The Herschel-PACS Grating Drive and its controller

The Photodetector Array Camera and Spectrometer (PACS), on board the Herschel Space Observatory, is designed for imaging and spectroscopy in the wavelength region between 60 and 210 gm. This paper reports the design of the grating cryogenic mechanism of the PACS spectrometer and its remote controller (DEC/MEC) located on the outside of the Herschel cryostat. The PACS grating shall be capable of accurate positioning (4 arcsec) within a large angular stroke (40 arcdeg) in cryogenic environment (4.2 K). Technologies of actuators, position sensors, pivots, dry lubricants, servo-control and cryogenic test set-up are presented. At the other end of the Herschel 10-meter high-impedance cryogenic harness, the DEC/MEC consists of DSP processor-based electronics that control and synchronise the cold focal plane mechanisms and infrared detector arrays. The DEC/MEC operates an on board software running under a real-time operating system. Technologies involved in the control electronics are discussed and correlated to validation tests conducted with actual hardwares.