An x-ray transmission grating spectrometer for Lynx

NASA commissioned four studies for large astrophysics missions in different wavebands to provide input for the 2020 Decadal Survey of Astronomy and Astrophysics. The X-ray concept under study is called “Lynx”. The Lynx Science and Technology Definition Team has formulated science requirements and we need to evaluate the optical technologies that can meet these requirements. In this context, we present a draft design for a Lynx X-ray grating spectrometer (XGS) using critical-angle transmission (CAT) gratings on a Rowland torus. One of the most important parameters for the achievable spectral resolving power R is the mirror point-spread function (PSF). Different mirror technologies can lead to different PSF characteristics, e.g. the PSF might be dominated by scattering and figure errors or by the misalignment of many different mirror shells. The benefits of using sub-apertures to increase the resolving power could differ widely between different scenarios. Our study avoids mirror-specific assumptions and looks at how we can adjust the design to any of the proposed technologies. Size and placement of the CAT gratings have strong impact on spectral resolving power. We show how gratings can be arranged to meet the Lynx requirements. One consideration is how the presence of gratings impacts the effective area of the zeroth order. We discuss and trade-offs between resolving power and effective area in our design that can be adjusted to match the science requirements.