This quartz diagonal is built on the USA around a quartz flat and cated with an extremely bright silver coating with quartz protective overcoat. These coatings have an approximate reflectivity of 99%.
Mirrors are guarenteed to have interferometry results of at least 1/20th wave PV. Many are 1/25th or 1/30th or even better. Each diagonal is numbered and comes with an individual Zygo interferometer printout for proof of flatness of the quartz mirror. Considered to be the finest in the world.