Editing Extreme ultraviolet Imaging Telescope (section)

==Technology==

EIT is the first long-duration instrument to use [[normal incidence]] multilayer [[Optical coating|coated optics]] to image the [[Sun]] in  [[ultraviolet|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 [[angle of incidence (optics)|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 [[nanometer|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 reflector|Cassegrain]] or [[Ritchey–Chrétien telescope|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 rocket]]s, each of which flew on several five-minute missions into space. Multilayer EUV optics are also used in terrestrial [[nanolithography]] rigs for fabrication of [[integrated circuit|microchip]]s.

The EIT detector is a conventional [[Charge-coupled device|CCD]]s 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 [[1000000000 (number)|billion]] (10<sup>9</sup>) 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&nbsp;nm (0.2 micrometre) thick, and transmit about half of the incident EUV light while absorbing essentially all of the incident visible light.