1.25'' diameter simply attach to eyepiece cased in a sturdy transparent for case;standard filter thread for 1.25" eyepieces and other accessories. for background is darker;and contrast of emission nebulae are noticeably improved;the filter for viewing nebula from light polluted skies;or for boosting the contrast of nebula from dark for Optical glass lens;aluminum frame;significantly reduces light pollution;great for city and suburban observation. UHC or light pollution reduction are designed to selectively reduce the transmission of certain wavelengths of light;especially those produced by artificial light. Due to its ability to produce exceptionally for and clear images;this UHC filter is considered of the most popular astronomy accessories;it can be used not only for observations but also for astrophotography.