Extreme ultraviolet Imaging Telescope

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The Extreme ultraviolet Imaging Telescope (EIT) is an

(II), respectively. EIT is built as a single telescope with a quadrant structure to the entrance mirrors: each quadrant reflects a different colour 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.

The EIT wavelengths are of great interest to solar physicists because they are emitted by the very hot

.

Technology

EIT is the first long-duration instrument to use

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–Chrétien 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

, 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.

History

EIT was a difficult sell to the scientific funding agencies, as it was not clear in the early 1990s that simple imaging of the corona would be scientifically useful (most of the other instruments on board SOHO are spectrographs of various kinds). The EIT PI, Jean-Pierre Delaboudiniere, was forced to scrounge funding and resources from several locations to construct and launch the instrument. For example, EIT alone of the SOHO instruments does not have its own flight computer; it is connected to the LASCO instrument flight computer, and is treated operationally as an additional LASCO camera. No funding was available for a pointing adjustment mechanisms, so EIT is bolted directly to the spacecraft and hence forms the SOHO pointing reference: the other instruments all align themselves to the EIT images. Focus adjustment is achieved by thermal expansion: the internal survival heaters (found in most spaceborne instruments) are used to achieve microscopic changes in the size of the telescope structure and hence the mirror spacing. EIT was originally allocated only about 1 kbit/s of data—about the same speed as a 110 baud teletype—but after its utility became clear much more telemetry bandwidth was allocated to it.

Related instruments

The technology in EIT is based on prototype instruments that were flown on the

mission.

External links

• Latest EIT full-field images

References

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