The Orion 1.25" Dielectric Mirror Star Diagonal is an ideal choice for refractor and Cassegrain telescope owners who wish to obtain the brightest views possible while using a diagonal. This high-quality mirror diagonal features sophisticated, lab-tested dielectric coatings that boast an astounding 99% reflectivity. With such a high reflectivity diagonal, your telescopic views of starry skies will be bright, detailed and full of contrast. Dielectric mirror coatings are more durable than enhanced aluminum coatings, so you'll enjoy year after year of exceptional performance without degradation. The machined and anodized aluminum diagonal housing features machine-threaded internal baffles to ensure high-contrast performance. A non-marring compression collar is built-in to safely secure any inserted 1.25" telescope eyepiece without scratching. The diagonal's 1.25" nosepiece barrel is conveniently threaded to accept like-sized filters. Protective caps are included to keep the diagonal free of dust when not in use. Diagonals are not recommended for use with Newtonian reflector telescopes.