Recent advances in the fabrication of segmented silicon mirrors make it possible to build large-area, lightweight, high-resolution x-ray telescopes with arc-second angular resolution. To build such a telescope, we fabricate accurate silicon mirrors and develop alignment and bonding techniques to precisely align and integrate these silicon mirror segments into modular units. In this way, the processes of mirror fabrication, mirror alignment and bonding, and subsequent integration into units of successive larger scale are completely independent, and their technologies can be developed independently. In this paper, we present recent improvement in the precision of optical alignment and mirror bonding. We discuss the measurement of the mirror’s focusing in a parallel optical beam and address the practical challenges in bonding these mirrors into modules as an intermediate step to build up a full-scale telescope for space missions.
Astronomical observations of distant and faint X-ray sources will expand our understanding of the evolving universe. These challenging science goals require X-ray optical elements that are manufactured, measured, coated, aligned, assembled, and tested at scale. The Next Generation X-ray Optics (NGXO) group at NASA Goddard Space Flight Center is developing solutions to the challenges faced in planning, constructing, and integrating X-ray optics for future telescopes such as the Lynx Large Mission concept for the Astro2020 Decadal Survey on Astronomy and Astrophysics, Probe Mission concepts AXIS, TAP, and HEX-P, the Explorer Mission concepts STAR-X and FORCE and the sub-orbital mission OGRE. The lightweight mirror segments, efficiently manufactured from blocks of commercially available monocrystalline silicon, are coated, aligned, and fixed in modular form. This paper discusses our first attempt to encapsulate our technology experience and knowledge into a model to meet the challenge of engineering and production of the many modules required for a spaceflight mission. Through parallel lines of fabrication, assembly, and testing, as well as the use of existing high throughput industrial technologies, ∼104 coated X-ray mirror segments can be integrated into ∼103 modules adhering to a set budget and schedule that survive environmental testing and approach the diffraction limit.
The capability of an X-ray telescope depends on the quality of its mirror, which can be characterized by four quantities: point-spread-function, photon-collecting area, field of view, and energy bandwidth. In this paper, we report on our effort of developing an X-ray mirror technology that advances all of those four quantities for future X-ray astronomical missions. In addition, we have adopted a modular approach, capable of making mirror assemblies for missions of all sizes, from large missions like Lynx, to medium-sized Probes like AXIS, TAP, and HEX-P, to Explorers like STAR-X and FORCE, and to small sub-orbital missions like OGRE. This approach takes into account that all X-ray telescopes must be spaceborne and therefore require their mirror assemblies be lightweight. It is designed to make use of modern mass production techniques and commercial off-the-shelf equipment and materials to maximize production throughput and thereby to minimize implementation schedule and costs.
The Off-plane Grating Rocket Experiment (OGRE) is a soft X-ray spectroscopy suborbital rocket payload designed to obtain the highest-resolution soft X-ray spectrum of Capella to date. With a spectral resolution goal of R(λ/▵λ) < 2000 at select wavelengths in its 10-55 Å bandpass of interest, the payload will be able to study the line-dominated spectrum of Capella in unprecedented detail. To achieve this performance goal, the payload will employ three key technologies: mono-crystalline silicon X-ray mirrors developed at NASA Goddard Space Flight Center, reflection gratings manufactured at The Pennsylvania State University, and electron-multiplying CCDs developed by The Open University and XCAM Ltd. In this document, an updated optical design that can achieve the performance goal of the OGRE spectrometer and a new grating alignment concept to realize this optical design are described.
