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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.

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.

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.

Progress within the field of x-ray astronomy depends on astronomical x-ray observations of ever-increasing quality and speed. Fast and high-resolution x-ray observations over a broad spectral range promise amazing new discoveries. These observations, however, require a spaceborne x-ray telescope of unprecedented imaging power. Of the numerous technological concerns associated with the design and construction of such a telescope, the x-ray focusing optics present a particularly complex and arduous set of challenges. An x-ray optical assembly comprises many thousands of x-ray mirrors, a most critical element. Our group at NASA Goddard Space Flight Center (GSFC) pursues the development of an x-ray mirror manufacturing process capable of meeting the stringent quality, production time, and cost requirements of the next-generation of x-ray telescopes. The manufacturing process employs monocrystalline silicon: a lightweight, stiff, thermally conductive, and readily available material which is free of internal stress; it is a nearly ideal material for a thin mirror substrate. The process involves various traditional optical fabrication techniques adapted to x-ray mirror geometry. Presently, our process is capable of fabricating sub-arcsecond half-powerdiameter (HPD) resolution mirror pairs (primary and secondary) at a mirror thickness of 0.5 mm and of virtually any x-ray optical design (e.g. Wolter-I, Wolter-Schwarzschild, etc.). The mirror substrate surface quality is comparable to, and sometimes exceeding, that of the mirrors on the Chandra X-ray Observatory. This paper describes the various manufacturing steps involved in the production of x-ray mirror substrates and a present status report.

X-ray astronomy critically depends on X-ray optics. The capability of an X-ray telescope is largelydetermined by the point-spread function (PSF) and the photon-collection area of its mirrors, the same astelescopes in other wavelength bands. Since an X-ray telescope must be operated above the atmosphere inspace and that X-rays reflect only at grazing incidence, X-ray mirrors must be both lightweight and thin, bothof which add significant technical and engineering challenge to making an X-ray telescope. In this paper wereport our effort at NASA Goddard Space Flight Center (GSFC) of developing an approach to making an Xraymirror assembly that can be significantly better than the mirror assembly currently flying on the ChandraX-ray Observatory in each of the three aspects: PSF, effective area per unit mass, and production cost per uniteffective area. Our approach is based on the precision polishing of mono-crystalline silicon to fabricate thinand lightweight X-ray mirrors of the highest figure quality and micro-roughness, therefore, having thepotential of achieving diffraction-limited X-ray optics. When successfully developed, this approach will makeimplementable in the 2020s and 2030s many X-ray astronomical missions that are currently on the drawingboard, including sounding rocket flights such as OGRE, Explorer class missions such as STAR-X andFORCE, Probe class missions such as AXIS, TAP, and HEX-P, as well as large missions such as Lynx.

At NASA Goddard Space Flight Center, we consistently produce affordable lightweight sub-arcsecond X-ray mirrors made of directly polished single crystal silicon. Silicon is favored for its high stiffness, low density, high thermal conductivity, zero internal stress, and commercial availability. Our manufacturing process includes traditional grinding, lapping, and polishing methods adapted to X-ray mirror geometry. These mirrors promise to meet the stringent requirements of various planned X-ray telescope missions. Presently, we are refining the many steps involved in our manufacturing process. This paper reports an overview of our mirror manufacturing process and the most recent results.

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 m 2 ) 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).

The Off-plane Grating Rocket Experiment (OGRE) is a soft X-ray spectroscopy suborbital rocket payload scheduled for launch in Q3 2020 from Wallops Flight Facility. The payload will serve as a testbed for several key technologies which can help achieve the desired performance increases for the next generation of X-ray spectrographs and other space-based missions: monocrystalline 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. With these three technologies, OGRE hopes to obtain the highest-resolution on-sky soft X-ray spectrum to date. We discuss the optical design of the OGRE payload.

We present the development of the reflective coating by magnetron sputtering deposition onto precisely-fabricated thin X-ray mirrors. Our goal is to remove distortion induced by the coating and then keep their surface pro les. We first addressed the uniform coating to minimize the distortion by introducing a mask to control the spatial distribution of the coating thickness. The uniformity was finally achieved within 1%. We next tried a platinum single-layer coating on a glass substrate with a dimension of 200 mm 125 mm. The distortion caused by the frontside coating with a thickness of 320 A was found to be at most 1 m, smaller than the previous results obtained from the non-uniform coating. We then carried out the platinum coating with the same amount of the thickness on the backside surface of the glass substrate. The surface pro le of the glass substrate was fully recovered, indicating that the residual stress was successfully balanced by the backside coating. Furthermore, we tried to an iridium single-layer coating with a thickness of 150 Aon the silicon mirrors. The frontside coating caused the degradation of the imaging quality by 7:5 arcsec in half-power width. However, the backside coating with the same amount of the thickness reduced this degradation to be 3:4 arcsec. Finally, an additional backside coating with a thickness of 100 A and the annealing to relax the residual stress were found to eliminate the distortion completely; the final degradation of the imaging quality was only 0:4 arcsec.

