This 1.5"/38mm internal diameter cap allows you to protect Televue Type 6 Nagler, Panoptic 19 & 24mm, Nagler 20mm Type 5, Takahashi LE and Vixen LV eyepieces. It fits on the objective end of the eyepiece that you place your eye to. For a complete list see our eyepiece compatibility chart on the home page. It is made of soft yellow plastic so it is quick to put on and take off, and easy to see in the dark.