In-Flight Performance of the HERSCHEL Sorption Coolers – One Year of Operation
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HERSCHEL, the fourth ‘cornerstone’ mission in the ESA science program, was launched on 14 May 2009 from Kourou, French Guyana. With a 3.5 m Cassegrain telescope it is the largest space telescope ever launched. HERSCHEL is performing photometry and spectroscopy in the electromagnetic spectrum for wavelengths 55-672 μm. This bridges the gap between earlier infrared space missions and ground-based facilities. HERSCHEL’s payload consists of three instruments built by international scientific consortia: HIFI (Heterodyne Instrument for the Far Infrared), PACS (Photo-conductor Array Camera & Spectrometer) and SPIRE (Spectral and Photometric Imaging REceiver). Two of these instruments, SPIRE and PACS, use bolometric detectors cooled to 300 mK. The HERSCHEL cryogenic subsystem relies on passive cooling down to liquid nitrogen temperatures for both the cryostat & telescope. It features a 2367 liter superfluid helium tank that vents to space and provides the instruments with cooling at four interface temperatures between 1.6 K and 15 K. Two dedicated sorption coolers are then used to lower the PACS and SPIRE bolometer temperatures to below 300 mK. These units are single shot devices and need to be recycled on a regular basis. Their typical hold time is over 48 hours for less than 2 hours recycling time. To date these systems have been in operation for approximately a year. As of April 2010, over 150 re-cycles have been successfully performed and the performance of these coolers is stable and fully in line with predictions.