Forming mandrels for making lightweight x-ray mirrors
Future x-ray astronomical missions, similar to the proposed International X-ray Observatory (IXO), will utilize replicated mirrors to reduce both mass and production costs. Accurately figured and measured molds (called mandrels) - on which the mirror substrates are thermally formed, replicating the surface of the mandrels - are essential to enable these missions. The Optics Branches of the Goddard Space Flight Center (GSFC) and Marshall Space Flight Center (MSFC) have developed fabrication processes along with metrologies that yield high-precision mandrels; and through the SBIR program, they encourage small businesses to attack parts of the remaining problems. The Goddard full-aperture mandrel polisher (the MPM-500) has been developed to a level where mandrel surfaces match the 1.5 arcsec HPD level allocation in a 5 arcsec telescope program. This paper reviews this current technology and describes a pilot program to design a suite of machine tools and process parameters capable of producing many hundreds of these precision objects. A major challenge is to keep mid-spatial frequency errors below 2 nm rms - a severe specification; but we must also note the factors which work to our advantage: e.g., how the figure departs from a pure cone by only one micron, and how the demanding figure specifications which apply in the axial direction are relaxed by an order of magnitude in the azimuthal. Careful study of other large optical fabrication programs in the light of these challenges and advantages has yielded a realistic plan for the economical production of mandrels that meet program requirements in both surface and quantity.