Quaternary Mirror
The quaternary mirror directs light from the tertiary mirror into the laser enclosure, in a 90-degree bend. The mirror must be a high-power dielectric because the incident laser intensity is still rather high at this point. The mirror is to be suspended in a mount that attaches to the back-port bolt-circle, only part of which is accessible (due to our own laser enclosure).
The footprint of the beam at this point is about 2.6 by 3.7 inches (elliptical), so at least a 4-inch flat is required, with a dielectric coating. We need to price round vs. elliptical mirror shapes. My guess is that round will win, even though it's larger in area. The mirror needs to have some crude occasional adjustment capability. We will shroud this mount in a baffle box that lets no laser light out into the dome enclosure. This means good (flexible) seals between the baffle box and mating surfaces.
The rotation in the "azimuth" direction is not exactly 90°. In other words, the beam does not penetrate the enclosure in a perpendicular manner. Rather, the beam enters/exits the enclosure slightly lower than the perpendicular point.