What is the purpose of the flat cuts on the TECHSPEC Monolithic Reflective Beam Expanders (Mark 1)?

The flat cuts are optical and mechanical reference surfaces for alignment and mounting. These flats are cut in the same diamond turning operation as the parabolic surfaces of the laser beam expander, and as such are perpendicular to the optical axis of these mirrors within the machine capability. They are designed to serve as retroreflection alignment fiducials; once your laser beam is being reflected back upon itself, the beam is properly aligned with the optical axis of the input mirror.