Low-Stress Iridium Coatings for Thin-Shell X-Ray Telescopes

We propose to develop and commercialize a new type of low-stress iridium (Ir) X-ray mirror coating technology that can be used for the construction of high-resolution X-ray telescopes comprising thin-shell mirror substrates, such as the Soft X-ray Telescope (SXT) currently being developed for the Constellation-X mission. The urgent need for low-stress Ir coating technology is driven by the current limitations on telescope angular resolution resulting from substrate distortions caused by conventional reflective Ir coatings that have high stress. In particular, we have measured film stresses in excess of 3 GPa in the case of 30 nm Ir films deposited by conventional magnetron sputtering techniques. The distortions in thin glass mirror shells (such as those suitable for the Constellation-X SXT) resulting from these extremely large coating stresses presently make the largest contribution to the SXT telescope imaging error budget, of order 10 arcsec or more. Consequently, it will be difficult, if not impossible, to meet the imaging requirements of Constellation-X, or other future high-resolution X-ray missions, unless high-quality Ir coatings having significantly lower stresses can be developed. The development of such coatings is precisely the aim of our proposal.