Contamination control and material screening for the extreme ultraviolet coronal diagnostic spectrometer on SOHO

The Coronal Diagnostic Spectrometer (CDS) to be flown on the Solar Heliospheric Observatory (SOHO) satellite makes use of both grazing and normal incidence optics operating in the extreme ultraviolet wavelength range from 15 to 80 nm. Its optical components include a Wolter Schwartzschild type 2 telescope and grazing and normal incidence reflective diffraction gratings. The instrument is thus highly sensitive to contamination both from particulate debris of a kind readily controlled by use of clean rooms, but also from offgassed and outgassed molecular species arising from materials used in clean room construction and from components within the instrument itself. We describe the contamination control protocol used to constrain the contamination, both particulate and molecular, to levels that will maintain the instrument performance for the expected duration of the SOHO mission. We also describe and report some results from a parallel activity in which we have screened materials for inclusion in CDS construction, by making use of a purpose built ultra high vacuum system to provide a contamination source and surface science diagnostics. In this apparatus material samples are held at elevated temperature to expel, in a controlled manner, outgas products which are allowed to contaminate previously characterized gold coated witness mirrors. The outgas products being monitored by residual gas analysis and a quartz crystal microbalance. The contaminated witness mirror surfaces are chemically characterized without breaking vacuum using X-ray photoelectron spectroscopy (XPS), and the change in reflectivity at ultraviolet wavelengths is also measured in the same apparatus.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.