  Several high priority subjects in astrophysics can be addressed by a
state-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 highly
feasible based on Critical-Angle Transmission (CAT) gratings, even for
telescopes with angular resolution of 5-10 arcsec. Significantly higher
performance can be provided by a CAT XGS on an X-ray-Surveyor-type mission. CAT
gratings combine the advantages of blazed reflection gratings (high efficiency,
use of higher diffraction orders) with those of transmission gratings (low
mass, relaxed alignment and temperature requirements, transparent at high
energies) with minimal mission resource demands. They are high-efficiency
blazed transmission gratings that consist of freestanding, ultra-high
aspect-ratio grating bars made from SOI wafers using anisotropic dry and wet
etch techniques. Blazing is achieved through reflection off grating bar
sidewalls. Silicon is well matched to the soft x-ray band, and existing silicon
CAT gratings exceed 30% absolute diffraction efficiency, with clear paths for
improvement. CAT gratings coated with heavier elements allow extension of the
CAT grating principle to higher energies and larger angles, enabling higher
resolving power at shorter wavelengths. We show x-ray data from CAT gratings
coated with platinum using atomic layer deposition, and demonstrate blazing to
higher energies and much larger blaze angles than possible with silicon. We
measure resolving power of a CAT XGS consisting of a Wolter-I focusing mirror
pair from GSFC and CAT gratings, performed at the MSFC SLF. Measurement of the
Al Ka doublet in 18th order shows resolving power > 10,000, based on
preliminary analysis. This demonstrates that currently fabricated CAT gratings
are compatible with the most advanced XGS designs for future soft x-ray
spectroscopy missions.
