The NuSTAR from NASA

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 (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).

The optics are produced, at Goddard Space Flight Center, by heating thin (210 µm) sheets of flexible glass in an oven so that they slump over precision-polished cylindrical quartzmandrels of the appropriate radius. The coatings are applied by a group at the Danish Technical University.

The shells are then assembled, at the Nevis Laboratories of Columbia University, using graphite spacers machined to constrain the glass to the conical shape, and held together by epoxy. There are 4680 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 shell 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 focusing hard-X-ray optics, though up to two orders of magnitude worse than the best resolution achieved at longer wavelengths by Chandra.

The optics have a 10.15-metre focal length, and so are held at the end of a long deployable mast; a laser metrology system is used to determine the exact relative positions of the optics and the focal plane at all times, 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.