Metal multilayer mirrors for EUV wide field telescopes

Metal multilayer mirrors have been designed for the ALEXIS satellite, which is to carry six wide field telescopes to perform an all-sky survey in three or four narrow wavelength bands in the EUV. Comprised of alternating layers of molybdenum and silicon, the mirrors are optimized to provide maximum reflectivity at angles from 11.5 to 17 degrees off normal incidence and at wavelengths of 133, 171, or 186 Angstrom. Simultaneously, the mirrors use a wavetrap described below to suppress reflectivity at 304 Angstrom, where the extremely strong geocoronal line of He II causes severe background problems. This paper reports that low reflectivity at 304 Angstrom is achieved by superposing two layer pairs that provide destructive interference with an effective 2d spacing of 152 Angstrom. The Mo layers in this wavetrap must be very thin, about 10 Angstrom each, in order to allow the shorter wavelengths desired for peak reflectivity to penetrate without significant attenuation. Because refraction changes the effective angle of passage through the wavetrap, a joint optimization between layer thicknesses in the deep layers and the wavetrap layers must be performed for each target peak wavelength. For the 186 Angstrom mirror, the optimum design from substrate upward is 40 layer pairs, each 74 Angstrom Si and 31 Angstrom Mo, followed by 2 layer pairs, each 55 Angstrom Si and 10 Angstrom Mo