This 1.875"/48mm internal diameter cap allows you to protect many Televue and older Japanese made Meade eyepieces. Here's a list: Original Meade Ultrawide 8.8, 14mm, All Radians, Panoptic 41,35,27,22mm, Nagler Type 4 22,17,12mm, Nagler Type 5 31,26mm, Televue Plossl 50,40,32mm, Televue Click Stop Zoom, Meade QX 26,36mm. See our eyepiece compatibility chart on the home page for a complete list. It fits on the objective (eye) end of the eyepiece that you place your eye to. It is made of soft yellow plastic so it is quick to put on and take off, and easy to see in the dark.