Fabrication of lightweight silicon x-ray mirrors

Numerous scientific questions at the forefront of our modern physical understanding require enhanced x-ray astronomical observations beyond the capabilities of existing x-ray telescopes. These observations demand spaceborne x-ray telescopes of unprecedented imaging power. x-ray focusing optics which are simultaneously light weight and of sufficient quality are presently impossible to realize, given real-world constraints and current manufacturing technology. Our group at the National Aeronautics and Space Administration (NASA) Goddard Space Flight Center (GSFC) investigates the development of an x-ray mirror manufacturing process capable of meeting the stringent quality, production time, and cost requirements of the next generation of x-ray telescopes. This process employs monocrystalline silicon: a lightweight, stiff, thermally conductive, and readily available material which is free of internal stress. These properties combined make silicon a nearly ideal material for thin mirror substrates. Presently, our process is capable of fabricating sub-arcsecond Half-Power-Diameter (HPD) resolution mirror pairs (primary and secondary) at a sub-millimeter mirror thicknesses and of virtually any x-ray optical design (e.g. Wolter-I, Wolter-Schwarzschild, etc.). Further, the substrates include three-dimensional structures on their backsides to aid their alignment and bonding. The mirror substrate surface quality is generally superior to that of the mirrors on the Chandra X-ray Observatory. This paper describes the various manufacturing steps involved in the production of x-ray mirror substrates and a present status report.