The fundamental developmental issue facing the next generation of X-ray astronomical telescopes is the manufacturability, assembly, and structural robustness of the grazing incidence optics. Combining the high angular resolution requirements with large effective areas and physical launch vehicle restrictions leads to very thin shelled optics that must remain very stable. Meeting these stability requirements while also surviving launch and space environments presents a significant engineering challenge. Over the last few years, the Next Generation X-ray Optics (NGXO) team at NASA Goddard has been developing thin segmented silicon optics that are assembled into both modules and meta-shells, which show great promise in meeting these challenges. This paper summarizes the analytical approaches, as well as the environmental tests, used to assess such assemblies. Many parameters in the design space of the assembly have been assessed and optimized using Finite Element (FE) models and ray trace algorithms. The results of these analyses have helped shape reasonable and justifiable error budgets, as well as guide the team’s decision making in both near and long term processes. The structural integrity of an assembly has been assessed both with testing and FE models. Preliminary strength testing has been conducted on the basic components used in the assembly.
Future astronomical X-ray spectrometer missions call for high spectral resolution in conjunction with high throughput. To achieve both of these requirements simultaneously, many grating elements must be aligned such that their diffraction arcs overlap at the focal plane. Methods for the alignment of reflection gratings operated in the extreme off-plane mount are being developed at The Pennsylvania State University in support of the Off-plane Grating Rocket Experiment. We report on the alignment methodology and performance tests of an aligned reflection grating module.
The Off-plane Grating Rocket Experiment (OGRE) is a sounding rocket payload designed to obtain a high-resolution soft X-ray spectrum of Capella. OGRE’s optical system uses new technologies including state-of-the-art X-ray optics, custom arrays of reflection gratings, and an array of EM-CCDs. Many of these technologies will be tested for the first time in flight with OGRE. To achieve the high performance that these new technologies are capable of, the payload components must be properly aligned to meet high tolerances. This paper will outline OGRE’s opto-mechanical design for achieving alignment within these tolerances. Specifically, the design of the X-ray grating arrays will be discussed.
We describe an approach to build an x-ray mirror assembly that can meet Lynx’s requirements of high-angular resolution, large effective area, light weight, short production schedule, and low-production cost. Adopting a modular hierarchy, the assembly is composed of 37,492 mirror segments, each of which measures ∼100 mm × 100 mm × 0.5 mm. These segments are integrated into 611 modules, which are individually tested and qualified to meet both science performance and spaceflight environment requirements before they in turn are integrated into 12 metashells. The 12 metashells are then integrated to form the mirror assembly. This approach combines the latest precision polishing technology and the monocrystalline silicon material to fabricate the thin and lightweight mirror segments. Because of the use of commercially available equipment and material and because of its highly modular and hierarchical building-up process, this approach is highly amenable to automation and mass production to maximize production throughput and to minimize production schedule and cost. As of fall 2018, the basic elements of this approach, including substrate fabrication, coating, alignment, and bonding, have been validated by the successful building and testing of single-pair mirror modules. In the next few years, the many steps of the approach will be refined and perfected by repeatedly building and testing mirror modules containing progressively more mirror segments to fully meet science performance, spaceflight environments, as well as programmatic requirements of the Lynx mission and other proposed missions, such as AXIS.
Recent advances in the fabrication of silicon mirrors and their alignment and integration methods make it possible to build large-area, lightweight, high-resolution x-ray telescopes with arc-second angular resolution. Such a telescope, having simultaneously arc-second resolution and large (> 1 m2 ) collecting area, has never been built before and it will revolutionize high energy astronomy. For such optics, the challenges are twofold: fabrication of high quality mirror segments and precise integration of thousands of these mirrors to a common sharp focus. In this paper, we address the technology for the mirror integration carried out at Goddard Space Flight Center and report the recent result of making such high-resolution optics. We address the crucial technology components: positioning a mirror, measuring its focus, adjusting its mount pointsto optimize the focus, bonding the mirror, and co-alignment of mirrors. We also present the latest x-ray test results that demonstrate the efficacy of such methods and address areas for further improvement. Presently, mirrors built this way have a resolution of 2²-3² HPD (half-ower diameter).
