Abstract: The cryogenic thermal vacuum/thermal balance test of the James Webb Space Telescope (JWST) combined Optical Telescope Element (OTE)/Integrated Science Instrument Module (ISIM), known as the OTIS, at the Johnson Space Center (JSC) Chamber A in 2017 was likely the most complex test ever performed by NASA for an unmanned mission. The test of the combined flight Optical Telescope and ISIM elements was prefaced by years of modifications to chamber facilities, and included three extensive precursor…