We describe the fabrication of the two NuSTAR flight optics modules. The NuSTAR optics modules are glass-graphiteepoxy composite structures to be employed for the first time in space-based X-ray optics by NuSTAR, a NASA Small Explorer schedule for launch in February 2012. We discuss the optics manufacturing process, the qualification and environmental testing performed, and briefly discuss the results of X-ray performance testing of the two modules. The integration and alignment of the completed flight optics modules into the NuSTAR instrument is described as are the optics module thermal shields. OVERVIEW OF THE OPTICS MODULESThe Nuclear Spectroscopy Telescope Array (NuSTAR) is a NASA Small Explorer (SMEX) satellite mission scheduled for launch in February 2012. The NuSTAR experiment contains two telescopes each consisting of an optic and a CdZnTe focal plane detector separated from each other by a 10-meter deployable mast (figure 1). The experiment is an extension and improvement on the design successfully employed in the HEFT balloon experiment (Harrison et al. 2005 1 ). NuSTAR will operate in the 6-79 keV energy band. More details on the mission, the overall instrument design and performance requirements and scientific objectives can be found in Harrison et al. 2010 2 .A blowup of an individual optics module is also shown in figure 1. Each layer of the optic has an upper and lower conic shell (equivalent to the parabola-hyperbola sections of a Wolter-I optic). Each shell is composed of multiple thermally formed glass segments. Each piece of glass is coated with a depth-graded multilayer. The enhanced reflectivity provided by the multilayers, along with the shallow graze angles afforded by the focal length of the optics (10.15 meter) provide high effective area over the NuSTAR energy band of 6-79 keV, and a field of view of 12 arcminutes by 12 arcminutes. There are 133 concentric layers which together form each optic. The glass layers (a glass-epoxy-graphite composite structure) are built up on a Titanium mandrel. Titanium support spiders located on the top and bottom of each optic connect it to the optical bench. The compliant, radially-symmetric spiders accommodate thermal expansion effects as well as dynamic loading. Thin x-ray transparent thermal covers on the entrance and exit apertures of the optic reduce thermal gradients by blocking direct view of the sun and deep space. Two flight modules, FM1 and FM2, were fabricated. A third module, FM0, was fabricated earlier and has Pt/SiC multilayers on the inner 89 layers. FM0 is a potential flight spare and is available to provide for more extensive X-ray characterization than is permitted for either of the flight modules, given the compressed delivery schedule of the optics.

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Next generation's lightweight, high resolution, high throughput optics for x-ray astronomy requires integration of very thin mirror segments into a lightweight telescope housing without distortion. Thin glass substrates with linear dimension of 200 mm and thickness as small as 0.4 mm can now be fabricated to a precision of a few arc-seconds for grazing incidence optics. Subsequent implementation requires a distortion-free deposition of metals such as iridium or platinum. These depositions, however, generally have high coating stresses that cause mirror distortion. In this paper, we discuss the coating stress on these thin glass mirrors and the effort to eliminate their induced distortion. It is shown that balancing the coating distortion either by coating films with tensile and compressive stresses, or on both sides of the mirrors is not sufficient. Heating the mirror in a moderately high temperature turns out to relax the coated films reasonably well to a precision of about a second of arc and therefore provide a practical solution to the coating problem.

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X-ray astronomy depends on the availability of telescopes with high resolution and large photon collecting areas. Since x-ray observation can only be carried out above the atmosphere, these telescopes must be necessarily lightweight. Compounding the lightweight requirement is that an x-ray telescope consists of many nested concentric shells, which further require that x-ray mirrors must also be geometrically thin to achieve high packing efficiency. This double lightweight and geometrically thin requirement poses significant technical challenges in fabricating the mirrors and in integrating them into mirror assemblies. This paper reports on the approach, strategy and status of our x-ray optics development program whose objective is to meet these technical challenges at modest cost to enable future x -ray missions, including small Explorer missions in the near term, probe class missions in the medium term, and large flagship missions in the long term.