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.
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.
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 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.
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.
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.
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.
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.
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.
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.
Optics for the next generation’s high-resolution, high throughput x-ray telescope requires fabrication of wellformed 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 (half power diameter) for a single pair of mirrors tested at soft x-rays.
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.
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.