UHC filters (Ultra high contrast), or narrow band nebular, are designed to restrict the passage of light at those wavelengths where streetlights emit light that pollute the sky. Especially mercury and sodium vapor emissions. This achieves a high contrast of the images of the nebulae, both in visual and in photography.

They typically screw into telescope eyepieces, either 1.25″ or directly into the inside of some dslr’s. They can also be installed on filter-holder wheels for astrophotography.