Ryan S. McClelland

First results from a next-generation off-plane X-ray diffraction grating

Future NASA X-ray spectroscopy missions will require high throughput, high resolving power grating spectrometers. Off-plane reflection gratings are capable of meeting the performance requirements needed to realize the scientific goals of these missions. We have identified a novel grating fabrication method that utilizes common lithographic and microfabrication techniques to produce the high fidelity groove profile necessary to achieve this performance. Application of this process has produced an initial pre-master that exhibits a radial (variable line spacing along the groove dimension), high density (> 6000 grooves/mm), laminar profile. This pre-master has been tested for diffraction efficiency at the BESSY II synchrotron light facility and diffracts up to 55 % of incident light into usable spectral orders. Furthermore, tests of spectral resolving power show that these gratings are capable of obtaining resolving powers well above 1300 (λ/Δλ) with limitations due to the test apparatus, not the gratings. Obtaining these results has provided confidence that this fabrication process is capable of producing off-plane reflection gratings for the next generation of X-ray observatories.

Mirror Technology Development for the International X-ray Observatory Mission

The International X-ray Observatory mission is a collaborative effort of NASA, ESA, and JAXA. It will have unprecedented capabilities in spectroscopy, imaging, timing and polarization measurement. A key enabling element of the mission is a flight mirror assembly providing unprecedented large effective area (3 m2) and high angular resolution of (5 arcseconds half-power diameter). In this paper we outline the conceptual design of the mirror assembly and development of technology to enable its construction.

Next generation astronomical x-ray optics: high angular resolution, light weight, and low production cost

X-ray astronomy depends upon the availability of telescopes with high resolution and large photon colleX-ray astronomy depends upon the availability of telescopes with high resolution and large photon collecting areas. As astronomical x-ray observations can only be carried out above the atmosphere, these telescopes must necessarily be lightweight. Compounding the lightweight requirement is that an x-ray telescope consists of many nested concentric shells, which further requires that x-ray mirrors must be geometrically thin to achieve high packing efficiency. This double requirement—lightweight and geometrically thin—poses significant technical challenges in fabricating the mirrors and in integrating them into mirror assemblies. This paper reports on the approach, strategy, and status of our program to develop x-ray optics meeting these technical challenges at modest cost. The objective of this technology program is to enable future x-ray missions—including small Explorer missions in the near term, probe class missions in the medium term, and large flagship missions in the long term.ing areas. As astronomical x-ray observations can only be carried out above the atmosphere, these telescopes must necessarily be lightweight. Compounding the lightweight requirement is that an x-ray telescope consists of many nested concentric shells, which further requires that x-ray mirrors must be geometrically thin to achieve high packing efficiency. This double requirement—lightweight and geometrically thin—poses significant technical challenges in fabricating the mirrors and in integrating them into mirror assemblies. This paper reports on the approach, strategy, and status of our program to develop x-ray optics meeting these technical challenges at modest cost. The objective of this technology program is to enable future x-ray missions—including small Explorer missions in the near term, probe class missions in the medium term, and large flagship missions in the long term.

Lightweight and high angular resolution x-ray optics for astronomical missions

X-ray optics of both high angular resolution and light weight are essential for advancing x-ray astrophysics. High angular resolution is important for avoiding source confusion and reducing background, thus allowing observation of the most distant objects in the early Universe. It is also important in enabling gratings to achieve high spectral resolution, to study the myriad plasmas in planetary, stellar, and galactic environments, as well as inter-planetary, inter-stellar, and inter-galactic media. Light weight is essential for further increasing photon collection area: X-ray observations must be performed from space, where mass available for a telescope has always been and is expected to continue to be quite limited. This paper reports on a program to develop x-ray optics satisfying these two requirements. The objective of this technology program is to enable Explorer-class missions in the near term and facility-class missions in the long term.

