Figuring, polishing, metrology and performance-analyses of Wolter type 1 forming mandrels for the Constellation - X mirror development program

摘要

Constellation - X, NASA's next major X-ray observatory, is planned to be launched in 2012/2013. Each of the four identical spacecraft contains a large diameter, spectroscopic X-ray telescope (SXT). The mirror assembly is composed of many densely nested Wolter type 1 mirror reflectors, having segment angles of 30 or 60 degrees. The reflectors will be made of thin, accurately shaped glass sheets, onto which the reflective mirror surface is replicated from high precision, super polished Zerodur mandrels. One key issue for high-performance mirrors is the exact shape of the glass substrates. They are produced at NASA/GSFC by a dedicated hot forming process (slumping) from precision forming mandrels. The hot forming process requires special materials for the glass substrates and the forming mandrels. In this paper we report on figuring (precision machining), polishing, metrology and performance analyses of Wolter type 1 forming mandrels. The mandrels are characterized by the following features: Mandrel dimensions: 450 x 640 mm~2; radius of curvature: ～ 800 mm; segment angle: ～ 40 deg; shape: hyperboloid or paraboloid; material: Zerodur K20 (Keatite) or fused silica (quartz).