Development of a flat-field spectrograph with a wide-band multilayer grating and prefocusing mirror covering 2-4 keV

A flat-field spectrograph equipped with a wide-band multilayer grating and prefocusing mirror covering 2-4 keV without any mechanical movement has been developed. To realize this, a new multilayer structure consisting of W and B4C layers has been invented, which enhances the diffraction efficiency of the grating over the whole energy range at a fixed angle of incidence as well as the reflectivity of the prefocusing mirror. The multilayer has been deposited on a laminar-type varied-line-spacing holographic replica grating and a spherical mirror substrate. The diffraction efficiency of the multilayer grating varies between 1.2% and 3.3% at 88.65 degrees in the 2.1-4.0 keV range. Also the reflectivity of the prefocusing mirror varies between 2.7% and 12% at 88.00 degrees in the same range. The overall throughput of the spectrograph with the multilayer optics is 10(4) times higher than that of gold-coated optics.