Critical-angle x-ray transmission grating spectrometer with extended bandpass and resolving power > 10,000

Abstract

A number of high priority subjects in astrophysics can be addressed by a state-of-the-art soft x-ray grating spectrometer, such as the role of Active Galactic Nuclei in galaxy and star formation, characterization of the Warm-Hot Intergalactic Medium and the missing baryon problem, characterization of halos around the Milky Way and nearby galaxies, as well as stellar coronae and surrounding winds and disks. An Explorer-scale, largearea (> 1,000 cm 2 ), high resolving power (R = λ/∆λ > 3,000) soft x-ray grating spectrometer is highly feasible based on Critical-Angle Transmission (CAT) grating technology, even for telescopes with angular resolution of 5-10 arcsec. Still, significantly higher performance can be provided by a CAT grating spectrometer on an X-raySurveyor-type mission. CAT gratings combine the advantages of blazed reflection gratings (high efficiency, use of higher diffraction orders) with those of conventional transmission gratings (low mass, relaxed alignment tolerances and temperature requirements, transparent at higher energies) with minimal mission resource requirements. They are high-efficiency blazed transmission gratings that consist of freestanding, ultra-high aspect-ratio grating bars fabricated from silicon-on-insulator (SOI) wafers using advanced anisotropic dry and wet etch techniques. Blazing is achieved through grazing-incidence reflection off the smooth grating bar sidewalls. The reflection properties of silicon are well matched to the soft x-ray band, and existing silicon CAT gratings can exceed 30% absolute diffraction efficiency, with clear paths for further improvement. Nevertheless, CAT gratings with sidewalls made of higher atomic number elements allow extension of the CAT grating principle to higher energies and larger dispersion angles, thus enabling higher resolving power at shorter wavelengths. We show x-ray data from CAT gratings coated with a thin layer of platinum using atomic layer deposition, and demonstrate efficient blazing to higher energies and much larger blaze angles than possible with silicon alone. We also report on measurements of the resolving power of a breadboard CAT grating spectrometer consisting of a Wolter-I slumped-glass focusing mirror pair from Goddard Space Flight Center and CAT gratings, performed at the Marshall Space Flight Center Stray Light Facility. Measurement of the Al K α doublet in 18 th diffraction order shows resolving power > 10,000, based on conservative preliminary analysis. This demonstrates that currently fabricated CAT gratings are compatible with the most advanced grating spectrometer instrument designs for future soft x-ray spectroscopy missions.

Lynx Mission concept status

“…This readout detector array must be able to count individual X-ray photons and determine their energies well enough to separate overlapping spectral orders diffracted by the grating. The same technology used for HDXI will be used for this readout Two different grating technologies, both with active research programs, are being considered for use on Lynx: Off-Plane Gratings (OPGs) (PSU) 18,19 and Critical Angle Transmission (CAT) Gratings (MIT) 20,21 . The MSFC ACO is studying both of these technologies as each has the capacity to meet Lynx requirements.…”

Arcus: the soft x-ray grating explorer

“…This is a modular instrument consisting of four optical channels (OCs) and two detector subsystem assemblies (DSAs). The OCs limit the angular span of the telescope aperture to <40°, an approach known as subaperturing [18,23,29] allows the design to exploit the anisotropy of the individual SPO MM point spread function (PSF). Aligning the spectrometer's dispersion axis to the narrow axis of the telescope PSF results in a narrow line spread function (LSF) and yields high spectral resolving power.…”

“…CAT gratings are blazed transmission gratings that combine the high efficiency (30-40%) and resolution (>3000) of blazed reflection gratings with the relaxed alignment tolerances and thermal requirements of conventional transmission gratings [17,18,33]. The Arcus optical system is designed to take advantage of these relaxed tolerances to simplify integration and reduce costs.…”

Ray-tracing critical-angle transmission gratings for the X-ray Surveyor and Explorer-size missions

“…Experimentally R > 10, 000 has already been demonstrated at 0.83 nm wavelength with CAT gratings. 16…”