A Test Cryostat for Herschel-PACS Mechanism

The PACS Grating Assembly consists of a flat ruled grating of 320x80 mm²; its positioning mechanism; and launch-locking system. The Grating Assembly is part of the PACS Focal Plane Unit (FPU) and is operated at liquid helium temperature (4K). It is remote-controlled from the Detector & Mechanism Controller (DEC/MEC), which is located on the Herschel Service Module.
The grating shall be capable of accurate positioning (4 arcsec) within a large angular throw (40 arcdeg) and restricted dissipation to the 4K heat sink (< 5 mW).
The development of the Grating mechanism controller has urged the need for a cryogenic simulator of the Grating Assembly with regard to the large change in value of mechanical and electrical characteristics between room temperature and liquid helium temperature. The simulator consists of a functional prototype of the Grating Assembly and a dedicated helium cryostat with appropriate interfaces.
The test cryostat is a homemade facility (CSL) designed to cool down the prototype (and subsequent models) to liquid helium temperature. The test set-up also provides relevant instrumentation such as cryogenic temperature read-out, electrical feedthroughs toward the payload and optical viewport.