X-ray astronomy critically depends on X-ray optics. The capability of an X-ray telescope is largely determined by the point-spread function (PSF) and the photon-collection area of its mirrors, the same as telescopes in other wavelength bands. Since an X-ray telescope must be operated above the atmosphere in space and that X-rays reflect only at grazing incidence, X-ray mirrors must be both lightweight and thin, both of which add significant technical and engineering challenge to making an X-ray telescope. In this paper we report our effort at NASA Goddard Space Flight Center (GSFC) of developing an approach to making an Xray mirror assembly that can be significantly better than the mirror assembly currently flying on the Chandra X-ray Observatory in each of the three aspects: PSF, effective area per unit mass, and production cost per unit effective area. Our approach is based on the precision polishing of mono-crystalline silicon to fabricate thin and lightweight X-ray mirrors of the highest figure quality and micro-roughness, therefore, having the potential of achieving diffraction-limited X-ray optics. When successfully developed, this approach will make implementable in the 2020s and 2030s many X-ray astronomical missions that are currently on the drawing board, including sounding rocket flights such as OGRE, Explorer class missions such as STAR-X and FORCE, Probe class missions such as AXIS, TAP, and HEX-P, as well as large missions such as Lynx.
Next Generation X-ray Optics (NGXO) team at the Goddard Space Flight Center (GSFC) has been developing a new silicon-based grazing incidence mirror technology for future high resolution X-ray astronomical missions. Recently, the GSFC team completed the construction of first few mirror modules that contain one pair of mirrors. One of the mirror pairs was tested in GSFC 600-m long beamline facility and PANTER (Neuried, Germany) 120-m long X-ray beamline facility. Both full aperture X-ray tests, Hartmann tests, and focal plane sweeps were completed. In this paper we present the data analysis process and compare the results from our models to measured X-ray centroid data, X-ray performance data, and out of focus images of the mirror pair.
Lynx is a concept under study for prioritization in the 2020 Astrophysics Decadal Survey. Providing orders of magnitude increase in sensitivity over Chandra, Lynx will examine the first black holes and their galaxies, map the large-scale structure and galactic halos, and shed new light on the environments of young stars and their planetary systems. In order to meet the Lynx science goals, the telescope consists of a high-angular resolution optical assembly complemented by an instrument suite that may include a High Definition X-ray Imager, X-ray Microcalorimeter and an X-ray Grating Spectrometer. The telescope is integrated onto the spacecraft to form a comprehensive observatory concept. Progress on the formulation of the Lynx telescope and observatory configuration is reported in this paper.
Angular resolution and photon-collecting area are the two most important factors that determine the power of an X-ray astronomical telescope. The grazing incidence nature of X-ray optics means that even a modest photon-collecting area requires an extraordinarily large mirror area. This requirement for a large mirror area is compounded by the fact that X-ray telescopes must be launched into, and operated in, outer space, which means that the mirror must be both lightweight and thin. Meanwhile the production and integration cost of a large mirror area determines the economical feasibility of a telescope. In this paper we report on a technology development program whose objective is to meet this three-fold requirement of making astronomical X-ray optics: (1) angular resolution, (2) photon-collecting area, and (3) production cost. This technology is based on precision polishing of monocrystalline silicon for making a large number of mirror segments and on the metashell approach to integrate these mirror segments into a mirror assembly. The meta-shell approach takes advantage of the axial or rotational symmetry of an X-ray telescope to align and bond a large number of small, lightweight mirrors into a large mirror assembly. The most important features of this technology include: (1) potential to achieve the highest possible angular resolution dictated by optical design and diffraction; and (2) capable of implementing every conceivable optical design, such as Wolter-I, WolterSchwarzschild, as well as other variations to one or another aspect of a telescope. The simplicity and modular nature of the process makes it highly amenable to mass production, thereby making it possible to produce very large X-ray telescopes in a reasonable amount of time and at a reasonable cost. As of June 2017, the basic validity of this approach has been demonstrated by finite element analysis of its structural, thermal, and gravity release characteristics, and by the fabrication, alignment, bonding, and X-ray testing of mirror modules. Continued work in the coming years will raise the technical readiness of this technology for use by SMEX, MIDEX, Probe, as well as major flagship missions.
