Unlike others, Tele Vue prisms and mirror diagonals begin life as one solid chunk of metal-which is then continually refined to produce what you see before you now.  This is a huge asset because there's nothing within the prism or diagonal which can unscrew and cause a heavy, expensive eyepiece to drop.

Tele Vue's Everbrite diagonals provide whiter and brighter images than any other diagonal.  The exotic Everbrite coating is 99% reflective across the entire visual spectrum.  Painstakingly applied, the Everbrite is offered in both 1.25" and 2.00" models.  The bodies are machined from solid aluminum blocks, ensuring permanent alignment as well as not allowing heavy accessories to unscrew the body from the barrel.

90° Diagonals such as this render upright-but-laterally reversed images, which in turn gives a brighter image than if the device yielded an upright and laterally correct image.  This is important when you're trying to muscle every last bit of magnification that you can out of your scope given the object's size, the scope's aperture and the seeing conditions.