The O-III filter (Oxygen filter) only allows the passage of doubly ionized oxygen. This means that there is an enormous increase in contrast with diffuse, planetary and extremely dim nebulae. This is a special narrowband filter which allows an even smaller range of wavelengths than a regular UHC filter. However, there are innumerable astronomical objects, which can be observed in detail with this filter. A prime example is the Veil nebula in the constellation of Cygnus.
An O-III filter is not only useful at light-polluted sites; the contrast is also increased under dark sky locations.
