Lester M. Cohen

Constellation-X to Generation-X: evolution of large collecting area moderate resolution grazing incidence x-ray telescopes to larger area high-resolution adjustable optics

Large collecting area x-ray telescopes are designed to study the early Universe, trace the evolution of black holes, stars and galaxies, study the chemical evolution of the Universe, and study matter in extreme environments. The Constellation-X mission (Con-X), planned for launch in 2016, will provide ~ 104 cm2 collecting area with 15 arc-sec resolution, with a goal of 5 arc-sec. Future missions require larger collecting area and finer resolution. Generation-X (Gen-X), a NASA Visions Mission, will achieve 100 m2 effective area at 1 keV and angular resolution of 0.1 arc-sec, half power diameter. We briefly describe the Con-X flowdown of imaging requirements to reflector figure error. To meet requirements beyond Con-X, Gen-X optics will be thinner and more accurately shaped than has ever been accomplished. To meet these challenging goals, we incorporate for the first time active figure control with grazing incidence optics. Piezoelectric material will be deposited in discrete cells directly on the back surface of the optical segments, with the strain directions oriented parallel to the surface. Differential strain between the two layers of the mirror causes localized bending in two directions, enabling local figure control. Adjusting figure on-orbit eases fabrication and metrology. The ability to make changes to mirror figure adds margin by mitigating risk due to launch-induced deformations and/or on-orbit degradation. We flowdown the Gen-X requirements to mirror figure and four telescope designs, and discuss various trades between the designs.

Space telescope design considerations

The design considerations for astronomical space telescopes cover many disciplines but can be simplified into two overarching constraints: the desire to maximize science while adhering to budgetary constraints. More than ever, understanding the cost implications up front will be critical to success. Science performance can be translated into a set of simple performance metrics that set the requirements for design options. Cost is typically estimated by considering mass, complexity, technology maturity, and heritage. With this in mind, we survey the many diverse design considerations for a space telescope and, where appropriate, relate them to these basic performance metrics. In so doing, we hope to provide a roadmap for future space telescope designers on how best to optimize the design to maximize science and minimize total cost.

Constellation-X spectroscopy X-ray telescope (SXT)

We provide an overview of the Constellation-X SXT development program. We describe the performance requirements and goals, and the status of the technology development program. The SXT has a 1.6-meter diameter, a 10-meter focal length, and is to have an angular resolution exceeding 15 arc seconds. It has a modular design, incorporting lightweight, multiply nested, segmented Wolter Type I x-ray mirrors. All aspects of the design lend themselves to mass-production. The reflecting surfaces are produced by epoxy replication off precision mandrels onto glass substrates that have been accurately formed by thermal slumping. Coalignment of groups of relfectors to the required sub-micron accuracy is assisted by precison silicon micorstructures. Optical alignment is performed using the Centroid Detector Assembly originally developed for aligning the Chandra mirror. Recent efforts have concentrated on the producotin of an Engineering Unit, incorporating the components for the first time into a flight-like configuration. We summarize the status of the development of the processes for the key components and the initial metrology results of the Engineering Unit.

High-accuracy x-ray foil optic assembly

Achieving arcsecond angular resolution in a grazing-incidence foil optic X-ray telescope, such as the segmented mirror approach being considered for the Constellation-X Spectroscopy X-Ray Telescope (SXT), requires accurate placement of individual foils. We have developed a method for mounting large numbers of nested, segmented foil optics with sub- micrometer accuracy using lithographically defined and etched silicon alignment micro-structures. A system of assembly tooling, incorporating the silicon micro-structures, is used to position the foils which are then bonded to a flight structure. The advantage of this procedure is that the flight structure has relaxed tolerance requirements while the high accuracy assembly tooling can be reused. A companion paper by Bergner et al. discusses how our process could be used for the SXT. We have built an assembly truss with a simplified rectilinear geometry designed to experimentally test this alignment and mounting technique. We report results of tests with this system that demonstrate its ability to provide sub- micrometer alignment of rigid test optics.

JWST mirror production status

The James Webb Space Telescope (JWST) is an on axis three mirror anastigmat telescope with a primary mirror, a secondary mirror, and a tertiary mirror. The JWST mirrors are constructed from lightweight beryllium substrates and the primary mirror consists of 18 hexagonal mirror segments each approximately 1.5 meters point to point. Ball Aerospace and Technologies Corporation leads the mirror manufacturing team and the team utilizes facilities at six locations across the United States. The fabrication process for each individual mirror assembly takes approximately six years due to limitations dealing with the number of segments and manufacturing & test facilities. The primary mirror Engineering Development Unit (EDU) recently completed the manufacturing process with the final cryogenic performance test of the mirror segment assembly. The 18 flight primary mirrors segments, the secondary mirror, and the tertiary mirror are all advanced in the mirror production process with many segments through the final polishing process, coating process, final assembly, vibration testing, and final acceptance testing. Presented here is a status of the progress through the manufacturing process for all of the flight mirrors.

