Filter for nebulae observations. Barrel diameter: 2". Levenhuk Ra O-III 2" Filter is perfect for observation of nebulae due to its capability to suppress light that lies outside its 4900-5010 angstrom (490-501nm) passband and transmit wavelengths of the visible spectrum within 150 angstrom (15nm) range. This filter is recommended for observation under urban city illumination. It's highly effective in reducing light pollution and enhancing the contrast of produced images. However, don't forget that this filter gives a green tint to observed objects. It has a barrel diameter of 1.25" so you can use it with any type of telescope.