Very often when viewing the moon, its light is so intense in the telescope you need a filter to bring down the amount of light entering your eye. Think of it like a pair of sunglasses for your telescope. You will see more detail on crators, more surface detail and a much more comfortable eye at the eyepiece. Celestrons 1.25-inch moon filter is an economical eyepiece filter for reducing the brightness of the moon and improving contrast. This allows surface detail to be observed more readily. The clear aperture is 21mm for this 1.25-inch filter. Light transmission is about 18 percent. * Takes down the brightness of the magnified lunar surface * 1.25 inch format moon filter * Threads easily into the bottom of most 1.25 inch eyepieces * Much more comfortable viewing of the moon * See more surface detail and greatly improve contrast