Gray-green moon filter for astronomical telescopes. It works with glasses in the 1.25 "standard. Manufacturer: Celestron.
The connection of ALL astronomical filters, with the exception of solar foil filters (which are filters for the lens), is made by screwing the filter into the eyepiece frame from the side opposite to the eye you put in (i.e. from the side you put into the eyepiece extractor) ). Filters can be additionally joined together as they have threads on both sides of the housing.