In order to advance significantly scientific objectives, future x-ray astronomy missions will likely call for x-ray telescopes
with large aperture areas (≈ 3 m2) and fine angular resolution (≈ 12). Achieving such performance is programmatically
and technologically challenging due to the mass and envelope constraints of space-borne telescopes and to the need for
densely nested grazing-incidence optics. Such an x-ray telescope will require precision fabrication, alignment, mounting,
and assembly of large areas (≈ 600 m2) of lightweight (≈ 2 kg/m2 areal density) high-quality mirrors, at an acceptable cost
(≈ 1 M$/m2 of mirror surface area). This paper reviews relevant programmatic and technological issues, as well as possible
approaches for addressing these issues-including direct fabrication of monocrystalline silicon mirrors, active (in-space
adjustable) figure correction of replicated mirrors, static post-fabrication correction using ion implantation, differential
erosion or deposition, and coating-stress manipulation of thin substrates.
We describe an approach to building mirror assemblies for next generation X-ray telescopes. It incorporates knowledge and lessons learned from building existing telescopes, including Chandra, XMM-Newton, Suzaku, and NuSTAR, as well as from our direct experience of the last 15 years developing mirror technology for the Constellation-X and International X-ray Observatory mission concepts. This approach combines single crystal silicon and precision polishing, thus has the potential of achieving the highest possible angular resolution with the least possible mass. Moreover, it is simple, consisting of several technical elements that can be developed independently in parallel. Lastly, it is highly amenable to mass production, therefore enabling the making of telescopes of very large photon collecting areas.
Lightweight and high resolution optics are needed for future space-based x-ray telescopes to achieve advances in highenergy astrophysics. Past missions such as Chandra and XMM-Newton have achieved excellent angular resolution using a full shell mirror approach. Other missions such as Suzaku and NuSTAR have achieved lightweight mirrors using a segmented approach. This paper describes a new approach, called meta-shells, which combines the fabrication advantages of segmented optics with the alignment advantages of full shell optics. Meta-shells are built by layering overlapping mirror segments onto a central structural shell. The resulting optic has the stiffness and rotational symmetry of a full shell, but with an order of magnitude greater collecting area. Several meta-shells so constructed can be integrated into a large x-ray mirror assembly by proven methods used for Chandra and XMM-Newton.
The mirror segments are mounted to the meta-shell using a novel four point semi-kinematic mount. The four point mount deterministically locates the segment in its most performance sensitive degrees of freedom. Extensive analysis has been performed to demonstrate the feasibility of the four point mount and meta-shell approach. A mathematical model of a meta-shell constructed with mirror segments bonded at four points and subject to launch loads has been developed to determine the optimal design parameters, namely bond size, mirror segment span, and number of layers per meta-shell. The parameters of an example 1.3 m diameter mirror assembly are given including the predicted effective area. To verify the mathematical model and support opto-mechanical analysis, a detailed finite element model of a meta-shell was created. Finite element analysis predicts low gravity distortion and low sensitivity to thermal gradients.
High-resolution, high throughput optics for x-ray astronomy entails fabrication of well-formed mirror segments and their integration with arc-second precision. In this paper, we address issues of aligning and bonding thin glass mirrors with negligible additional distortion. Stability of the bonded mirrors and the curing of epoxy used in bonding them were tested extensively. We present results from tests of bonding mirrors onto experimental modules, and on the stability of the bonded mirrors tested in x-ray. These results demonstrate the fundamental validity of the methods used in integrating mirrors into telescope module, and reveal the areas for further investigation. The alignment and integration methods are applicable to the astronomical mission concept such as STAR-X, the Survey and Time-domain Astronomical Research Explorer.