Arcus X-ray telescope performance predictions and alignment requirements

Arcus is a concept for a National Aeronautics and Space Administration probe-class X-ray mission to deliver high-resolution Far Ultraviolet and X-ray spectroscopy with two separate instruments. We focus on the X-ray spectrograph (XRS). It consists of four spectral channels arranged in a double-tilted Rowland torus geometry. It combines cost-effective silicon pore optics with high-throughput critical-angle transmission gratings to achieve at least R>3000 in a bandpass from 12 to 50 Å. We present ray-tracing studies to derive performance characteristics such as the spectral resolving power and effective area and look at the best positioning of the four channels to improve the resiliency toward misalignments and reduce the overall impact of chip gaps. We study the effect of misalignments on the performance and present alignment requirements in 6 degrees of freedom for all optical elements in the XRS. We conclude that most tolerances can be achieved with mechanical means alone.