Device for Measuring Stress Stability in Reflective Coatings for Thin-Shell X-Ray Mirrors

The proposed Lynx X-Ray Observatory aims to achieve an on-axis point spread function (PSF) of about 0.5 arc-second half-power diameter (HPD) and a 2 m^2 effective area. Thousands of extremely thin silicon mirrors with reflective x-ray coatings are needed to reach these goals and reduce mass per area. The coatings create film stress that can deform the mirrors. Stress relaxation in the film can cause the mirrors to disfigure over time and broaden the PSF. Typically, coating stability is measured by coating a mirror and measuring over a period of years in an x-ray beam line, or by measuring bending of flat mirror coupons. We discuss a novel device to measure the stress stability of reflective coatings that does not require flight mirrors or x-rays, and has a higher sensitivity compared to substrate bending. The device measures the resonant frequency of coated silicon membranes which can be related to film stress.