Nebular filters, also called Light Pollution Reduction (LP or LPR) filters, are used to view faint, deep-sky objects in your telescope if you cannot access a dark-sky location outside of a city. They are a nice-to-have telescope accessory. If you can only pick one filter, get a narrowband filter. Be sure and get one that is labelled UHC or Ultra-High Contrast.

LP filters work by blocking all wavelengths of light except those transmitted by the ionized atoms of hydrogen, oxygen and sulfur found within nebulae and supernovae remnants.