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 Sky background is darker;and contrast of emission nebulae are noticeably improved;the perfect filter for viewing nebula from light polluted skies for boosting the contrast of nebula from dark for sky Due to its ability to produce exceptionally for sharp 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. Optical glass lens;aluminum frame;significantly reduces light pollution;great for city and suburban observation. UHC filters light pollution reduction filters are designed to selectively reduce the transmission of certain wavelengths of light;especially those produced by artificial light.