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FUSE Spectrograph at University of Colorado prior to shipment to NASA |
8. ULTRAVIOLET TELESCOPES:
"Near" ultraviolet (UV) radiation with wavelengths from 3000 - 4000 Angstroms, can pass through the Earth's atmosphere, giving us sunburn on sunny days. But atoms and molecules absorb "far" UV radiation (wavelengths < 3000 A) more easily than any other kind of radiation, so the Earth's atmosphere is very opaque to such radiation. Even balloons and aircraft cannot fly high enough to observe the far-UV sky; we need to put telescopes in space.
Because atoms and molecules absorb UV radiation so well, UV telescopes equipped with spectrometers are the best way to observe atoms in the universe -- to see what chemical elements stars and other objects are made of and to detect diffuse interstellar and intergalactic gas.
The most powerful UV spectrometer in space is the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope. Many astronomers at CU use this instrument. I use it myself to observe Supernova 1987A, as you will soon learn. The STIS takes spectra of optical and ultraviolet wavelengths from 1200 A to 10,000 A. But it is insensitive to UV radiation with wavelengths < 1200 A because the mirrors of HST do not reflect such radiation.
Although the STIS is a very powerful instrument, it is not sensitive enough to observe very faint objects, especially distant galaxies and quasars. The STIS loses sensitivity because the UV light must reflect off several mirrors before it reaches the detector. Therefore, when NASA announced a competition to build new instruments for the HST, the University of Colorado proposed to build a new, simpler spectrometer for HST that has only one reflection (after the reflections from the HST primary and secondary mirrors). We call it the Cosmic Origins Spectrograph (COS). We were delighted to learn in August 1997 that the CU proposal was judged the best of the 8 proposals competing for this opportunity, and that NASA has awarded CU $25 million to build this instrument in collaboration with Ball Aerospace of Boulder. Astronauts will install COS on the HST in July 2003.
Some of the most valuable information about atoms and molecules in space is found in the UV spectrum in the wavelength range 912 - 1200 A, which the HST cannot observe. Therefore, in June 1999 NASA launched the Far Ultraviolet Space Explorer (FUSE), which is specifically designed to observe at these wavelengths. The ultraviolet spectrometer on FUSE was built here at CU by a team led by Dr. Jim Green of our department.
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FUSE Spectrograph at University of Colorado prior to shipment to NASA |
Because of our long involvement with UV spectrometry and our current involvement with STIS, FUSE, and COS, the University of Colorado is one of the world's leading centers for research in UV space astronomy.
Radiation with wavelengths between 70 and 912 A is called Extreme Ultraviolet Radiation. A few years ago, NASA launched the Extreme Ultraviolet Explorer (EUVE) satellite, which mapped the sky at these wavelengths.
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Ultraviolet Telescopes in Space |
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Name |
Operated by |
Dates |
Effective Diameter |
Spectral Range |
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NASA |
1990 - |
2.4 m |
> 1200 A |
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NASA |
1999 - |
0.7 m |
912 - 3000 A |
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NASA |
1992 - 97 |
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70 - 760 A |
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Last modified January 18, 2002
Copyright by Richard McCray
