Most effective when used with larger, fast focus telescope in a light polluted sky. These filters dramatically improve image contrast by removing most of the "noise" or ambient light from the background sky. Light from natural oxygen emission, ionized mercury and sodium (vapor lamps) are selectively removed, while emission nebula wavelengths are allowed through. The effect of the filter is that nebular light reaching the eye is observed visually in dramatically increased detail, while the effects of unwanted city light are greatly reduced 31.7mm (1.25") Eyepiece filter.