Spectroscopic investigations with a reflection-grating spectrometer

The reflection grating spectrometer (RGS) designed for the ESA X-ray multimirror (XMM) mission is described. The RGS is one of the two identical instruments of the RGS experiment, placed behind two of the three high-throughput X-ray telescopes of the XMM mission. The RGS design incorporates an array of reflection gratings in the converging beam at the exit from the X-ray telescope. The effective area of the RGS is more than two orders of magnitude larger than those of the grating spectrometers on the Einstein and the Exosat satellites. The large wavelength dependent resolving power of the XMM RGS will make it possible to measure many of the expected discrete soft X-ray spectral features in most sources.