Osmium atomic-oxygen protection by an iridium overcoat for increased extreme-ultraviolet grating efficiency

The deposition of a 30- ?-thick layer of iridium upon a 250- ?-thick osmium reflective layer for use as a diffraction grating in the Cosmic Hot Interstellar Plasma Spectrometer (CHIPS) satellite observatory has provided sufficient protection from an expected maximum orbital atomic-oxygen fluence of 1 x 10^16 atoms /cm^2 . The grating parameters of groove constant and depth, efficiencies of zeroth-order, first and second inside orders, and first inside-order efficiency positional uniformity as well as stray light near the first inside order of the Ir-Os-coated grating were measured within a CHIPS spectral bandpass of 90 -260 ?. Stray-light measurements were also made near the first inside spectral order at 304, 584, and 1216 ?. The results make the Ir-Os coat an acceptable grating reflectivity layer for CHIPS and other spaceborne extreme-ultraviolet spectrometers that employ grazing-incidence reflection optics.