Marton V. Sharpe

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.

The Nuclear Spectroscopic Telescope Array (NuSTAR): optics overview and current status

The Nuclear Spectroscopic Telescope Array (NuSTAR) is a NASA Small Explorer mission scheduled for launch in February 2012. NuSTAR will deploy two imaging CdZnTe spectrometers in the 6-79 keV energy band. The two NuSTAR optics utilize multilayer-coated, thermally-slumped glass integrated into a titanium-glass-epoxy-graphite composite structure, along with an extendable mast, to obtain 10.15 meter focal length. Using this approach, the NuSTAR optics will obtain subarcminute imaging with large effective area over its entire energy band. NuSTARs conic-approximation Wolter-I optics are the first true hard X-ray focusing optics to be deployed on a satellite experiment. We report on the design of the NuSTAR optics, present the status of the two flight optics under construction, and report preliminary measurements that can be used to predict performance.

Fabrication of the NuSTAR flight optics

We describe the fabrication of the two NuSTAR flight optics modules. The NuSTAR optics modules are glass-graphiteepoxy composite structures to be employed for the first time in space-based X-ray optics by NuSTAR, a NASA Small Explorer schedule for launch in February 2012. We discuss the optics manufacturing process, the qualification and environmental testing performed, and briefly discuss the results of X-ray performance testing of the two modules. The integration and alignment of the completed flight optics modules into the NuSTAR instrument is described as are the optics module thermal shields.

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.

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.

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.

Preserving accurate figures in coating and bonding mirrors for lightweight x-ray telescopes

Lightweight, high-resolution, high throughput optics for x-ray astronomy requires fabrication and integration of thin mirrors segments with arc-second precision. In this paper, we present results on our effort leading to the most recent two test modules achieving the intermediate goal of 10 arc-second resolution. We will address issues of coating and bonding thin glass mirrors with negligible distortion. Annealing of sputtered high-density metallic films was found to be sufficiently accurate. We will present result from tests of bonding mirrors onto experimental strongbacks, as well as the sensitivity on bonding procedure, bond parameters and environment.

Progress on the fabrication of high resolution and lightweight monocrystalline silicon x-ray mirrors

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.