Design and analysis of the International X-Ray Observatory mirror modules

The Soft X-Ray Telescope (SXT) modules are the fundamental focusing assemblies on NASAs next major X-ray telescope mission, the International X-Ray Observatory (IXO). {su12]The preliminary design and analysis of these assemblies has been completed, addressing the major engineering challenges and leading to an understanding of the factors effecting module performance. Each of the 60 modules in the Flight Mirror Assembly (FMA) supports 200–300 densely packed 0.4 mm thick glass mirror segments in order to meet the unprecedented effective area required to achieve the scientific objectives of the mission. Detailed Finite Element Analysis (FEA), materials testing, and environmental testing have been completed to ensure the modules can be successfully launched. Resulting stress margins are positive based on detailed FEA, a large factor of safety, and a design strength determined by robust characterization of the glass properties. FEA correlates well with the results of the successful modal, vibration, and acoustic environmental tests. Deformation of the module due to on-orbit thermal conditions is also a major design driver. A preliminary thermal control system has been designed and the sensitivity of module optical performance to various thermal loads has been determined using optomechanical analysis methods developed for this unique assembly. This design and analysis furthers the goal of building a module that demonstrates the ability to meet IXO requirements, which is the current focus of IXO FMA technology development team.

High resolution and high throughput x-ray optics for future astronomical missions

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.

Design concept for the international x-ray observatory Flight Mirror Assembly

The Flight Mirror Assembly (FMA) mechanical design for NASAs next major x-ray telescope mission, the International X-Ray Observatory (IXO), recently kicked off at NASA. The design presents some unique engineering challenges requiring a novel mirror design due to the high angular resolution and large effective area required to achieve the desired scientific objectives [1]. The Wolter-I x-ray telescope optical design requires about 14,000 0.4mm thick glass mirror segments to be densely packed into a 3.2m diameter FMA and supported with micron level accuracy and stability. Key challenges addressed by the FMA design concept include bonding the mirrors into the module without distortion, designing the segment support for glass survivability, keeping the structure light enough to launch, providing a large effective area, and preventing unacceptable thermal distortion. The thin mirror segments are mounted into intermediate wedge shaped structures called modules. Modules are kinematically mounted to the FMA primary structure which is being optimized for minimum mass and maximum projected area in the focal plane. The current design approach appears feasible without new technology development beyond that currently in process.

Mechanical overview of the International X-ray Observatory

The International X-ray Observatory (IXO) is a new collaboration between NASA, ESA, and JAXA which is under study for launch in 2020. IXO will be a large 6600 kilogram Great Observatory-class mission which will build upon the legacies of the Chandra and XMM-Newton X-ray observatories. It combines elements from NASAs Constellation-X program and ESAs XEUS program. The observatory will have a 20–25 meter focal length, which necessitates the use of a deployable instrument module. Currently the project is actively trading configurations and layouts of the various instruments and spacecraft components. This paper will provide a snapshot of the latest observatory configuration under consideration and summarize the observatory from the mechanical engineering perspective.

Coating thin mirror segments for lightweight x-ray optics

Next generation’s lightweight, high resolution, high throughput optics for x-ray astronomy requires integration of very thin mirror segments into a lightweight telescope housing without distortion. Thin glass substrates with linear dimension of 200 mm and thickness as small as 0.4 mm can now be fabricated to a precision of a few arc-seconds for grazing incidence optics. Subsequent implementation requires a distortion-free deposition of metals such as iridium or platinum. These depositions, however, generally have high coating stresses that cause mirror distortion. In this paper, we discuss the coating stress on these thin glass mirrors and the effort to eliminate their induced distortion. It is shown that balancing the coating distortion either by coating films with tensile and compressive stresses, or on both sides of the mirrors is not sufficient. Heating the mirror in a moderately high temperature turns out to relax the coated films reasonably well to a precision of about a second of arc and therefore provide a practical solution to the coating problem.

Reflective Coating for Lightweight X-Ray Optics

X-ray reflective coating for next generation’s lightweight, high resolution optics for astronomy requires thin-film deposition that is precisely fine-tuned so that it will not distort the thin sub-mm substrates. Film of very low stress is required. Films with multi-layer or bi-layer can be deposited to give an effective low stress which cause negligible distortion. Alternatively, mirror distortion can be cancelled by precisely balancing the deformation by coating films on both sides of the substrates. We have been developing techniques to coat glass substrates that can provide good reflectivity in the soft x-ray band below 10 keV, and yet introduce negligible surface distortion for arc-second optics. These efforts include: low-stress deposition by magnetron sputtering and atomic layer deposition of the metals, balancing of gross deformation with two-layer depositions of opposite stresses and with depositions on both sides of the thin mirrors.