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Info on Telescopes
Herschel's Predecessors
In 1983 the US-Dutch-British
IRAS satellite inaugurated infrared space astronomy by mapping
250 000 cosmic infrared sources and large areas of extended
emission.
In November 1995 ESA launched
its Infrared Space Observatory, ISO, which has allowed a much
more detailed study of the infrared sky. ISO observed in the
wavelength range from 2.5 to 240 µm and achieved an one thousand
fold increase in sensitivity and a one hundred fold improvement
in angular resolution (at 12 µm) compared to IRAS. ISO's
operational lifetime was one year longer than planned, ending in
May 1998.
The Spitzer Space Telescope
(formerly SIRTF, the Space Infrared Telescope Facility) was
launched on 25 August 2003. During its 2.5-year mission,
Spitzer will obtain infrared images and spectra in the
wavelength range 3 to 180 µm. Consisting of a 0.85 metre
telescope and three science instruments operating at cryogenic
temperatures, Spitzer is the largest infrared space telescope
constructed to date. Rather than operating at L2, as Herschel
will do, Spitzer is in an Earth trailing heliocentric orbit.
Why Observe in the Infrared?
Large parts of the universe are
too cold to radiate in the visible wavelength range or at
shorter wavelengths. Study of these cooler objects is only
possible by observing in the infrared spectrum or at even longer
(sub-millimetre) wavelengths. Bodies with temperatures between
five and fifty Kelvin have radiative emission peaks in the
wavelength range observed by Herschel, and gases with
temperatures between ten and a few hundred Kelvin exhibit their
brightest molecular and atomic emission lines at these
wavelengths.
Additionally, many objects of
great interest to astronomers are concealed within or behind
clouds of gas and dust. In the early stages of their formation,
stars and planets are surrounded by the gas and dust clouds from
which they are being created. Galactic cores and most of the
remnants of the early universe are also hidden from view by dust
clouds. The dust particles in these clouds are comparable in
size to the wavelength of visible light and are therefore
efficient at scattering or absorbing radiation at these
wavelengths. Infrared radiation is less affected by these clouds
- the longer the wavelength, the thicker the dust cloud that it
can penetrate.
Why Observe in Space?
Water vapour in the Earth's
atmosphere absorbs radiation across large parts of the infrared
and sub-millimetre wavebands, making ground based observations
at these wavelengths impossible. Limited observations can be
made using techniques such as high altitude balloons but a
space-based observatory is the only truly satisfactory solution
to this problem.
By orbiting at L2, some 1.5
million kilometres from Earth, Herschel will avoid problems
caused by infrared radiation from the Earth interfering with
observations. The L2 orbit also prevents the occurrence of
temperature changes due to the spacecraft moving in and out of
eclipse in an Earth orbit, which are a particular problem for
infrared instruments requiring extreme thermal stability.
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