The 3" ES Star Diagonal from Explore Scientific features a dielectric-coated mirror with 99% light transmission. This reflectiveness translates to minimal light loss to maintain image clarity and brightness. The insert barrel has a double taper that makes it easier to insert and remove from the scope's focuser. The holder accepts 3" eyepieces, and it comes with a 2" and 1.25" adapter to use the more common smaller diameter eyepieces.