Herschel flight models sorption coolers

The Herschel and Planck satellites will be jointly launched on an ARIANE 5 in 2008. The Herschel payload consists of three instruments built by international scientific consortia, heterodyne instrument for first (HIFI), photo-conductor array camera and spectrometer (PACS) and spectral and photometric imaging receiver (SPIRE). The spacecraft provides the environment for astronomical observations in the infrared and sub-millimeter wavelength range requiring cryogenic temperatures for the cold focal plane units. The spectral and photometric imaging receiver (SPIRE) will cover the 200–670 μm spectral range using bolometric detectors, as the photo-conductor array camera and spectrometer (PACS) will cover the 60–210 μm spectral range. Both instruments SPIRE and PACS feature detectors operating at 300 mK. This cooling will be effected by two helium sorption coolers developed at the Service des Basses Températures of the Commissariat à l’Energie Atomique (CEA-SBT). These coolers based on an evaporative cooling cycle features no moving parts and can be recycled indefinitely pending the availability of a cold heat sink at temperature below 3 K. Several models were developed in the course of the Herschel program and this paper deals with the design, manufacturing and qualification of the flight model coolers.