SV188P star diagonal dielectric coatings; super flat mirror with dielectric coatings to reduce reflected light loss; provide an over 99% reflective surface; good for astronomical viewing SV188P star diagonal 90 degree viewing angle; diagonal reflects the telescope's light path 90 degree to provide a more comfortable viewing angle; best for observing objects overhead SV188P star diagonal brass compression ring; Built in brass ring and thumbscrew accessories avoid eyepiece to rotate during adjustments; effortless fixes more firmly and will not scratch your eyepieces barrels Dovetail base work with finder scope or reflex sight bracket; include two large thumb screw locks; so you can securely attach an aiming device to any telescope Durable metal design; made of high strength aluminum with anodizing aluminium process; ultra-light weight and ultra-thin body