The exactly focusing spherical grating monochromator for undulator radiation at BESSY

The design goals and the performance data for the exactly focusing spherical grating monochromator at BESSY which is installed on a crossed field undulator for circularly polarized undulator radiation is described. By employing a variable deviation angle at the grating it is possible to exactly focus the entrance slit on the exit slit at all wavelengths. In addition, the optical aberrations are very small, leading to a high resolution over the energy range of the monochromator. The performance has been verified by analysis of the He (1s−1 np) and the Kr (3d−15/2,3/2 np) resonance series. A resolution of E/ΔE≊17 500 at 65 eV in second order has been achieved. Since commissioning, this monochromator has been used for a variety of experiments requiring both plane- and circularly polarized undulator radiation.