Grating arrays for high-throughput soft x-ray spectrometers

Cosmic soft X-ray spectroscopy exploits principal transitions of astrophysically abundant elements to infer physical properties of objects in the sky. Most of these transitions, however, fall well below 2 keV, or 6 Angstroms. Consquently, grating spectrometers offer the current, best means by which to analyze soft X-rays from such sources, where throughput and resolving power must be maximized together. We describe grating spectrometer design candidates for the future mission Constellation-X, and how the grating array on board (~1000 gratings in a 1600mm diameter, each for 4 instruments) may be implemented. Grating fabrication and grating alignment approaches require special consideration (over the XMM-Newton RGS experience), because of grating replication fidelity and instrument mass constraints.