The Lumicon 1.25" LumiBrite Dielectric Coated Diagonal reflects 99 percent of the light from the telescope's primary optics into your eyepiece. Its single 1/10 wave precision polished surface is superior to the three surfaces of prism diagonals. The Dielectric Coating is smoother and even more durable than an enhanced aluminum coating, providing maximum image contrast. Lumicon collimates each and every LumiBrite Star Diagonal with a double-pass collimator, ensuring that the light reflected from the diagonal is perfectly perpendicular to the telescope's optical axis. The telescope-side of all of Lumicon's diagonals are threaded for filters, so you can switch eyepieces while leaving your favorite Lumicon filter in place.