Transmission grating arrays for the X-ray spectrometer on Arcus Probe

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Abstract
The Arcus Probe mission concept has been submitted as an Astrophysics Probe Explorer candidate. It features two co-aligned high-resolution grating spectrometers: one for the soft X-ray band and one for the far ultraviolet. Together, these instruments can provide unprecedented performance to address important key questions about the structure and dynamics of our universe across a large range of length scales. The X-ray spectrometer consists of four parallel optical channels, each featuring an X-ray telescope with a fixed array of 216 lightweight, high-efficiency, blazed transmission gratings, and two charge-coupled device readout arrays. Average spectral resolving power λ/Δλ>2500 (3500 expected) across the 12 to 50 Å band and combined effective area >350 cm2 (>470 cm2 expected) near OVII wavelengths are predicted, based on the measured X-ray performance of the spectrometer prototypes and detailed ray trace modeling. We describe the optical and structural design of the grating arrays, from the macroscopic grating petals to the nanoscale gratings bars, grating fabrication, alignment, and X-ray testing. Recent X-ray diffraction efficiency results from chemically thinned grating bars are presented and show performance above mission assumptions.

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