Fabrication of the NuSTAR flight optics

Abstract

We describe the fabrication of the two NuSTAR flight optics modules. The NuSTAR optics modules are glass-graphiteepoxy composite structures to be employed for the first time in space-based X-ray optics by NuSTAR, a NASA Small Explorer schedule for launch in February 2012. We discuss the optics manufacturing process, the qualification and environmental testing performed, and briefly discuss the results of X-ray performance testing of the two modules. The integration and alignment of the completed flight optics modules into the NuSTAR instrument is described as are the optics module thermal shields. OVERVIEW OF THE OPTICS MODULESThe Nuclear Spectroscopy Telescope Array (NuSTAR) is a NASA Small Explorer (SMEX) satellite mission scheduled for launch in February 2012. The NuSTAR experiment contains two telescopes each consisting of an optic and a CdZnTe focal plane detector separated from each other by a 10-meter deployable mast (figure 1). The experiment is an extension and improvement on the design successfully employed in the HEFT balloon experiment (Harrison et al. 2005 1 ). NuSTAR will operate in the 6-79 keV energy band. More details on the mission, the overall instrument design and performance requirements and scientific objectives can be found in Harrison et al. 2010 2 .A blowup of an individual optics module is also shown in figure 1. Each layer of the optic has an upper and lower conic shell (equivalent to the parabola-hyperbola sections of a Wolter-I optic). Each shell is composed of multiple thermally formed glass segments. Each piece of glass is coated with a depth-graded multilayer. The enhanced reflectivity provided by the multilayers, along with the shallow graze angles afforded by the focal length of the optics (10.15 meter) provide high effective area over the NuSTAR energy band of 6-79 keV, and a field of view of 12 arcminutes by 12 arcminutes. There are 133 concentric layers which together form each optic. The glass layers (a glass-epoxy-graphite composite structure) are built up on a Titanium mandrel. Titanium support spiders located on the top and bottom of each optic connect it to the optical bench. The compliant, radially-symmetric spiders accommodate thermal expansion effects as well as dynamic loading. Thin x-ray transparent thermal covers on the entrance and exit apertures of the optic reduce thermal gradients by blocking direct view of the sun and deep space. Two flight modules, FM1 and FM2, were fabricated. A third module, FM0, was fabricated earlier and has Pt/SiC multilayers on the inner 89 layers. FM0 is a potential flight spare and is available to provide for more extensive X-ray characterization than is permitted for either of the flight modules, given the compressed delivery schedule of the optics.

First results from the ground calibration of the NuSTAR flight optics

“…6 Here we briefly summarize the aspects of the optics design and fabrication that are of particular importance to calibration of the effective area and point spread response function (PSF). The NuSTAR optics are composed of 133 concentric layers of conical-approximation Wolter I mirrors aligned and held together by epoxy and precisely machined graphite spacers that run along the optic axis.…”

“…Gravitational effects will not be present on orbit, and the optics were installed and mechanically aligned in the spacecraft with the optical axes aligned vertically (i.e., along the gravity vector) with a technique to avoid introducing stress on the optics. 6 Detailed raytrace simulations are required to fully quantify the systematic error and potentially introduce small corrections.…”

Next generation astronomical x-ray optics: high angular resolution, light weight, and low production cost

“…As shown in Figure 2 (right panel), it has been able to make mirror substrates consistently at -6.5 are-seconds (HPD); Each entry in the histogram represents a substrate pair which has been precisely measured with optical metrology [19,20,21,22,23,24] and whose x-ray imaging performance calculated with standard performance prediction techniques. The glass slumping process is capable of making substrates the meet the requirements of a sub-l0 arc-second mirror assembly: It has been used to making the more than 10,000 substrates for the NuST AR mission [7].…”

“…Suzaku [6] and NuSTAR [7] represent the state of the art oflightweight x-ray telescope making. They have angular resolutions of approximately 120 and 60 are-seconds, respectively.…”

Evaluation of the surface strength of glass plates shaped by hot slumping process

“…Such telescopes assembling will be based on a principle of hierarchical integration: single mirrors segments will be integrated into elemental modules, usually called X-ray Optical Unit (XOU), then assembled and aligned in the Flight Mirror Assembly (FMA) through intermediate azimuthal structures to reestablish the cylindrical symmetry of the nested telescope with Wolter I (or a Wolter I approximation) optical design [8]. The slumping technology has already been successfully employed for the production of NuSTAR telescope, launched in 2012 [9], [10], able to deliver an image quality at focus of 45 arcsec HEW [11]. Additional studies are currently on going both by American and European research groups to further expand the technology in terms of achievable optical quality (the goal being 5 arcsec HEW) [12], [13].…”