Todd R. Decker

NuSTAR hard x-ray optics

The Nuclear Spectroscopic Telescope Array (NuSTAR) is a small explorer (SMEX) mission currently under an extended Phase A study by NASA. NuSTAR will be the first satellite mission to employ focusing optics in the hard X-ray band (8-80 keV). Its design eliminates high detector backgrounds, allows true imaging, and permits the use of compact high performance detectors. The result: a combination of clarity, sensitivity, and spectral resolution surpassing the largest observatories that have operated in this band by orders of magnitude. We present an overview of the NuSTAR optics design and production process. We also describe the progress of several components of our independent optics development program that are beginning to reach maturity and could possibly be incorporated into the NuSTAR production scheme. We then present environmental test results that are being conducted in preparation of full space qualification of the NuSTAR optics.

Development and production of hard X-ray multilayer optics for HEFT

The High Energy Focusing Telescope (HEFT) will observe a wide range of objects including young supernova remnants, active galactic nuclei, and galaxy clusters at energies between 20 and 70 keV. Large collecting areas are achieved by tightly nesting layers of grazing incidence mirrors in a conic approximation Wolter-I design. The segmented mirrors that form these layers are made of thermally formed glass substrates coated with depth-graded multilayer films for enhanced reflectivity. The mirrors are assembled using an over-constraint method that forces the overall shape of the nominally cylindrical substrates to the appropriate conic form. We will present performance data on the HEFT optics and report the current status of the assembly production.

Hard x-ray optics for the HEFT balloon-borne payload: prototype design and status

We report on the current status and performance of prototype hard x-ray optics we are producing for use on the high energy focusing telescope (HEFT) experiment. The baseline substrates are thermally formed glass mirrors that are overcoated with multilayers to provide good performance throughout the 20-80 keV bandpass. Progress made in the thermal forming process as well as in the multilayer performance has allowed production of optics that meet or exceed all HEFT requirements. We present metrology on the substrates and result from x-ray characterization. A novel mounting scheme for the individual telescope shells is currently being tested. If successful the mounting technique will produce a monolithic, extremely stiff and robust optic.

Substrates and mounting techniques for the High-Energy Focusing Telescope (HEFT)

The high energy focusing telescope (HEFT) is a balloon-borne system for obtaining arcminute imagery in the 20 - 100 keV energy band. The hard x-ray optics are baselined to use thin epoxy-replicated aluminum foil substrates coated with graded-d multilayers, and we show some results on x-ray performance of prototype foil substrates. We also propose an extremely promising alternative substrate -- thermally formed glass. The advantages of thermally formed glass substrates, their fabrication and preliminary metrology on sample pieces are discussed. If ultimately feasible, the thermally formed glass is a better substrate due to its superior hard x-ray reflectivity and scattering properties in comparison to similarly coated epoxy-replicated aluminum foil. We also discuss some preliminary work on the HEFT mirror mounting concept and the associated angular resolution error budget.

NuSTAR ground calibration: The Rainwater Memorial Calibration Facility (RaMCaF)

The Nuclear Spectroscopic Telescope Array (NuSTAR) is a NASA Small Explorer mission that will carry the first focusing hard X-ray (5-80 keV ) telescope to orbit. The ground calibration of the three flight optics was carried out at the Rainwater Memorial Calibration Facility (RaMCaF) built for this purpose. In this article we present the facility and its use for the ground calibration of the three optics.

