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.
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.
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.
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.
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.
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.
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 <1 mm and further processed to yield a mirror segment with <1 arcsecond RMS slope errors. Furthermore, our experiments suggest that this mirror production process requires ~2 days to produce a mirror segment and is easily integrated into a cost-reducing parallel processing scheme. Presently, there is strong evidence that the mirror production process described in this paper will meet the stringent requirements of future x-ray missions.
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.
The future of x-ray astronomy depends upon development of x-ray telescopes with larger aperture areas (≈ 3 m2) and
fine angular resolution (≈ 1″). 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 (≈ 600 m2) of
lightweight (≈ 1 kg/m2 areal density) high-quality mirrors at an acceptable cost (≈ 1 M$/m2 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.
Future x-ray astronomical missions require x-ray mirror assemblies that provide both high angular resolution and large photon collecting area. In addition, as x-ray astronomy undertakes more sensitive sky surveys, a large field of view is becoming increasingly important as well. Since implementation of these requirements must be carried out in broad political and economical contexts, any technology that meets these performance requirements must also be financially affordable and can be implemented on a reasonable schedule. In this paper we report on progress of an x-ray optics development program that has been designed to address all of these requirements. The program adopts the segmented optical design, thereby is capable of making both small and large mirror assemblies for missions of any size. This program has five technical elements: (1) fabrication of mirror substrates, (2) coating, (3) alignment, (4) bonding, and (5) mirror module systems engineering and testing. In the past year we have made progress in each of these five areas, advancing the angular resolution of mirror modules from 10.8 arc-seconds half-power diameter reported (HPD) a year ago to 8.3 arc-seconds now. These mirror modules have been subjected to and passed all environmental tests, including vibration, acoustic, and thermal vacuum. As such this technology is ready for implementing a mission that requires a 10-arc-second mirror assembly. Further development in the next two years would make it ready for a mission requiring a 5-arc-second mirror assembly. We expect that, by the end of this decade, this technology would enable the x-ray astrophysical community to compete effectively for a major x-ray mission in the 2020s that would require one or more 1-arc-second mirror assemblies for imaging, spectroscopic, timing, and survey studies.
X-ray optics is an essential component of every conceivable future x-ray observatory. Its astronomical utility is measured with two quantities: angular resolution and photon collecting area. The angular resolution determines the quality of its images and the photon collecting area determines the faintest sources it is capable of detecting and studying. Since it must be space-borne, the resources necessary to realize an x-ray mirror assembly, such as mass and volume, are at a premium. In this paper we report on a technology development program designed to advance four metrics that measure the capability of an x-ray mirror technology: (1) angular resolution, (2) mass per unit photon collecting area, (3) volume per unit photon collecting area, and (4) production cost per unit photon collecting area.
We have adopted two approaches. The first approach uses the thermal slumping of thin glass sheets. It has advantages in mass, volume, and cost. The objective for this approach is improving its angular resolution. As of August 2013, we have been able to consistently build and test with x-ray beams modules that contain three co-aligned Wolter-I parabolichyperbolic mirror pairs, achieving a point spread function (PSF) of 11 arc-second half-power diameter (HPD), to be compared with the 17 arc-seconds we reported last year. If gravity distortion during x-ray tests is removed, these images would have a resolution of 9 arc-seconds, meeting requirements for a 10 arc-second flight mirror assembly. These modules have been subjected to a series of vibration, acoustic, and thermal vacuum tests.
The second approach is polishing and light-weighting single crystal silicon, a material that is commercially available, inexpensive, and without internal stress. This approach has advantages in angular resolution, mass, and volume, and objective is reducing fabrication cost to make it financially feasible to fabricate the ~103 m2 mirror area that would be required for a future major x-ray observatory.
The overall objective of this technology program is to enable missions in the upcoming years with a 10 arc-second angular resolution, and missions with ~1 arc-second angular resolution in the 2020s.
Our development of ultra light-weight X-ray micro pore optics based on MEMS (Micro Electro Mechanical System)
technologies is described. Using dry etching or X-ray lithography and electroplating, curvilinear sidewalls
through a flat wafer are fabricated. Sidewalls vertical to the wafer surface are smoothed by use of high temperature
annealing and/or magnetic field assisted finishing to work as X-ray mirrors. The wafer is then deformed to
a spherical shape. When two spherical wafers with different radii of curvature are stacked, the combined system
will be an approximated Wolter type-I telescope. This method in principle allows high angular resolution and
ultra light-weight X-ray micro pore optics. In this paper, performance of a single-stage optic, coating of a heavy
metal on sidewalls with atomic layer deposition, and assembly of a Wolter type-I telescope are reported.
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.
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.
In recent years, X-ray telescopes have been shrinking in both size and weight to reduce cost and volume on
space flight missions. Current designs focus on the use of MEMS technologies to fabricate ultra-lightweight and
high-resolution X-ray optics. In 2006, Ezoe et al. introduced micro-pore X-ray optics fabricated using anisotropic
wet etching of silicon (110) wafers. These optics, though extremely lightweight (completed telescope weight 1
kg or less for an effective area of 1000 cm2), had limited angular resolution, as the reflecting surfaces were flat
crystal planes. To achieve higher angular resolution, curved reflecting surfaces should be used.
Both silicon dry etching and X-ray LIGA were used to create X-ray optics with curvilinear micro-pores;
however, the resulting surface roughness of the curved micro-pore sidewalls did not meet X-ray reflection criteria
of 10 nm rms in a 10 μm2 area. This indicated the need for a precision polishing process. This paper describes
the development of an ultra-precision polishing process employing an alternating magnetic field assisted finishing
process to polish the micro-pore side walls to a mirror finish (< 4 nmrms). The processing principle is presented,
and a polishing machine is designed and fabricated to explore the feasibility of this polishing process as a possible
method for processing MEMS X-ray optics to meet X-ray reflection specifications.