and Cosmology

1. Introduction and Overview
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Fig. 1.31. The Paranal Observatory after
completion of the domes for the four VLT
unit telescopes. The tracks seen in the foreground
were installed for additional smaller
telescopes that are now jointly used with
the VLT unit telescopes for interferometric
observations in the NIR
1.3.4 UV Telescopes
Radiation with a wavelength shorter than λ 0.3 μm =
3000 Å cannot penetrate the Earth’s atmosphere but is
instead absorbed by the ozone layer, whereas radiation
at wavelengths below 912 Å is absorbed by neutral hydrogen
in the interstellar medium. The range between
these two wavelengths is the UV part of the spectrum,
in which observation is only possible from space.
The Copernicus satellite (also known as the Orbiting
Astronomical Observatory 3, OAO-3) was the
first long-term orbital mission designed to observe
high-resolution spectra at ultraviolet wavelengths. In
addition, the satellite contained an X-ray detector.
Launched on 21 August, 1972, it obtained UV spectra
of 551 sources until its decommissioning in 1981.
Among the achievements of the Copernicus mission are
the first detection of interstellar molecular hydrogen H 2
and of CO, and measurements of the composition of the
interstellar medium as well as of the distribution of OVI,
i.e., five-time ionized oxygen.
The IUE (International Ultraviolet Explorer) operated
between 1978 and 1996 and proved to be
a remarkably productive observatory. During its more
than 18 years of observations more than 10 5 spectra
of galactic and extragalactic sources were obtained.
In particular, the IUE contributed substantially to our
knowledge of AGN.
The HST, with its much larger aperture, marks
the next substantial step in UV astronomy, although
no UV instrument is operational onboard HST after
the failure of STIS in 2004. Many new insights
were gained with the HST, especially through spectroscopy
of quasars in the UV, insights into both the
the quasars themselves and, through the absorption
lines in their spectra, into the intergalactic medium
along the line-of-sight towards the sources. In 1999
the FUSE (Far Ultraviolet Spectroscopic Explorer)
satellite was launched. From UV spectroscopy of
absorption lines in luminous quasars this satellite
provided us with a plethora of information on the
state and chemical composition of the intergalactic
medium.