Soft X-Ray Spectroscopy With Transmission Gratings

Abstract

Transmission gratings have become increasingly important in x-ray astronomy. Gratings were used on the Einstein Observatory and EXOSAT, and they are planned for SPEKTROSAT and AXAF. Gratings with ruling densities as high as 5000 1/mm have been developed at MIT for the AXAF 'High Energy Transmission Grating' (HETG). This grating operates over the range 0.4-8 key with a resolving power of 100-1000. At MPI we concentrate on the development of gratings with moderate line densities (1000-2000 1/mm) but very high optical performance. The first diffraction order efficiency is very close to the theoretical optimum, even orders are suppressed by a proper adjustment of the wire-slit ratio. Spectral resolving power of several thousands can be achieved due to the very precise mechanical ruling and replication process, which has been developed by a commercial manufacturer, the Dr. Johannes Heidenhain GmbH. The grating is freestanding, self-supported by a coarse grid, which obstructs only 20% of the whole area, but enables the grating to withstand the enormous acoustic loads during a rocket launch. A 1000 1/mm grating will be supplied in collaboration with the Laboratory for Space Research at Utrecht for AXAF ('Low Energy Transmission Grating', LETG). The LETG will cover the low energy range down to 80 eV.