A high vacuum housing with a first and second housing part and in which the housing parts are secured together to form an interior housing chamber. In order to seal the housing parts, an annular outwardly extending sealing ring having a flat radial surface is formed on one of the housing parts. An annular alignment recess is formed on the other housing part which registers with and receives the alignment ring. A thin walled seal is disposed between the housing parts. When the housing parts are secured together by bolts, an outer periphery of the seal is compressibly sandwiched between the flat radial surface on the sealing ring and the flat surface on the second housing part thus sealing the window to the housing.