These 1.25" Lumicon Dust Plugs help keep your scope free from environmental contaminants.  They're inserted into 1.25" focusers and eyepiece holders, sealing the opening where dust and grime can invade and the glass and other internal components.  Lumicon also has dust covers, which insert onto the 1.25" eyepiece barrel to repel dust from eyepiece elements and filter threads.