Small-animal radionuclide imaging with focusing gamma-ray optics

Significant effort currently is being devoted to the development of noninvasive imaging systems that allow in vivo assessment of biological and biomolecular interactions in mice and other small animals. While physiological function in small animals can be localized and imaged using conventional radionuclide imaging techniques such as single-photon emission tomography (SPECT) and positron emission tomography (PET), these techniques inherently are limited to spatial resolutions of 1-2 mm. For this reason, we are developing a small animal radionuclide imaging system (SARIS) using grazing incidence optics to focus gamma-rays emitted by 125I and other radiopharmaceuticals. We have developed a prototype optic with sufficient accuracy and precision to focus the 27.5 keV photons from 125I onto a high-resolution imaging detector. Experimental measurements from the prototype have demonstrated that the optic can focus X-rays from a microfocus X-ray tube to a spot having physical dimensions (approximately 1500 microns half-power diameter) consistent with those predicted by theory. Our theoretical and numerical analysis also indicate that an optic can be designed and build that ultimately can achieve 100 μm spatial resolution with sufficient efficiency to perform it in vivo single photon emission imaging studies in small animal.

Development of precision hard x-ray multilayer optics with sub-arcminute performance

A new generation of hard X-ray telescopes using focusing optics are poised to dramatically improve the sensitivity and angular resolution at energies above 10 keV to levels that were previously unachievable by the past generation of background-limited collimated and coded-aperture instruments. Active balloon programs (HEFT, InFocus), possible Explorer-class satellites, and major X-ray observatories (Constellation-X, XEUS) using focusing optics will play a major role in future observations of a wide range of objects including young supernova remnants, active galactic nuclei, and galaxy clusters. These instruments call for grazing incidence optics coated with depth-graded multilayer films to achieve large collecting areas. To accomplish the ultimate goals of the more advanced satellite missions such as Constellation-X, lightweight and low-cost substrates with angular resolution well below an arcminute must be developed. Recent experimental results will be presented on the development of improved substrates and precision mounting techniques that yield sub-arcminute performance.

XMM flight mirror modules with reflection grating assembly and x-ray baffle testing

In the frame of the XMM project, several test campaigns are accomplished to qualify the optical elements of the mission. The test described in this paper are performed on a XMM flight model mirror module added with a reflection grating assembly (RGA). The mirror module contains 58 x-ray optical quality shells, an x-ray baffle (XRB) to reduce the straylight. This complete XMM flight model mirror assembly (MA) is tested in a vertical configuration at CSL, in a full aperture or partial EUV collimated beam illumination, and with an x-ray pencil beam. One of the advantages of the EUV collimated beam is to verify the correct position of the RGA when integrated in flight configuration on the mirror module structure. This is not possible in x-ray with a finite source distance. The partial EUV illumination is performed to verify the correct integration of the RGA grating stacks. The pencil beam allows to make an accurate metrology of the XRB position, and to verify the positions of the 0, 1 and 2 diffraction order foci. In this paper, the tested module is first exposed, and the approach to qualify the instrument is described. The analysis of the results achieved over the different test configurations is presented. The impact of the environmental test on the reflection grating box is also diagnosed.

Metrology system for measuring mast motions on the NuSTAR mission

A metrology system designed and built for the NuSTAR mission is described. The NuSTAR mission is an orbiting X-ray telescope with a 10 meter focal length. The system consists of two laser pointers mounted rigidly together with a star tracker and the X-ray optics. The focused laser beams illuminates two metrology detectors mounted rigidly with the X-ray detectors. The detectors and optics/lasers are separated by a ∼10 meter deployable (and somewhat flexible) carbon fiber mast. Details about the implementation of the metrology system is discussed in this paper. 12

The Rainwater Memorial Calibration Facility for X-Ray Optics

The Nuclear Spectroscopic Telescope ARray (NuSTAR) is a NASA Small Explorer mission that will carry the first focusing hard X-ray (5–80 keV) telescope to orbit. The ground calibration of the optics posed a challenge as the need to suppress finite source distance effects over the full optic and the energy range of interest were unique requirements not met by any existing facility. In this paper we present the requirements for the NuSTAR optics ground calibration, and how the Rainwater Memorial Calibration Facility, RaMCaF, is designed to meet the calibration requirements. The nearly 175 m long beamline sports a 48 cm diameter 5–100 keV X-ray beam and is capable of carrying out detailed studies of large diameter optic elements, such as the NuSTAR optics, as well as flat multilayer-coated Silicon wafers.