Orion 8880 1.25-Inch Dielectric Mirror Star Diagonal 90 diagonal features sophisticated, lab-tested dielectric coatings that boast an astounding 99 percent reflectivity Dielectric mirror coatings are more durable and more reflective than enhanced aluminum coatings Diagonal provides a comfortable 90-degree viewing orientation when observing the night sky through a refractor, Maksutov-Cassegrain (Mak-Cass) or Schmidt-Cassegrain telescope Machined and anodized aluminum housing features machine-threaded baffles for optimized contrast 1.25" nosepiece barrel is threaded to accept like-sized Orion eyepiece filters