The finest telescope in the world is part of a system. The telescope is certainly the most critical component of the system, but the mount, drive, diagonal, lenses, and any filters are all part of the total system. There is an old saying that a chain is no stronger than its weakest link, and this certainly applies in astronomy. The humble mirror diagonal is a case in point. A good diagonal should be optically flat to avoid distortion in the optical chain, it should have high reflectivity (to avoid wasting what light your telescope collects, and the mirror should be mounted very accurately to maintain collimation.