Precise angular positioning at 6K: the FIFI-LS grating assembly

The Field Imaging Far Infrared Line Spectrometer (FIFI-LS) obtains spectral data within two wavelength ranges. The observed wavelengths are set by rotating the two diffraction gratings to specific angles. This paper describes on the grating assemblies, designed to rotate and stabilize the gratings. First the assembly itself and its special environment inside FIFI-LS is explained. Then a method is layed out how to monitor the performance of the drive and how to detect upcoming failures before they happen. The last chapter is dedicated to first inflight measurements of the position stability of the grating.