The Kowa TSN-CV-66 See Through Protective Cover provides a clear optical protection cover to the front of the eyepiece. It serves a function similar to the protection filters used with SLR lenses. It is especially ideal for eyeglass users. If the cover should get scratched, you still have a clear undamaged eyepiece. For use with eyepieces used with TSN-600 and TSN-660 series (60/66mm) scopes.