Lynx soft x-ray critical-angle transmission grating spectrometer

Abstract

Lynx is one of four Surveyor-class mission concept studies for the 2020 Astrophysics Decadal Survey. It features an x-ray telescope with an unprecedented collecting area of 2 m 2 at 1 keV and a point-spread function of 0.5 arc sec. We describe the status of critical-angle transmission (CAT) grating technology development and perform ray-traces for a CAT grating x-ray spectrometer that can reach high spectral resolving power λ∕Δλ > 5000 (often exceeding 7500) and effective area around 4000 cm 2 in the soft x-ray band (0.2 to 2 keV). To achieve these characteristics, about two-thirds of the aperture must be covered with gratings. CAT gratings are mostly transparent at high energies, and thus hard x-rays can still be used for simultaneous imaging spectroscopy using a microcalorimeter array. We simulate several design scenarios and investigate how subaperturing can be most effectively used to increase performance. For large gratings, the resolving power is limited by the deviation of flat gratings from the ideal Rowland torus surface. Chirped gratings, i.e., gratings where the spacing of grating bars is variable, can overcome this limitation. Alignment tolerances in many degrees of freedom can be achieved with machining tolerances. We outline the development path to CAT grating performance improvements and discuss future ray-trace work to refine the design of the spectrometer.

Toward volume manufacturing of high-performance soft x-ray critical-angle transmission gratings

“…The ability to chirp would allow us to make larger (and thus fewer) gratings for Lynx, without introducing unacceptable losses in R caused by aberrations due to the deviation of flat gratings from the Rowland torus surface. 12,31 We continue to work on transferring more of the fabrication steps onto 200 mm tools for faster and cheaper production. Process uniformity over the large area of a 200 mm wafer remains a challenge and may limit the number of quality gratings that can be extracted from a single wafer.…”

“…One of them, the Lynx 11 x-ray observatory, features a retractable grating array for large-area (> 4000 cm 2 ) high-resolution (R > 5000) soft x-ray spectroscopy, covering a fraction of a large x-ray optic. A CAT grating spectrometer made of ∼ 2000 large-area gratings 12,13 was selected for the Design Reference Mission described in the Lynx X-ray Observatory Report. 14 As part of this effort we developed a detailed Technology Development Roadmap that can take CAT grating technology from today's state-of-the-art to Technology Readiness Level (TRL) 6 for Lynx by 2024.…”

Lynx X-Ray Observatory: an overview

“…7,8 Similarly, multiple technologies were studied for the large-scale active sensor pixel array, dubbed the high-definition x-ray imager (HDXI) [9][10][11][12] and for the XGS. 13,14 The Lynx x-ray microcalorimeter (LXM) [15][16][17][18][19][20][21] is singular but has elements that have multiple candidate technologies. LXM also leverages heritage from Astro-H and Astro-E and design features from ATHENA.…”

“…All LXM elements are detailed in multiple papers of this section, as is a comprehensive overview of this ambitious, yet highly feasible instrument. [14][15][16][17][18][19][20] 3 Lynx Mission…”

Limiting Spectral Resolution of a Reflection Grating Made via Electron-Beam Lithography

“…The instrument optical designs for both Lynx and LUVOIR benefit from grating customization. For example, introducing a grating 'chirp,' where the period is intentionally varied across the grating, theoretically increases the spectral resolution of the transmission grating spectrometer concept for Lynx (Günther & Heilmann (2019)). Furthermore, the POLLUX instrument, a UV-spectropolarimeter baselined for LUVOIR, employs gratings patterned on freeform optics i.e., optics with arbitrary deviations from a plane or curved surface (Muslimov et al (2018a)).…”