Segmented x-ray mirror development for Constellation-X

Segmented mirrors are one of the two approaches being investigated for both the Spectroscopy X-ray Telescope (SXT) and the Hard X-ray Telescope (HXT) on Constellation-X. Mirrors based on the grazing incidence foil optics pioneered by GSFC will meet the stringent Constellation-X SXT weight requirement, but the currently achieved resolution falls short of the 15 inch half-power diameter (HPD) required for Constellation-X. Significant contributions to the blur arise from the figure of individual reflectors and from inaccurate mounting. Only a small contribution to the HPD of the existing mirrors arises from the conical approximation. In this paper, we describe our program for improving the spatial resolution of segmented mirrors to meet the COnstellation-X requirement. Our effort incorporates accurately figured replication mandrels, mechanically more robust reflector substrates, high accuracy alignment, and ultimately a transition from conical to curved reflecting surfaces.

Focus and alignment of the AXAF optics

We discuss the x-ray measurement of the focus and alignment of the AXAF (Advanced X-ray Astrophysics Facility) x-ray optics. The high resolution mirror assembly (HRMA) consists of four nested Wolter type I x-ray optics. The attainment of the program goals for high resolution imaging requires that the mirror foci be coincident, both axially and laterally; in addition, the relative tilts between optics within each mirror pair must be small. The mirror tilts and the parfocalization were measured at the X-Ray Calibration Facility (XRCF) at the Marshall Space Flight Center in Huntsville, Alabama during a series of tests in the winter/spring of 1996/1997. The x-ray measurements are compared to the optical alignment data obtained by Eastman Kodak using the HRMA Alignment and Test System (HATS) during HRMA assembly. From these data a preliminary model for the relative location and rigid-body orientation of the individual mirror elements is developed; this mirror model is a component of the SAO high fidelity HRMA raytrace model.

ATLAST ULE mirror segment performance analytical predictions based on thermally induced distortions

The Advanced Technology Large-Aperture Space Telescope (ATLAST) is a concept for a 9.2 m aperture space-borne observatory operating across the UV/Optical/NIR spectra. The primary mirror for ATLAST is a segmented architecture with pico-meter class wavefront stability. Due to its extraordinarily low coefficient of thermal expansion, a leading candidate for the primary mirror substrate is Corning’s ULE® titania-silicate glass. The ATLAST ULE® mirror substrates will be maintained at ‘room temperature’ during on orbit flight operations minimizing the need for compensation of mirror deformation between the manufacturing temperature and the operational temperatures. This approach requires active thermal management to maintain operational temperature while on orbit. Furthermore, the active thermal control must be sufficiently stable to prevent time-varying thermally induced distortions in the mirror substrates. This paper describes a conceptual thermal management system for the ATLAST 9.2 m segmented mirror architecture that maintains the wavefront stability to less than 10 pico-meters/10 minutes RMS. Thermal and finite element models, analytical techniques, accuracies involved in solving the mirror figure errors, and early findings from the thermal and thermal-distortion analyses are presented.

Lessons we learned designing and building the Chandra telescope

2014 marks the crystal (15th) anniversary of the launch of the Chandra X-ray Observatory, which began its existence as the Advanced X-ray Astrophysics Facility (AXAF). This paper offers some of the major lessons learned by some of the key members of the Chandra Telescope team. We offer some of the lessons gleaned from our experiences developing, designing, building and testing the telescope and its subsystems, with 15 years of hindsight. Among the topics to be discussed are the early developmental tests, known as VETA-I and VETA-II, requirements derivation, the impact of late requirements and reflection on the conservatism in the design process.

Successful production of the engineering development unit (EDU) primary mirror segment and flight unit tertiary mirror for JWST

During 2009, Tinsley finished most of the Configuration 1 pre-cryo test Computer Controlled Optical Surfacing (CCOS) operations on the James Webb Space Telescope primary mirror segments and in mid-2009 we began the Configuration 2 post-cryo test CCOS operations. After completing the grinding and polishing operations, including final figuring to a cryo-null target, we delivered the finished Engineering Development Unit (EDU) to Ball Aerospace Technology Corporation on 4 December 2009. Achieving fabrication and metrology conditions to meet the specifications for this off-axis ~1.5 m hexagonal point-to-point segmented mirror required special methods. Achieving repeatable and accurate interferometric alignment of the off-axis aspherical mirror surface and stable thermal gradient control of the beryllium substructure during tests required rigorous component and system-level validation. Final optical wavefront measurements over the various spatial frequency ranges have demonstrated that all of the requirements are met. This success has validated our processes of fabrication and metrology and allows us to proceed with the production of the 18 flight mirror segments. The first finished flight mirror, the Tertiary Mirror, was shipped to BATC on 24 February, 2010. Performance of that mirror is reported here also.