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Extreme ultraviolet imaging telescope on SOHO to view the solar corona.

The Extreme ultraviolet Imaging Telescope (EIT) is an instrument on the SOHO spacecraft used to obtain high-resolution images of the solar corona in the ultraviolet range. The EIT instrument is sensitive to light of four different wavelengths: 17.1, 19.5, 28.4, and 30.4 nm, corresponding to light produced by highly ionized iron (XI)/(X), (XII), (XV), and helium (II), respectively. EIT is built as a single telescope with a quadrant structure to the entrance mirrors: each quadrant reflects a different color of EUV light, and the wavelength to be observed is selected by a shutter that blocks light from all but the desired quadrant of the main telescope.

Telescope.
Sun as seen from SOHO spacecraft.

The EIT wavelengths are of great interest to solar physicists because they are emitted by the very hot solar corona but not by the relatively cooler photosphere of the Sun; this reveals structures in the corona that would otherwise be obscured by the brightness of the Sun itself. EIT was originally conceived as a viewfinder instrument to help select observing targets for the other instruments on board SOHO, but EIT is credited with a good fraction of the original science to come from SOHO, including the first observations of travelling wave phenomena in the corona, characterization of coronal mass ejection onset, and determination of the structure of coronal holes. It currently (2006) produces an Fe XII (19.5 nm wavelength) image of the Sun about four times an hour, around the clock; these are immediately uplinked as time-lapse movies to the SOHO web site for immediate viewing by anyone who is interested. The images are used for long-duration studies of the Sun, for detailed structural analyses of solar features, and for real-time space weather prediction by the NOAA Space Weather Prediction Center.

History of the Extreme ultraviolet Imaging Telescope.

EIT is the first long-duration instrument to use normal incidence multilayer coated optics to image the Sun in extreme ultraviolet. This portion of the spectrum is extremely difficult to reflect, as most matter absorbs the light very strongly. Conventionally these wavelengths have been reflected either using grazing incidence (as in a Wolter telescope for imaging X-rays) or a diffraction grating (as in the infamous overlappograph flown on Skylab in the mid 1970s). Modern vacuum deposition technology allows mirrors to be coated with extremely thin layers of nearly any material. The multilayer mirrors in an EUV telescope are coated with alternate layers of a light "spacer" element (such as silicon) that absorbs EUV light only weakly, and a heavy "scatterer" element (such as molybdenum) that absorbs EUV light very strongly. Perhaps 100 layers of each type might be placed on the mirror, with a thickness of around 10 nm each! The layer thickness is tightly controlled, so that at the desired wavelength, reflected photons from each layer interfere constructively. In this way, reflectivities of up to ~50% can be attained.

The multilayer technology allows conventional telescope forms (such as the Cassegrain or Ritchey-Chretien designs) to be used in a novel part of the spectrum. Solar imaging with multilayer EUV optics was pioneered in the 1990s by the MSSTA and NIXT sounding rockets, each of which flew on several five-minute missions into space. Multilayer EUV optics are also used in terrestrial nanolithography rigs for fabrication of microchips.

The EIT detector is a conventional CCDs that are back-illuminated and specially thinned to admit the EUV photons. Because the detector is about equally sensitive to EUV and visible photons, and the Sun is about one billion (109) times brighter in visible light than in EUV, special thin foil filters are used to block the visible light while admitting the EUV. The filters are made of extremely thin aluminum foil, about 200 nm (0.2 micrometre) thick, and transmit about half of the incident EUV light while absorbing essentially all of the incident visible light.

Related instruments to the Extreme ultraviolet Imaging Telescope.

The technology in EIT is based on prototype instruments that were flown on the sounding rocket payloads MSSTA and NIXT. The first multilayer telescope to image the full disk of the Sun in EUV was flown by A.B.C. Walker and team in 1987. The TRACE spacecraft (launched into LEO in the late 1990s) carries a similar multilayer imager, as do the planned STEREO and Solar Dynamics Observatory missions.


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