Lynx grating spectrometer design: Optimizing chirped transmission gratings

TL;DR

This paper explores the design of chirped transmission gratings for NASA's Lynx X-ray spectrometer, demonstrating that larger, chirped gratings can significantly reduce component count and increase effective area without sacrificing resolution.

Contribution

It introduces the use of chirped gratings in X-ray spectrometers, showing they enable larger gratings, fewer components, and improved effective area compared to traditional flat gratings.

Findings

Using chirped gratings reduces the number of gratings from thousands to hundreds.

Chirped gratings increase the effective area by 25%.

Bending gratings further enhances effective area by 5-10%.

Abstract

Lynx is one of four large-mission concept studies for NASA's 2020 Decadal survey. The design reference mission includes an X-ray grating spectrometer (XGS) based on critical-angle transmission (CAT) gratings. In the past we studied different grating sizes and arrangements using traditional flat CAT gratings with constant bar spacing. However, new technology development brings chirped gratings in reach. Using chirped gratings where the grating bar spacing varies over a grating allows us to fill the aperture with larger gratings because the chirp can compensate for some aberrations caused by the deviation of large flat gratings from the Rowland torus. This reduces the area blocked by grating support structures. Using larger gratings also carries potential cost savings. We use ray-tracing to study an XGS design with chirped grating and find that using chirped gratings of $80 * 160$ mm size allows us to reduce the number of gratings from a few thousand to a few hundred, while simultaneously increasing the effective area by 25% and keeping the resolving power constant. Bending those gratings to maintain a constant blaze angle over the entire grating increases the effective area by another 5-10%.