We present a vacuum window mechanism that is useful for applications requiring two different vacuum windows in series, with one of them movable and resealable. Such applications include space borne instruments that can benefit from a thin vacuum window at low ambient pressures, but must also have an optically open aperture at atmospheric pressures. We describe the implementation and successful operation with the EBEX balloon-borne payload, a millimeter-wave instrument designed to measure the polarization of the cosmic microwave background radiation.
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