Tyler Evans

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.

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.

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.

Alignment and integration of lightweight mirror segments

The optics for the International X-Ray Observatory (IXO) require alignment and integration of about fourteen thousand thin mirror segments to achieve the mission goal of 3.0 square meters of effective area at 1.25 keV with an angular resolution of five arc-seconds. These mirror segments are 0.4 mm thick, and 200 to 400 mm in size, which makes it difficult not to impart distortion at the sub-arc-second level. This paper outlines the precise alignment, permanent bonding, and verification testing techniques developed at NASAs Goddard Space Flight Center (GSFC). Improvements in alignment include new hardware and automation software. Improvements in bonding include two module new simulators to bond mirrors into, a glass housing for proving single pair bonding, and a Kovar module for bonding multiple pairs of mirrors. Three separate bonding trials were x-ray tested producing results meeting the requirement of sub ten arc-second alignment . This paper will highlight these recent advances in alignment, testing, and bonding techniques and the exciting developments in thin x-ray optic technology development.

Arc-second alignment of International X-Ray Observatory mirror segments in a fixed structure

The optics for the International X-Ray Observatory (IXO) require alignment and integration of about fourteen thousand thin mirror segments to achieve the mission goal of 3.0 square meters of effective area at 1.25 keV with an angular resolution of five arc-seconds. These mirror segments are 0.4 mm thick, and 200 to 400 mm in size, which makes it hard to meet the strict angular resolution requirement of 5 arc-seconds for the telescope. This paper outlines the precise alignment, verification testing, and permanent bonding techniques developed at NASAs Goddard Space Flight Center (GSFC). These techniques are used to overcome the challenge of transferring thin mirror segments from a temporary mount to a fixed structure with arc-second alignment and minimal figure distortion. Recent advances in technology development in addition to the automation of several processes have produced significant results. Recent advances in the mirror fixture process known as the suspension mount has allowed for a mirror to be mounted to a fixture with minimal distortion. Once on the fixture, mirror segments have been aligned to around 5 arc-seconds which is halfway to the goal of 2.5 arc-seconds per mirror segment. This paper will highlight the recent advances in alignment, testing, and permanent bonding techniques as well as the results they have produced. 12

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.

Arc-second alignment and bonding of International X-Ray Observatory mirror segments

The optics for the International X-Ray Observatory (IXO) require alignment and integration of about fourteen thousand thin mirror segments to achieve the mission goal of 3.0 square meters of effective area at 1.25 keV with an angular resolution of five arc-seconds. These mirror segments are 0.4 mm thick, and 200 to 400 mm in size, which makes it hard not to impart distortion at the sub-arc-second level. This paper outlines the precise alignment, verification testing, and permanent bonding techniques developed at NASAs Goddard Space Flight Center (GSFC). These techniques are used to overcome the challenge of aligning thin mirror segments and bonding them with arc-second alignment and minimal figure distortion. Recent advances in technology development in the area of permanent bonding have produced significant results. This paper will highlight the recent advances in alignment, testing, and permanent bonding techniques as well as the results they have produced.

ICESat-2 ATLAS telescope testing

Many lessons were learned in the comprehensive testing of the one meter Beryllium flight telescope for the ICESat-2 mission. This paper will focus on testing areas of encircled energy analysis, plate scale measurements, and boresight alignment for alignment of fiber optic cables. The Optical Development System Lab (ODSL) at NASAs Goddard Space Flight Center (GSFC) was developed to build up experience using engineering test units. This experience was applied to testing the flight telescope. Several tests were able to be performed on the telescope itself, helping drive down risk, cost, and schedule during the integration phase of the telescope onto the instrument and box structure. The main ICESat-2 instrument is the Advanced Topographic Laser Altimeter System (ATLAS). It measures ice elevation by transmitting laser pulses, and collecting the reflection in a telescope. Because so few photons return from each pulse, the alignment of each receiver channel fiber is critical as well as minimizing the distortion. The lab consisted of a clean room with a one meter parabola collimator system with a point source fiber-coupled 532nm laser and a CCD detector. This was used to feed collimated light into the telescope that was recorded with a CCD detector in the telescope focal plane. A large one meter flat mirror was used to certify the collimator system. Fiber optic cables were also used to back-illuminate the telescope and image in the collimator focal plane. The telescope was mounted in a gimbal that allowed for three degrees of rotational freedom allowing the telescope to be steered to each respective science field point. The setup worked well for accomplishing the testing. Through well written procedures and prior experience, the testing was carried out according to plan and on schedule despite obstacles along the way such as late ground support equipment and tests that needed to be repeated. The objective of this paper is to share those lessons learned for optical alignment of a receiver telescope assembly to promote future mission success.

Optical Development System life cycle for the ICESat-2 ATLAS instrument

The Optical Development System Lab (ODSL) completed a cycle of prototype testing and engineering model integration models to successfully test out a closed loop optical design. Through this process many lessons have been learned that are being directly applied to flight integration and testing, helping drive down risk, cost, and schedule for the instrument. The ODSL was used to test and understand the functional performance of the transmitter to receiver alignment system concepts. Advanced Topographic Laser Altimeter System (ATLAS) is the instrument being tested by the ODSL and is the main instrument on the ICESat-2 mission. It measures ice elevation by transmitting laser pulses, and collecting the reflection in a telescope. Because the round trip time is used to calculate distance, alignment between the outgoing transmitter beam and the incoming receiver beams are critical. An automated closed loop monitoring control system is currently being tested at the engineering model level to prove out implementation for the final spacecraft. To achieve an error of less than 2 micro-radians, an active deformable mirror was used to correct the lab wave front from the collimated “ground reflection” beam. The lab includes a focal plane assembly set up, a one meter diameter collimator optic, a 0.8 meter flight spare telescope for alignment, and the appropriate flight software to control the transmit beam path. By having a fully integrated system with prototypes and engineering units, lessons were learned before flight designs were finalized. The flight telescope is now ready to be accepted and put through tests that were refined with the ODS lab.