Optical layout and resolution of the ASTRID plane folding mirror spherical grating monochromator

We present the optical layout, the theoretically expected and experimentally obtained resolution of a plane, folding mirror spherical grating monochromator at the ASTRID storage ring in Denmark. The monochromator has a plane mirror in front of the grating and uses a variable included angle on the grating to obtain a zero defocus term. The advantages of this design are a fixed exit slit position, which makes it easier to refocus behind the exit slit, and the possibility to optimize the resolution for the energy regions, using different ranges of included angles. The maximum resolving power EΔE for 10 μm slits with full vertical acceptance increases from about 5700 for the high-energy grating to about 14 000 for the low-energy grating. The resolution has been tested using absorption features of helium, argon and nitrogen. A resolution of 68 meV was obtained at the N2 K edge around 400 eV. At the He double resonances at 65 eV the 17+ structure was observed. In second order, at the He 7+ double resonance, a resolution of 1.4 meV was obtained. The resolving power EΔE at this energy (32 eV) is around 23 000.