Melinda Hong

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.

Grazing incidence wavefront sensing and verification of x-ray optics performance

Evaluation of interferometric mirror metrology data and characterization of a telescope wavefront can be powerful tools in understanding image characteristics of an x-ray optical system. In the development of soft x-ray telescope for the International X-Ray Observatory (IXO), we have developed new approaches to support the telescope development process. Interferometrically measuring the optical components over all relevant spatial frequencies can be used to evaluate and predict the performance of an x-ray telescope. Typically, the mirrors are measured using a mount that minimizes the mount and gravity induced errors. In the assembly and mounting process the shape of the mirror segments can dramatically change. We have developed wavefront sensing techniques suitable for the x-ray optical components to aid us in the characterization and evaluation of these changes. Hartmann sensing of a telescope and its components is a simple method that can be used to evaluate low order mirror surface errors and alignment errors. Phase retrieval techniques can also be used to assess and estimate the low order axial errors of the primary and secondary mirror segments. In this paper we describe the mathematical foundation of our Hartmann and phase retrieval sensing techniques. We show how these techniques can be used in the evaluation and performance prediction process of x-ray telescopes.

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.

Affordable and Lightweight High-Resolution X-ray Optics for Astronomical Missions

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.

Metrology of IXO mirror segments

For future x-ray astrophysics mission that demands optics with large throughput and excellent angular resolution, many telescope concepts build around assembling thin mirror segments in a Wolter I geometry, such as that originally proposed for the International X-ray Observatory. The arc-second resolution requirement posts unique challenges not just for fabrication, mounting but also for metrology of these mirror segments. In this paper, we shall discuss the metrology of these segments using normal incidence metrological method with interferometers and null lenses. We present results of the calibration of the metrology systems we are currently using, discuss their accuracy and address the precision in measuring near-cylindrical mirror segments and the stability of the measurements.

Opto-mechanics of the Constellation-X SXT Mirrors: Challenges in Mounting and Assembling the Mirror Segments

The Constellation-X Spectroscopy X-Ray Telescopes consists of segmented glass mirrors with an axial length of 200 mm, a width of up to 400 mm, and a thickness of 0.4 mm. To meet the requirement of < 15 arc-second half-power diameter with the small thickness and relatively large size is a tremendous challenge in opto-mechanics. How shall we limit distortion of the mirrors due to gravity in ground tests, that arises from thermal stress, and that occurs in the process of mounting, affixing and assembling of these mirrors? In this paper, we will describe our current opto-mechanical approach to these problems. We will discuss, in particular, the approach and experiment where the mirrors are mounted vertically by first suspending it at two points.