A model D168 photomultiplier tube made by Thorn EMI Electron Tubes Ltd, similar to those used on the Hipparcos astronomy satellite. A photomultiplier is an instrument containing a photoelectric cell and a series of electrodes used to detect and amplify very faint light. The Hipparcos space astrometry satellite was launched during the night of 8th August, 1989 on an Ariane 4 launcher. The principal objective of this European Space Agency (ESA) mision was to produce a star catalogue of unprecedented precision. In its four years of operation, Hipparcos collected information on the positions, distances, motions, brightnes and colours of over 100,000 stars, the results of which were published by ESA in a series of catalogues in 1997.
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