Manufacture of mirror glass substrates for the NuSTAR mission

Abstract

The NuSTAR (Nuclear Spectroscopy Telescope Array) observatory (Harrison et al. 2009), expected to be launched into an equatorial low earth orbit in 2011, will have two mirror assemblies capable of imaging X-rays in the hard X-ray band between 5 keV and 80 keV. It will be the first X-ray observatory using multilayer coatings to significantly expand the bandwidth of the typical X-ray telescope of 0.1 keV to 10 keV. The mirror assemblies use a segmented design to simplify the construction process, as such they require 4,680 mirror substrates coated with appropriately designed multilayers to enhance reflectivity for hard X-rays. These substrates are produced by slumping commercially available thin glass sheets. In this paper we report on our work of manufacturing these substrates at NASA Goddard Space Flight Center.The NuSTAR mirror assemblies have their heritage in the previously flown balloon payload HEFT (High Energy Focusing Telescope) (Koglin et al. 2005). They use the conical approximation to the traditional Wolter-I optical design. The conical approximation is implemented in two steps: (1) starting with cylindrically-shaped mirror segments whose radii of curvature are within 1 mm of their respective design values; (2) the alignment and integration process bends the cylindrical mirror segments into the required conical shapes and attaches them to the mirror assemblies under the monitoring of real-time metrology. As such the first step of making these mirror assemblies is the manufacture of cylindrically shaped glass substrates.The NuSTAR substrate production is based on a glass slumping technique developed for the International X-ray Observatory (IXO mission, formerly known as the Constellation-X (Zhang et al, 2006) mission. It uses commercially available thin borosilicate glass sheets and slump them on precisely polished and figured fused quartz mandrels. As a result, the finished curved glass is a replica of the mandrel. It takes up the overall shape of the mandrel while preserving the excellent microroughness of the glass sheets.

Affordable and lightweight high-resolution x-ray optics for astronomical missions

“…Over the last decade we have developed and perfected the slumping process [12,13,14,15,16,17,18,19,20,21,22,24] and successfully made thousands of 0.2 mm thick substrates [11] for the NuSTAR mission which was launched in June 2012 and has been in successful operation since. To date, we have been able to consistently make substrates with a figure quality between 4 and 8 arc-seconds HPD (two reflection equivalent), with the most probable (or average) value of 6 arc-seconds HPD.…”

X-ray telescope mirrors made of slumped glass sheets

“…An advantage of this method is that the micro roughness of the mould does not affect the optical surface and is proven to be damped by almost a factor of 10 by the glass thickness. One could imagine the opposite scenario, and in fact this is being followed by some other groups [10,11]: the direct slumping uses a convex mould and the optical surface is in direct contact with the mould surface. This has the disadvantage that the micro roughness of the mould surface affects directly the optical surface of the mirror glass.…”

“…by NuSTAR [10], a NASA X-ray telescope launched in 2012, to be a technology well suited for X-ray telescope missions.…”

Ultralightweight deformable mirrors

“…This process, known as slumping, will cause the substrate to take the curvature of the mold (without sacrificing the optically smooth surface finish of the original wafer). It was used for the mirrors of the NuSTAR x-ray telescope [37]. It has been demonstrated that under carefully controlled conditions silicon can be slumped as well, at around 900°C [38].…”