L. Olsen

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.

An approach for alignment, mounting, and integration of IXO mirror segments

The telescope on the International X-ray Observatory (IXO) comprises nearly 15 thousand thin glass mirror segments, each of them is capable of reflecting board-band soft x-rays at grazing angles. These mirror segments form densely packed, two-staged shells, in a Wolter type I optical design, in which each pair of the mirrors focus x-ray onto the focal plane in two reflections. The requirement in angular resolution of the IXO telescope is 5 arc-seconds. This requirement places severe challenges in forming precisely shaped mirror segments as well as in aligning and mounting these thin mirrors, which are 200 to 400 mm in size and 0.4 mm in thickness. In this paper, we will describe an approach for aligning and mounting the IXO mirror segments, in which no active adjustment is made to correct for any existing figure errors. The approach comprises processes such as suspension of a mirror under gravity to minimize gravity distortion, temporary bonding onto a strongback, alignment and transfer to a permanent structure and release of mirror from the temporary mount. Experimental results and analysis in this development are reported.

Monocrystalline Silicon and the Meta-Shell Approach to Building X-Ray Astronomical Optics

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.

Mounting and alignment of IXO mirror segments

A suspension-mounting scheme is developed for the IXO (International X-ray Observatory) mirror segments in which the figure of the mirror segment is preserved in each stage of mounting. The mirror, first fixed on a thermally compatible strongback, is subsequently transported, aligned and transferred onto its mirror housing. In this paper, we shall outline the requirement, approaches, and recent progress of the suspension mount processes.

The X-ray polarimeter instrument on board the Polarimeter for Relativistic Astrophysical X-ray Sources (PRAXyS) mission

The Polarimeter for Relativistic Astrophysical X-ray Sources (PRAXyS) is one of three Small Explorer (SMEX) missions selected by NASA for Phase A study. The PRAXyS observatory carries an X-ray Polarimeter Instrument (XPI) capable of measuring the linear polarization from a variety of high energy sources, including black holes, neutron stars, and supernova remnants. The XPI is comprised of two identical mirror-Time Projection Chamber (TPC) polarimeter telescopes with a system effective area of 124 cm2 at 3 keV, capable of photon limited observations for sources as faint as 1 mCrab. The XPI is built with well-established technologies. This paper will describe the performance of the XPI flight mirror with the engineering test unit polarimeter.

Reflective coatings for the future x-ray mirror substrates

We present the development of the reflective coating by magnetron sputtering deposition onto precisely-fabricated thin X-ray mirrors. Our goal is to remove distortion induced by the coating and then keep their surface profiles. We first addressed the uniform coating to minimize the distortion by introducing a mask to control the spatial distribution of the coating thickness. The uniformity was finally achieved within ±1%. We next tried a platinum single-layer coating on a glass substrate with a dimension of 200 mm × 125 mm. The distortion caused by the frontside coating with a thickness of 320 Å was found to be at most ∼ 1 μm, smaller than the previous results obtained from the non-uniform coating. We then carried out the platinum coating with the same amount of the thickness on the backside surface of the glass substrate. The surface profile of the glass substrate was fully recovered, indicating that the residual stress was successfully balanced by the backside coating. Furthermore, we tried to an iridium single-layer coating with a thickness of 150 Åon the silicon mirrors. The frontside coating caused the degradation of the imaging quality by 7.5 arcsec in half-power width. However, the backside coating with the same amount of the thickness reduced this degradation to be 3.4 arcsec. Finally, an additional backside coating with a thickness of 100 Å and the annealing to relax the residual stress were found to eliminate the distortion completely; the final degradation of the imaging quality was only 0.4 arcsec.

Alignment and distortion-free integration of lightweight mirrors into meta-shells for high-resolution astronomical x-ray optics

High-resolution, high throughput optics for x-ray astronomy requires fabrication of well-formed mirror segments and their integration with arc-second level precision. Recently, advances of fabrication of silicon mirrors developed at NASA/Goddard prompted us to develop a new method of mirror integration. The new integration scheme takes advantage of the stiffer, more thermally conductive, and lower-CTE silicon, compared to glass, to build a telescope of much lighter weight. In this paper, we address issues of aligning and bonding mirrors with this method. In this preliminary work, we demonstrated the basic viability of such scheme. Using glass mirrors, we demonstrated that alignment error of 1” and bonding error 2” can be achieved for mirrors in a single shell. We will address the immediate plan to demonstrate the bonding reliability and to develop technology to build up a mirror stack and a whole “meta-shell”.

Constellation-X Mirror Technology Development

As NASAs next major space X-ray observatory, the Constellation-X mission (Bookbinder et al. 2008) requires mirror assemblies with unprecedented characteristics that cannot be provided by existing optical technologies. In the past several years, the project has supported a vigorous mirror technology development program. This program includes the fabrication of lightweight mirror segments by slumping commercially available thin glass sheets, the support and mounting of these thin mirror segments for accurate metrology, the mounting and attachment of these mirror segments for the purpose of X-ray tests, and development of methods for aligning and integrating these mirror segments into mirror assemblies. This paper describes our efforts and developments in these areas.

X-ray imaging tests of Constellation-X SXT mirror segment pairs

The Constellation-X Spectroscopy X-ray Telescope (SXT) is a segmented, tightly nested Wolter-I telescope with a requirement of approximately 12.5 arcseconds HPD for the mirror system. The individual mirror segments are 0.4 mm thick, formed glass, making the task of mounting, alignment and bonding extremely challenging. Over the past year we have developed a series of tools to meet these challenges, the latest of which is an upgrade to the 600-meter x-ray beam line at GSFC. The new facilities allow us to perform full aperture and sub-aperture imaging tests of mirror segment pairs to locate the source of deformations and correlate them with our optical metrology. We present the optical metrology of the axial figure and Hartmann focus, x-ray imaging performance predictions based on analysis of the optical metrology, and both full aperture and sub-aperture x-ray imaging performance of test mirror segment pairs at 8.05 keV.