Abstract. The ESA X-ray Multi Mirror mission, XMM-Newton, carries two identical Reflection Grating Spectrometers (RGS) behind two of its three nested sets of Wolter I type mirrors. The instrument allows highresolution (E/∆E = 100 to 500) measurements in the soft X-ray range (6 to 38Å or 2.1 to 0.3 keV) with a maximum effective area of about 140 cm 2 at 15Å. 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.

We report on the design and performance of our second-generation 32-channel time-division multiplexer developed for the readout of large-format arrays of superconducting transition-edge sensors. We present design issues and measurement results on its gain, bandwidth, noise, and cross talk. In particular, we discuss noise performance at low frequency, important for long uninterrupted submillimeter/far-infrared observations, and present a scheme for mitigation of low-frequency noise. Also, results are presented on the decoupling of the input circuit from the first-stage feedback signal by means of a balanced superconducting quantum interference device pair. Finally, the first results of multiplexing several input channels in a switched, digital flux-lock loop are shown.

The current noise at the output of a microcalorimeter with a voltage biased superconducting transition edge thermometer is studied in detail. In addition to the two well-known noise sources: thermal fluctuation noise from the heat link to the bath and Johnson noise from the resistive thermometer, a third noise source strongly correlated with the steepness of the thermometer is required to fit the measured noise spectra. Thermal fluctuation noise, originating in the thermometer itself, fully explains the additional noise. A simple model provides quantitative agreement between the observed and calculated noise spectra for all bias points in the superconducting transition.

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We studied the phonon transport in free-standing 1μm thick silicon-nitride membranes at temperatures around 100 mK. By varying the geometry of the membranes and the dimensions of the heater element, we are able to distinguish between radiative and diffuse phonon transport. The data indicate that the transport is radiative ballistic with a lower limit to a phonon mean-free path of about 1 mm and that the probability for specular reflection from the surface is at least 0.99. The tested silicon-nitride membranes were grown on Si(100), Si(110), and polycrystalline-Si and the transport properties show no dependency on the substrate.