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 jocularly-termed 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–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 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.

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.

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, STEREO, and Proba-2 spacecraft (launched in 1998, 2006, and 2009, respectively) carry similar multilayer imagers, as does the Solar Dynamics Observatory mission.

• Latest EIT full-field images

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