The Extreme-ultraviolet Imaging Spectrometer (EIS) is one of the three major scientific instruments of the Solar-B. It utilizes an off-axis parabolic primary and a toroidal diffraction grating in a normal incidence optical layout with high-reflectance Mo/Si multi-layer coatings. The multi-layer coatings have high reflectance in two wavelength ranges, 170-210 Å and 250-290 Å, and these wavelength ranges are simultaneously observed with two large back-illuminated CCDs. Many EUV emission lines from the transition region, the corona, and flares are contained in these wavelength ranges and observers can select the number of spectral windows up to 25 in the imaging area of two CCDs. There is a slit/slot exchanger that contains two narrow slits (1" & 2" width) and two wide slots (40" & 266" width) at the prime focus of the primary mirror, and two dimensional EUV images are obtained with one of the narrow slits in a raster observation by a pivot rotation of the primary mirror in the east-west direction or by one of the wide slot observation without primary-mirror motion, though the velocity information is convolved in the latter case. The center of the field of view can be changed by ±890" in the east-west direction by a translational motion of the primary mirror and it enables to see a high-altitude region of the corona at the limb or to see the region near the limb when the nominal observing region of the Solar-B is located near the center of the sun.
The EIS instrument was designed and developed by an international collaboration of Mullard Space Science Laboratory (MSSL), The University of Birmingham, Ratherford Appleton Laboratory (RAL), NASA MSFC/GSFC, NRL Hulbert Center for Space Research, University of Oslo, JAXA, and NAOJ.