitude,
owing to the higher sensitivity of ACS compared
to WFPC2.
Large Telescopes. For more than 40 years the 5-m telescope
on Mt. Palomar was the largest telescope in the
western world – the Russian 6-m telescope suffered
from major problems from the outset. The year 1993
saw the birth of a new class of telescope, of which the
two Keck telescopes (see Fig. 1.28) were the first, each
with a mirror diameter of 10 m.
The site of the two Kecks at the summit of Mauna
Kea (at an altitude of 4200 m) provides ideal observing
conditions for many nights per year. This
summit is now home to several large telescopes. The
new Japanese telescope Subaru, and Gemini North
are also located here, as well as the aforementioned
CFHT and JCMT. The significant increase in sensitivity
obtained by Keck, especially in spectroscopy,
permitted completely new insights, for instance through
absorption line spectroscopy of quasars. Keck was also
essential for the spectroscopic verification of innumer-