The detector is not sealed with a window and its photocathode material is sensitive to degradation in air, so it must be maintained in a vacuum at all times throughout the preparations for flight. Two redundant vacuum ion pumps are provided to maintain a vacuum within the spectrometer, and a valved pump-out port allows the use of an external pump during much of the ground operations. The two small internal pumps are 2 liter sec-1 Varian ``VacIon" pumps, which are powered by individual high voltage power supplies mounted on the spider arms. Several different power sources are available for these pumps. For orbital operations, the power is supplied by either the Spacelab Experiment Power Distribution system or by the Shuttle's payload cabin power. For ground operations, the power is also supplied by a number of external power supplies through various connectors including finally the T-0 umbilical when Astro is installed in the Shuttle.