High Angular Resolution and Lightweight X-Ray Optics for Astronomical Missions

X-ray optics with both high angular resolution and lightweight is essential for further progress in x-ray astronomy. High angular resolution is important in avoiding source confusion and reducing background to enable the observation of the most distant objects of the early Universe. It is also important in enabling the use of gratings to achieve high spectral resolution to study, among other things, the myriad plasmas that exist in planetary, stellar, galactic environments, as well as interplanetary, inter-stellar, and inter-galactic media. Lightweight is important for further increase in effective photon collection area, because x-ray observations must take place on space platforms and the amount of mass that can be launched into space has always been very limited and is expected to continue to be very limited. This paper describes an x-ray optics development program and reports on its status that meets these two requirements. The objective of this program is to enable Explorer type missions in the near term and to enable flagship missions in the long term. Introduction By any measure x-ray astronomy is enjoying its golden age. The three currently operating observatories, Chandra, XMM-Newton, and Suzaku, have advanced our understanding of the Universe to an unprecedented level. In the meantime they have also raised questions that can only be answered by future x-ray telescopes with one better angular resolution, larger photon collection area, higher energy spectral resolution. This x-ray optics technology development program was initiated in 2001 (Zhang et al. 2003) in direct support of the Constellation-X mission development (White & Tananbaum 2003). The Constellation-X mission, which later became the International X-ray Observatory (IXO), was designed to be the successor to the Chandra x-ray observatory. With its priority set on spectroscopic studies, IXO required a moderate angular resolution of ~5" HPD (half-power diameter) and a large photon collection area of> 1 m• Figure 1 places these requirements and the achievements of this program as of August 2011 in the context of the technologies that built the three telescopes: ground and polished Zerodur shells for Chandra (Gordon & Catching, 1994), electroformed nickel shells for XMM-Newton (Gondoin et al. 1994), and epoxy replicated aluminum foils for Suzaku (Serlemistsos et al. 2007). In general, four parameters characterize an x-ray optics technology: (1) angular resolution, (2) effective are per unit mass, (3) production cost per unit effective area, and (4) production rate or schedule. Figure 1 use the first two variables to show that the three current missions form more or less a line that demarcates the past and future of x-ray telescope making. Above and to the left of the line is the region representing the past and telescopes that are easy to build and less powerful, and therefore is of no interest for now. Below and to the right of the line is the region representing the future. Any telescope in this region requires technology development. This technology development program is based on the segmented approach to building x-ray telescopes, as shown in Figure 2. It is hierarchical and suited to building both small and large telescopes, which differ mainly in the number of modules that need to be built and assembled. In either case, the dimensions of the module and the number of mirror segments contained therein are substantially similar. The objective of this technology program is to develop all necessary techniques to construct mirror modules that meet x-ray performance requirements in angular resolution and effective area, and environment requirements. The process and components of building a module are illustrated in Figure 3. It consists of three main steps: (1) forming mandrel fabrication, (2) mirror segment fabrication, and (3) installation of mirror segments into module housing. Each of these main steps in tum consists of one or more smaller steps. This technology program's objective is to develop and perfect each of these steps so that they can be engineered to become highly accurate to meet x-ray optical requirements and highly reliable and efficient to minimize both cost and schedule. The totality of this process's qualification lies in the successful and repeated construction of modules that meet those requirements. The rest of this paper describes the requirements and status of each of these steps as of August 2011.