The reflection grating spectrometer on board XMM-Newton

The ESA X-ray Multi Mirror mission, XMM-Newton, carries two identical Reflection Crating Spectrometers (RGS) behind two of its three nested sets of Welter I type mirrors. The instrument allows high-resolution (E/DeltaE = 100 to 500) measurements in the soft X-ray range (6 to 38 Angstrom or 2.1 to 0.3 keV) with a maximum effective area of about 140 cm(2) at 15 Angstrom. Its design is optimized for the detection of the K-shell transitions of carbon, nitrogen, oxygen, neon, magnesium, and silicon, as well as the L shell transitions of iron. The present paper gives a full description of the design of the RGS and its operational modes. We also review details of the calibrations and in-orbit performance including the line spread function, the wavelength calibration, the effective area, and the instrumental background.