The TPO 2" Quartz Dieletric Diagonal features 99% reflectivity, dielectric coatings to maximize light output of your telescope system. Our custom made diagonal uses a quartz mirror polished to 10th wave accuracy (or better) for best possible image quality. This diagonal provides greater thermal stability than comparable glass diagonals, resulting in minimal thermal expansion and contraction. This quartz diagonal, therefore, minimizes focus position change due to variations in temperature. It also features a CNC machined housing which will provide outstanding results for years of use. The housing is designed to retain the optimal collimation of the mirror and assure the best optical performance. This diagonal also includes brass compression rings on both diagonal and the included 2” to 1.25” adapter, assuring gentle treatment of your eyepieces. Some other brands use set screw locking mechanisms that can mar the barrel surface of your favorite eyepieces. Avoid scratches and gouges by using the TPO dielectric series diagonals. TPO 2" Quartz Dieletric Diagonal provides outstanding performance with low prices to provide amateur astronomers with excellent value!