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

The deposition of a 30-Å-thick layer of iridium upon a 250-Å-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 × 1016 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 Å. Stray-light measurements were also made near the first inside spectral order at 304, 584, and 1216 Å. 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. © 2003 Optical Society of America.