Linette D. Kolos

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.

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.

Fabrication of single crystal silicon mirror substrates for X-ray astronomical missions

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.

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.

Fabrication of high resolution and lightweight monocrystalline silicon x-ray mirrors

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.

Aligning, bonding, and testing mirrors for lightweight x-ray telescopes

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.

Light weight mirrors from single crystal silicon

A process for fabricating high quality light weight mirrors from single crystal silicon is described. The process uses conventional fabrication techniques in an unconventional sequence. It is capable of producing mirrors with 1/4th the mass of an equivalent solid quartz mirror. Each mirror is a monolithic structure of single crystal silicon. Mirrors with optical figures better than 1/10th wave peak-to-valley (@633 nm) have been demonstrated.