Performance of a double tilted-Rowland-spectrometer on Arcus

摘要

Spectroscopy of soft X-rays is an extremely powerful tool to understand the physics of the hot plasma in the universe but in many cases, such as kinematic properties of stellar emission lines or weak absorption features, we have reached the limits of current instrumentation. Critical-angle transmission (CAT) gratings blaze the dispersed spectra into high orders and also offer a high throughput. We present detailed ray-traces for the Arcus mission, which promises an effective area > 0.5 m~2 and resolving power > 2500 in the soft X-rays. The mirror consists of Athena-like silicon pore optics (SPOs) arranged in four petals. Each petal spans an azimuth of about 30 degrees and thus offers a point-spread function that is significantly narrower in one dimension than a full mirror would provide. The four channels are split into two pairs, where each pair has its own optical axis. For each pair, CAT gratings are arranged on a tilted Rowland torus and the two separate tori are positioned to overlap in such a way that the dispersed spectra from both pairs can be imaged onto a common set of CCD detectors, while at the same time keeping the requirement of the spectroscopic focus. Our ray-traces show that a set of 16 CCDs is sufficient to cover both zeroths orders and over 90% of the dispersed signal. We study the impact of misalignment, finite size of components, and spacecraft jitter on the spectral resolution and effective area and prove that the design achieves R > 4000 even in the presence of these non-ideal effects