The Off-plane Grating Rocket Experiment (OGRE) is a sub-orbital rocket payload that will make the highest spectral resolution astronomical observation of the soft X-ray Universe to date. Capella, OGRE’s science target, has a well-defined line emission spectrum and is frequently used as a calibration source for X-ray observatories such as Chandra. This makes Capella an excellent target to test the technologies on OGRE, many of which have not previously flown. Through the use of state-of-the-art X-ray optics, co-aligned arrays of off-plane reflection gratings, and an X-ray camera based around four Electron Multiplying CCDs, OGRE will act as a proving ground for next generation X-ray spectrometers.

Single crystal silicon is an excellent X-ray mirror substrate material due to its high stiffness, low density, high thermal conductivity, zero internal stress, and commercial availability. At NASA Goddard Space Flight Center, we have been developing a process for producing high resolution and lightweight X-ray mirror segments at low cost and with high throughput. Previously we demonstrated the possibility of producing X-ray mirrors which meet the high demands of a future X-ray mission. Presently, we are producing lightweight X-ray mirror segments of unprecedented quality. This paper presents results from these recent investigations.

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.

Optics for the next generation's high-resolution, high throughput x-ray telescope requires fabrication of well-formed lightweight mirror segments and their integration at arc-second precision. Recent advances in the fabrication of silicon mirrors developed at NASA/Goddard prompted us to develop a new method of mirror alignment and integration. In this method, stiff silicon mirrors are aligned quasi-kinematically and are bonded in an interlocking fashion to produce a "meta-shell" with large collective area. We address issues of aligning and bonding mirrors with this method and show a recent result of 4 seconds-of-arc for a single pair of mirrors tested at soft x-rays.

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.

Monocrystalline silicon is an excellent X-ray mirror substrate material due to its high stiffness, low density, high thermal conductivity, zero internal stress, and commercial availability. Our work at NASA Goddard Space Flight Center focuses on identifying and developing a manufacturing process to produce high resolution and lightweight X-ray mirror segments in a cost and time effective manner. Previous efforts focused on demonstrating the feasibility of cylindrical silicon mirror polishing and lightweighting. Present efforts are aimed towards producing true paraboloidal and hyperboloidal mirror surfaces on the lightweight silicon segments. This paper presents results from these recent investigations, including a mirror which features a surface quality sufficient for a 3 arcsecond telescope.

Monocrystalline silicon as an x-ray mirror substrate material promises significant improvements over the x- ray mirror technologies used to date, since it is mechanically stiff, stress-free, highly thermally conductive, and widely commercially available. Producing highly accurate and lightweight x-ray mirrors from monocrystalline silicon requires a unique and specialized manufacturing process capable of producing mirrors quickly and cost effectively. The identification, development, and testing of this process is the focus of the work described in this proceeding. Monocrystalline silicon blocks were obtained, and a variety of processes (wire electro-discharge machining, etching, polishing) were applied to generate an accurate and stress-free cylindrical or Wolter-I mirror surface. The mirror surface is then sliced off at a thickness of

Five characteristics determine the utility of an x-ray optics technology for astronomy: (1) angular resolution, (2) field of view, (3) energy bandwidth, (4) mass per unit photon collecting area, and (5) production cost per unit photon collecting area. These five desired characteristics are always in conflict with each other. As a result, every past, current, and future x-ray telescope represents an astronomically useful compromise of these five characteristics. In this paper, we outline and report the proof of concept of a new approach of using single-crystal silicon to make lightweight x-ray optics. This approach combines the grinding polishing process, which is capable of making diffraction-limited optics of any kind, with the stress-free nature of single-crystal silicon, which enables post-fabrication light-weighting without distortion. As such this technology has the potential of making diffraction-limited lightweight x-ray optics for future astronomical missions, achieving unprecedented performance without incurring prohibitive mass and cost increase.

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.

The future of x-ray astronomy depends upon development of x-ray telescopes with larger aperture areas (approx. = 3 square meters) and fine angular resolution (approx. = 1 inch). Combined with the special requirements of nested grazing-incidence optics, the mass and envelope constraints of space-borne telescopes render such advances technologically and programmatically challenging. Achieving this goal will require precision fabrication, alignment, mounting, and assembly of large areas (approx. = 600 square meters) of lightweight (approx. = 1 kilogram/square meter areal density) high-quality mirrors at an acceptable cost (approx. = 1 million dollars/square meter of mirror surface area). This paper reviews relevant technological and programmatic issues, as well as possible approaches for addressing these issues-including active (in-space adjustable) alignment and figure correction.

