ELSEVIER ADVANCED TECHNOLOGY zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Optics & Laser Technology, Vol. 28, No. I, pp. 555-560, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0030-3992/96 $15.00 + 0.00 PII: SOO30-3992(96)00023-O Design and fabrication of low-cost X-ray mirrors A. AHMAD, D. ENGELHAUPT, C. FENG, Y. LI Traditional X-ray mirrors made from Zerodur and silicon carbide are quite expensive because of the fabrication cost involved in achieving a very high surface finish of the order of 15 A or better. The cost of X-ray mirrors can be greatly reduced by using replication methods for producing the grazing-incidence type mirrors. This paper presents the optical and optomechanical design for a Wolter type I mirror and its mount, and the fabrication method used to produce a low-cost replicated nickel mirror. The finite element analysis results for this mirror are also presented. The design and fabrication steps for the aluminium master0 mandrel used for replication are also discussed. A surface finish of better than 15 A was achieved for a such an electroformed mirror with a wall thickness of only 1 mm. Copyright @ 1996 Elsevier Science Ltd. KEYWORDS: grazing-incidence X-ray optics, optomechanical design, finite element analysis, diamond machining, nickel plating, replication Introduction The Center for Applied Optics (CAO) and NASA Marshall Space Flight Center (MSFC) are involved in the design and development of several X-ray telescopes such as the Advanced X-ray Astrophysics Facility (AXAF-I) and Solar X-ray Imager (SXI). The SXI telescope will be flown on a weather satellite primarily as a solar radiation warning device. To reduce the cost and time involved in producing the prototype mirror assemblies for these telescopes, the MSFC is developing the capability to design and build electroformed nickel replicated (ENR) optics. This effort was initiated during the development of the AXAF-Spectroscopic (AXAF-S), and is now being continued with an eye toward other future multiple-mirror systems. A small l/lOth scale ENR version of the smallest AXAF-I mirror is being developed now for testing the detector of the high resolution mirror assembly of AXAF-I. The fabrication of metal (aluminium and beryllium) mirrors by single-point diamond machining of the electroless nickel plated on the surface is a common practice. Typically, a surface finish of 20-50 A RMS can be achieved for such mirrors. The nickel alloy preferred for these applications is 11% by weight phosphorus, N&(P), where the parenthesis denotes an amorphous rather than a crystalline deposit. A low stress nickel deposit with sufficient hardness to obtain an excellent surface finish is achieved at low plating rates by The authors are at the Center for Applied Optics, 301 Sparkman Drive, The University of Alabama in Huntsville, Huntsville, AL 35899, USA. Received 2 November 1995. Revised 28 May 1996. suitable adjustment of the plating parameters. Also, the fabrication of a cylindrical X-ray mirror by electroforming nickel over an aluminium mandrel coated with electroless nickel has been reported previously’12. The results reported here extend beyond the previous efforts due to a more rigorous process control. Since the surface finish requirement for theoflight mirrors of X-ray telescopes is usually less than 10 A RMS, the stability of the polished N&(P) surface oxidation was examined by electrochemical polarization tests to determine the extent of subsequent process degradation on the polished and inspected surface. Also, the circularity requirements for the SXI mirror were such that even very low residual internal stresses in the electroformed nickel would cause excessive distortion if not controlled in real-time. This required a strict process control to achieve X-ray quality optical components using the first surface as a mandrel for replication. Optical design of X-ray mirrors The SXI telescope consists of a monolithic grazing incidence type mirror with paraboloid and hyperboloid sections in a Wolter type I configuration. The mirror focuses an image of the sun on an X-ray image detector through one of a set of filters. There is no commercially available (or published) program tailored for the analysis of grazing-incidence type optical systems. A modular user-friendly computer program for the modelling of grazing-incidence type X-ray optical systems has been developed by CA0 and MSFC3. This comprehensive computer software GRAZTRACE (GT2) covers the manipulation of input data, 555