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Several high priority subjects in astrophysics can be addressed by astate-of-the-art soft x-ray grating spectrometer (XGS). An Explorer-scale,large-area (> 1,000 cm2), high resolving power (R > 3,000) XGS is highlyfeasible based on Critical-Angle Transmission (CAT) gratings, even fortelescopes with angular resolution of 5-10 arcsec. Significantly higherperformance can be provided by a CAT XGS on an X-ray-Surveyor-type mission. CATgratings combine the advantages of blazed reflection gratings (high efficiency,use of higher diffraction orders) with those of transmission gratings (lowmass, relaxed alignment and temperature requirements, transparent at highenergies) with minimal mission resource demands. They are high-efficiencyblazed transmission gratings that consist of freestanding, ultra-highaspect-ratio grating bars made from SOI wafers using anisotropic dry and wetetch techniques. Blazing is achieved through reflection off grating barsidewalls. Silicon is well matched to the soft x-ray band, and existing siliconCAT gratings exceed 30% absolute diffraction efficiency, with clear paths forimprovement. CAT gratings coated with heavier elements allow extension of theCAT grating principle to higher energies and larger angles, enabling higherresolving power at shorter wavelengths. We show x-ray data from CAT gratingscoated with platinum using atomic layer deposition, and demonstrate blazing tohigher energies and much larger blaze angles than possible with silicon. Wemeasure resolving power of a CAT XGS consisting of a Wolter-I focusing mirrorpair from GSFC and CAT gratings, performed at the MSFC SLF. Measurement of theAl Ka doublet in 18th order shows resolving power > 10,000, based onpreliminary analysis. This demonstrates that currently fabricated CAT gratingsare compatible with the most advanced XGS designs for future soft x-rayspectroscopy missions.

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