Excerpt from NASA/GSFC web site... "Before most Goddard spacecraft components are cleared for launch, they go through final checkups in one of the center's many clean room facilities. With a massive 1.3 million cubic feet of space, the High Bay Clean Room, often called the world's largest, prevents dirt, dust and other contaminants from damaging spacecraft components, especially sensitive and carefully calibrated instruments. The clean room ventilation system circulates almost one million cubic feet of air every minute through 9,000 square feet of HEPA filters located along one wall."
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The orientation of the eighteen hexagonal, 1.3 meter-wide, gold plated beryllium segments that comprise the 6.5 meter main mirror reflected the High Bay viewing windows and the many visitors that came to see JWST before it is scheduled to leave Goddard for final I&T and its planned fall 2018 launch.