Baader 31.7mm Diascop Bayonet Ocular Adaptor: For astro useage, you need a quick eyepiece change, without varying the orientation of the Diascope. This 31.7mm Diascope eyepiece adapter comes without a mounted sealing ring so that the eyepieces can be switched easily. The bayonet has got an undercut, which serves to receive one of the two supplied O-rings. If you prefer a sealing ring, you can adjust the stiffness when switching eyepieces between tight (large O-ring) and light (small Oring) with these two different rings. The image shows the undercut to place the O-Rings