The constellation-X RGS options: Raytrace modeling of the off-plane gratings

The Reflection Grating Spectrometer of the Constellation-X mission has two strong candidate configurations. The first configuration, the in-plane grating (IPG), is a set of reflection gratings similar to those flown on XMM-Newton and has grooves perpendicular to the direction of incident light. In the second configuration, the off-plane grating (OPG), the grooves are closer to being parallel to the incident light, and diffract along a cone. It has advantages of higher packing density, and higher reflectivity. Confinement of these gratings to sub-apertures of the optic allow high spectral resolution. We have developed a raytrace model and analysis technique for the off-plane grating configuration. Initial estimates indicate that first order resolving powers in excess of 1000 (defined with half-energy width) are achievable for sufficiently long wavelengths (λ ≥ 12Å), provided separate accommodation is made for gratings in the subaperture region farther from the zeroth order location.