The Herschel-PACS grating mechanism: mechanical and optical performance

The Photodetector Array Camera and Spectrometer (PACS), on board the Herschel Space Observatory, is designed for imaging and low and medium resolution spectroscopy in the wavelength region between 57 and 210 μm. This paper reports the design and the testing results of the grating cryogenic mechanism of the PACS spectrometer. The PACS diffraction grating is made from an aluminium substrate, mechanically ruled with a periodicity of 8.5 grooves per mm and gold coated for optimum reflectivity at PACS operating wavelengths. The grating mechanism is capable of accurate positioning (4") of the flat diffraction grating within a large angular throw (44°) in cryogenic environment (4.2 K). Technologies of actuators, position sensors, bearings, servo-control and cryogenic test set-up are presented. The grating mechanism was thoroughly tested, alone and when integrated in the PACS Focal Plane Unit (FPU). The tests were performed in cryogenic conditions, in a set-up fully representative of the flight conditions. Actual mechanical and optical performance obtained with the Flight Model (FM) is presented in detail. Quality of the angular positioning of the mechanism, spectral resolution and optical quality of the grating are analysed.