A practical method for introducing stray light for testing the James Webb Space Telescope (JWST) at cryogenic temperatures using hollow shell spherical reflectors is described. Several alternate approaches to stray light testing are compared, including fiber sources, diffuse panels, and curved specular reflectors. Alignment of the sources can pose special difficulties when cooling to cryogenic temperatures, since the shape of the reflector, mounts, and support structure can all change. The hollow shell spherical reflectors do not have any of these difficulties, and can be mounted
so that they are automatically aligned by gravity. This also makes them insensitive to vibration, so they can be used with long detector integration times to provide adequate stray light signal to noise without interfering with other optical tests. The reflectors are positioned so as to work with test source(s) already in the cryo-vac chamber. The spherical reflectors operate at all wavelengths, reducing the number of reflectors required and providing operational flexibility in reflector placement. Electroplated stainless steel hollow shell reflectors are inherently compatible with cryogenic and vacuum environments. The reflectors are passive and have no thermal dissipation, eliminating impact on sensitive thermal tests.
Their light weight and single point suspension mounting minimize the dynamic and static loads. Finally, the reflector's simple geometry is inherently compatible with optical alignment metrology (e.g. LIDAR), making position measurements both more accurate and simpler to document.