William A. Podgorski

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.

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.

The GMT-CfA, Carnegie, Catolica, Chicago Large Earth Finder (G-CLEF): a general purpose optical echelle spectrograph for the GMT with precision radial velocity capability

The GMT-CfA, Carnegie, Catolica, Chicago Large Earth Finder (G-CLEF) is a fiber fed, optical echelle spectrograph that has undergone conceptual design for consideration as a first light instrument at the Giant Magellan Telescope. GCLEF has been designed to be a general-purpose echelle spectrograph with precision radial velocity (PRV) capability. We have defined the performance envelope of G-CLEF to address several of the highest science priorities in the Decadal Survey1. The spectrograph optical design is an asymmetric, two-arm, white pupil design. The asymmetric white pupil design is adopted to minimize the size of the refractive camera lenses. The spectrograph beam is nominally 300 mm, reduced to 200 mm after dispersion by the R4 echelle grating. The peak efficiency of the spectrograph is >35% and the passband is 3500-9500Å. The spectrograph is primarily fed with three sets of fibers to enable three observing modes: High-Throughput, Precision-Abundance and PRV. The respective resolving powers of these modes are R~ 25,000, 40,000 and 120,000. We also anticipate having an R~40,000 Multi-object Spectroscopy mode with a multiplex of ~40 fibers. In PRV mode, each of the seven 8.4m GMT primary mirror sub-apertures feeds an individual fiber, which is scrambled after pupil-slicing. The goal radial velocity precision of G-CLEF is ∂V <10 cm/sec radial. In this paper, we provide a flowdown from fiducial science programs to design parameters. We discuss the optomechanical, electrical, structural and thermal design and present a roadmap to first light at the GMT.

Development of adjustable grazing incidence optics for Generation-X

For X-ray astronomy, 0.1 arc-second imaging resolution will result in a significant advance in our understanding of the Universe. Similarly, the advent of low cost high performance X-ray mirrors will also increase the likelihood of more X-ray telescopes being funded and built. We discuss the development plans of two different types of adjustable grazing incidence optics: one being a tenth arc-second resolution bimorph mirror approach also suitable for extremely large collecting areas, and the second being a few arc-second radially adjustable mirror approach more suitable for modest sized telescopes. Bimorph mirrors will be developed using thin (0.1 - 0.4 mm) thermally formed glass or electroplated metal mirror segments with thin film piezo-electric actuators deposited directly on the mirror back surface. Mirror figure will be adjusted on-orbit. Radially adjustable mirrors will employ discreet radially electrostrictive actuators for mirror alignment and low spatial error frequency figure correction during assembly and alignment. In this paper we report on. In this paper we describe mirror design and our development plans for both mirror concepts.

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-Resolution X-Ray Telescopes

High-energy astrophysics is a relatively young scientific field, made possible by space-borne telescopes. During the half-century history of x-ray astronomy, the sensitivity of focusing x-ray telescopes-through finer angular resolution and increased effective area-has improved by a factor of a 100 million. This technological advance has enabled numerous exciting discoveries and increasingly detailed study of the high-energy universe-including accreting (stellarmass and super-massive) black holes, accreting and isolated neutron stars, pulsar-wind nebulae, shocked plasma in supernova remnants, and hot thermal plasma in clusters of galaxies. As the largest structures in the universe, galaxy clusters constitute a unique laboratory for measuring the gravitational effects of dark matter and of dark energy. Here, we review the history of high-resolution x-ray telescopes and highlight some of the scientific results enabled by these telescopes. Next, we describe the planned next-generation x-ray-astronomy facility-the International X-ray Observatory (IXO). We conclude with an overview of a concept for the next next-generation facility-Generation X. The scientific objectives of such a mission will require very large areas (about 10000 m2) of highly-nested lightweight grazing-incidence mirrors with exceptional (about 0.1-arcsecond) angular resolution. Achieving this angular resolution with lightweight mirrors will likely require on-orbit adjustment of alignment and figure.

