NuSTAR (the Nuclear Spectroscopic Telescope Array) is a space-based X-ray telescope that will use a Wolter telescope, a telescope for X-rays using only grazing incidence optics. It is to focus high energy X-rays from astrophysical sources; astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior.
NuSTAR employs two grazing incidence focusing optics each of which consists of 133 concentric shells. The particular innovation enabling NuSTAR is that the optics are coated with depth-graded multilayers (optimized for broadband response by alternating atomically thin layers of a high-density and low-density material); with NuSTAR’s choice of Pt/SiC and W/Si multilayers, this enables reflectivity up to 79 keV (the platinum K-edge energy; K-edge describes a sudden increase in the attenuation coefficient of photons occurring at a photon energy just above the binding energy of the K shell electron of the atoms interacting with the photons).
The optics are produced at Goddard Space Flight Center (GSFC), a major NASA space research laboratory established on May 1, 1959 as NASA’s first space flight center. It is made by heating thin (210 µm) sheets of flexible glass in an oven so that they slump over precision-polished cylindrical quartz mandrels of the appropriate radius; mandrels (arbors) is an object used to shape machined work, a tool component that grips or clamps materials to be machined, or a tool component that can be used to grip other moving tool components. The coatings, or a covering that is applied to the surface of an object, usually referred to as the “substrate,” are applied by a group at the Danish Technical University, often simply referred to as DTU, a university just north of Copenhagen, Denmark.
The shells are then assembled, at the Nevis Laboratories, a research center owned and operated by Columbia University, using graphite spacers machined to constrain the glass to the conical shape, and held together by epoxy. There are 4,680 mirror segments in total (the 65 inner shells each comprise six segments and the 65 outer shells twelve; there are upper and lower segments to each shell, and there are two telescopes), there are five spacers per segment. Since the epoxy takes 24 hours to cure, one shall is assembled per day—it takes four months to build up one optic.
The expected point spread function for the flight mirrors is 43 arc-seconds, giving a spot size of about two millimeters at the focal plane; this is unprecedentedly good resolution for refocusing hard X-ray optics, though up to two orders of magnitude worse than the best resolution achieved at longer wavelengths by the Chandra X-ray Observatory, a space telescope launched on STS-93 by NASA on July 23, 1999.
The optics have a 10.15-metre focal length, and so are held at the end of a long deployable mast, a structure that can change shape so as to significantly change units size; a laser metrology system is used to determine the exact relative positions of the optics and the focal plane at all time, so that each detected photon can be mapped back to the correct point on the sky even if the optics and the focal plane move relative to one another during an exposure.