Curved diffractive optics for high energy spectroscopy: current status and challenges

Improved X-ray/UV spectroscopy is needed for studies of exoplanet host systems, the evolution of galaxies, and the physics of extreme astrophysical conditions. Efficient, high-resolution X-ray / UV spectrometers would meet this need, but realizing such instruments, particularly in compact formats like SmallSats, are often gated by the fabrication of a grating that meets the size, shape, diffractive design, and performance requirements of the spectrometer. We report on an effort to make custom, high performance gratings on curved surfaces using electron-beam lithography (EBL). We have made modestly-sized gratings (39 mm X 20 mm) on cylindrical surfaces with sag comparable to that of a grazing-incidence optic (> 1 mm). Our approach uses interferometric measurements of diffracted orders to assess the fidelity of EBL in realizing the desired grating, providing a direct measurement of the technology's current capabilities and informing development efforts. We also report on the fabrication and testing of a diffractive silicon mirror operating at grazing incidence, as well as an optical design for a test of a novel, two-element spectrometer system.