The 1.25 Star Diagonal slides into the focuser ( 1.25 ) of your refractor or catadioptric telescope and accepts standard 1.25 eyepieces, allowing you to view objects more comfortably. A diagonal that is angled at 90 helps avoid neck strain when looking through through the eyepiece. Please note that a 90 diagonal will display images that are right side up, but left to right orientation will be reversed.