Atomic force microscopy characterization of Zerodur mirror substrates for the extreme ultraviolet telescopes aboard NASA's Solar Dynamics Observatory

The high-spatial frequency roughness of a mirror operating at extreme ultraviolet (EUV) wavelengths is crucial for the reflective performance and is subject to very stringent specifications. To understand and predict mirror performance, precision metrology is required for measuring the surface roughness. Zerodur mirror substrates made by two different polishing vendors for a suite of EUV telescopes for solar physics were characterized by atomic force microscopy (AFM). The AFM measurements revealed features in the topography of each substrate that are associated with specific polishing techniques. Theoretical predictions of the mirror performance based on the AFM-measured high-spatial-frequency roughness are in good agreement with EUV reflectance measurements of the mirrors after multilayer coating.

Constellation-X spectroscopy x-ray telescope optical assembly pathfinder image error budget and performance prediction

The Constellation-X mission is a follow-on to the current Chandra and XMM missions. It will place in orbit an array of four X-ray telescopes that will work in unison, having a substantial increase in effective area, energy resolution, and energy bandpass over current missions. To accomplish these ambitious increases new optics technologies must be exploited. The primary instrument for the mission is the Spectroscopy X-Ray Telescope (SXT), which covers the 0.21 to 10 keV band with a combination of two x-ray detectors: a reflection grating spectrometer with CCD readout and a micro-calorimeter. Mission requirements are an effective area of 15,000 cm2 near 1 keV and a 15 arc-sec (HPD) image resolution with a goal of 5 arc-sec. The Constellation-X SXT uses a segmented design with lightweight replicated optics. A technology development program is being pursued with the intent of demonstrating technical readiness prior to the program new start. Key elements of the program include the replication of the optical elements, assembly and alignment of the optics into a complete mirror assembly and demonstration of production techniques needed for fabrication of multiple units. These elements will be demonstrated in a series of engineering development and prototype optical assemblies which are increasingly flight-like. In this paper we present an image angular resolution error budgets for the SXT and for the Optical Assembly Pathfinder #2 (OAP2), the first of engineering development units intended to be tested in x-rays. We describe OAP2 image error sources and performance analyses made to assess error sensitivities. Finally we present an overall prediction of as-tested imaging performance in the x-ray test facility.

The interface region imaging spectrograph for the IRIS Small Explorer mission

The Interface Region Imaging Spectrograph (IRIS) is a NASA SMall EXplorer mission scheduled for launch in January 2013. The primary goal of IRIS is to understand how the solar atmosphere is energized. The IRIS investigation combines advanced numerical modeling with a high resolution UV imaging spectrograph. IRIS will obtain UV spectra and images with high resolution in space (0.4 arcsec) and time (1s) focused on the chromosphere and transition region of the Sun, a complex interface region between the photosphere and corona. The IRIS instrument uses a Cassegrain telescope to feed a dual spectrograph and slit-jaw imager that operate in the 133-141 nm and 278-283 nm ranges. This paper describes the instrument with emphasis on the imaging spectrograph, and presents an initial performance assessment from ground test results.

Constellation-X spectroscopy x-ray telescope segmented optic assembly and alignment implementation

The Constellation-X mission will perform X-Ray science with improvements in energy resolution and effective area over its predecessor missions. The primary instrument on each of the four Constellation-X spacecraft is the Spectroscopy X-Ray Telescope (SXT). The SXT is a 1.6m diameter grazing incidence mirror assembly comprised of approximately 4000 optic elements. In order for the optic elements to work together to achieve the required 15 arcsec image resolution for the telescope, each optic must be aligned very precisely. To enable the alignment of the optic elements to the required tolerances, new technology must be developed through a series of technology demonstrators. The first step in this process is the production of the Optical Assembly Pathfinder (OAP). The OAP represents a small section, or module, of the complete SXT and has been designed to facilitate the evaluation and development of the optic element support, alignment, and adjustment concepts, processes, and procedures. To do this, one pair of optic elements, primary and secondary, will be aligned using optical alignment methods including the Centroid Detector Assembly (CDA) and Interferometry. Ten Optic Adjustment Arms will support the optic elements such that their position and figures can be adjusted. Currently, one section, the primary section, of the OAP has been assembled and is awaiting the installation of an optic element for testing.