Designed for 1.25" eyepieces eyepieces with a barrel diameter of approximately 40.5-41.5mm, the Alpine Astronomical Baader Winged Rubber Eyecup replaces the stock eyecup on your eyepiece to block peripheral light from interfering with your views and to help eliminate distractions, improving image brightness and contrast while allowing you to concentrate on your subject. The soft silicone wing can be folded down and out of the way for those who require corrective eyewear and need to observe with their glasses on.