Over the past several years there has been increasing activity in spectroscopy from space at extreme ultraviolet (EUV) wavelengths (λ < 1200 Å). Because of poor reflectivities at such short wavelengths, reflective coatings offering improvements in this region are attractive. A coating found to have superior reflectivity shortward of 1000 Å is osmium. At 600 Å osmium has a reflectivity almost a factor of 2 better than gold or platinum. However, samples coated with osmium were flown on the early Shuttle test flights, and on the return of these samples, the osmium coating was found to have disappeared evidently due to the oxidation of the material in the atomic oxygen atmosphere. An instrument flown on the Spacelab 1 mission in late 1983 comprised an array of five spectrometers covering the extreme ultraviolet to near-IR wavelengths. The EUV spectrometer contained an osmium coated reflective grating located fairly deep within the instrument. In this paper we report the results of an assessment of the reflectivity over the course of the ten-day mission. The grating is found to be stable at 1216 Å from the time of the first operation of the EUV spectrometer on the second day of the mission through the final tenth day of the mission. The relative response of the EUV (osmium) and far-UV (MgF₂) spectrometers to 1216 Å is essentially unchanged from the preflight condition to within the 50% overall calibration uncertainty. We conclude that the grating was sufficiently shielded by the surrounding spectrometer structure to avoid significant erosion of the osmium surface.

© 1985 Optical Society of America

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