The advancement of X-ray astronomy largely depends on technological advances in the manufacturing of X-ray optics. Future X-ray astronomy missions will require thousands of nearly perfect mirror segments to produce an X-ray optical assembly with < 5 arcsecond resolving capability. Present-day optical manufacturing technologies are not capable of producing thousands of such mirrors within typical mission time and budget allotments. Therefore, efforts towards the establishment of a process capable of producing sufficiently precise X-ray mirrors in a time-efficient and cost-effective manner are needed. Single-crystal silicon is preferred as a mirror substrate material over glass since it is stronger and free of internal stress, allowing it to retain its precision when cut into very thin mirror substrates. This paper details our early pursuits of suitable fabrication technologies for the mass production of sub-arcsecond angular resolution single-crystal silicon mirror substrates for X-ray telescopes.

Microelectromechanical systems (MEMS) micropore X-ray optics were proposed as an ultralightweight, high- resolution, and low cost X-ray focusing optic alternative to the large, heavy and expensive optic systems in use today. The optic's monolithic design which includes high-aspect-ratio curvilinear micropores with minimal sidewall roughness is challenging to fabricate. When made by either deep reactive ion etching or X-ray LIGA, the micropore sidewalls (re ecting surfaces) exhibit unacceptably high surface roughness. A magnetic eld-assisted nishing (MAF) process was proposed to reduce the micropore sidewall roughness of MEMS micropore optics and improvements in roughness have been reported. At this point, the best surface roughness achieved is 3 nm Rq on nickel optics and 0.2 nm Rq on silicon optics. These improvements bring MEMS micropore optics closer to their realization as functional X-ray optics. This paper details the manufacturing and post-processing of MEMS micropore X-ray optics including results of recent polishing experiments with MAF.

A taper angle and X-ray reflectivity of MEMS-Based silicon X-ray mirrors is quantitatively measured using a parallel X-ray beam at Al K α 1.49 keV.

We report a novel micromachined X-ray optics using DRIE formed micro mirrors for future space astronomical missions. We have fabricated a test optics and measured its imaging quality using X-rays. We have successfully verified the X-ray focusing using this type of optics for the first time in the world. The obtained angular resolution was about 20 arcmin and 3.1 mm. This result was consistent with the expected angular resolution based on the surface roughness of the DRIE formed side walls. To achieve a better angular resolution, we will condition annealing and magnetic field assisted finishing processes.

X-ray optics based on MEMS technologies can provide future astronomical missions with ultra light-weight and high-performance optical systems. Curvilinear micropores vertical to a thin wafer are made by using DRIE (Deep Reactive Ion Etching) or X-ray LIGA. The side walls are smoothed by using magnetic field assisted finishing and annealing technologies in order that the walls can reflect X-rays. Two or four such wafers are bent to spherical shapes with different curvature of radii and stacked, to focus parallel X-rays from astronomical objects by multiple reflections. In this paper, the concept and recent advances of the MEMS X-ray optics are reviewed.

An X-ray imaging test for an X-ray optical system based on MEMS technologies was conducted at the ISAS 30 m beamline. An X-ray reflection and focusing were successfully verified at Al Kα 1.49 keV for the first time. The image quality estimated as a half power diameter was ~20 arcmin. This was consistent with the angular resolution estimated from the surface roughness of 200 nm rms at 100 μm scale. In this paper, the experimental setup and the result of X-ray imaging analysis are reported.

An alternating magnetic field assisted finishing (MAF) technique has been developed to finish the 5–20 μm wide pore sidewalls of micro-pore X-ray focusing optics fabricated using micro-electro-mechanical systems (MEMS) techniques. To understand the material removal mechanism, this MAF technique is used to finish a silicon MEMS micro-pore X-ray optic that had previously undergone a hydrogen annealing treatment. Compared to the unfinished surface, distinctive surface features are observed on the finished surfaces using scanning electron microscopy, optical profilometry, and atomic force microscopy. This demonstrates the finishing characteristics and reveals the material removal mechanism on the nanometer scale. Moreover, the representative unfinished and finished micro-pore sidewall surfaces show a reduction in roughness due to finishing from 1.72 to 0.18 nm Rq.Copyright © 2010 by ASME

We have been developing ultra light-weight X-ray optics using MEMS (Micro Electro Mechanical Systems) technologies.We utilized crystal planes after anisotropic wet etching of silicon (110) wafers as X-ray mirrors and succeeded in X-ray reflection and imaging. Since we can etch tiny pores in thin wafers, this type of optics can be the lightest X-ray telescope. However, because the crystal planes are alinged in certain directions, we must approximate ideal optical surfaces with flat planes, which limits angular resolution of the optics on the order of arcmin. In order to overcome this issue, we propose novel X-ray optics based on a combination of five recently developed MEMS technologies, namely silicon dry etching, X-ray LIGA, silicon hydrogen anneal, magnetic fluid assisted polishing and hot plastic deformation of silicon. In this paper, we describe this new method and report on our development of X-ray mirrors fabricated by these technologies and X-ray reflection experiments of two types of MEMS X-ray mirrors made of silicon and nickel. For the first time, X-ray reflections on these mirrors were detected in the angular response measurements. Compared to model calculations, surface roughness of the silicon and nickel mirrors were estimated to be 5 nm and 3 nm, respectively.