Celestron"s Moon Filters are economical eyepiece filters for reducing the brightness of the moon and improving contrast, so greater detail can be observed on the lunar surface. The clear aperture is 21mm for the 1?" filter. Transmission is about 18%.Eyepiece filters are an invaluable aid in lunar and planetary observing. They help to reduce glare and light scattering, increase contrast through selective filtration and increase definition and resolution. They also help reduce irradiation and lessen eye fatigue. Celestron"s filters are made of high quality, solid plane parallel glass with excellent homogeneity. They are anti-reflection coated, to prevent glaring and ghosting. All eyepiece filters are threaded to fit Celestron"s, and most other manufacturer"s, 1?" eyepieces. They offer a full 26mm clear aperture. Celestron filters are mounted in black anodized aluminum cells with the Kodak Wratten Series Number individually engraved. They also come with a plastic case for safe storage. The cells of each filter are double-threaded, so they can be stacked in various combinations. This allows you to create different color combinations and transmission characteristics, or to have the same color characteristic, but with a lower transmission. When stacking color filters, the effective transmission of the combination you create is equal to the product of the spectral transmission of each of the filters used. The clear aperture is 21mm and the transmission is about 18%. Moon Filter is 1-1/4". The effectiveness of the filters depends on several factors, including: the aperture and focal length of the telescope, the magnification being used, and seeing conditions.