Dark gray lunar filter in 2 inch frame. It lets 13 percent of the light from the falling beam. The right filter, especially for large telescopes in planetary observations, when the amount of light is even a hindrance to getting a good picture.
It greatly improves the contrast when observing our natural satellite, especially in the near full phase.
Connection of ALL astronomical filters with the exception of solar film filters (which are filters for the lens) is accomplished by screwing the filter into the frame of the eyepiece from the opposite side than applying the eye (ie from the side that we put in the eyepiece extractor) . The filters can be additionally connected with each other, because they have threads on both sides of the luminaire.