* Design may contain errors and things that can be simplified or optimized further. This is a highly experimental system and YMMV.
* HP 5501B lasers do not like light (at any wavelength) sent into their aperture; the stabilization circuit fails and the REF output becomes wrong. Workaround is to misalign the input beam, which obviously introduces cosine error. A better solution is needed, maybe rebuild a 2-frequency HeNe but use the waste beam of the tube for REF/intensity measurements unlike the HP design. The HeNe tube itself attenuates incoming light.