SII filter: The narrowband S-II 7nm filter passes light at 672nm wavelength with a bandpass of 3nm which is designed for nebula fotography.
It is suitable for most emission nebulae, planetary nebulae and supernova remnants.
f you want the look of Hubble Space Telescope (HST) images, such as the famous “Pillars of Creation” (the Eagle Nebula, Messier 16), then the next filter to consider adding to your collection after H-a and OIII is the SII (es-two) or sulfur filter. The SII filter, like H-a, is a deep red filter near 672 nm. H-a, OIII and SII filters provide that Hubble look for many emission nebula. Again, tri-color narrowband imaging with these three filters can be done with the moon up, so your equipment is not sitting dormant for several weeks.
OIII Filter: The narrowband O-III 3nm filter is designed for nebula observation allowing 3nm bandwidth of light centered on a wavelength of 500nm through, which corresponds to OIII emission lines, blocking out all other light.
The newly introduced ColourMagic Super D 2" Duo from Askar is a particularly high-quality narrowband filter that has been specially developed for deep sky photography.
The ColourMagic Super D2 Duo narrowband filter (OⅢ+SⅡ) has an extremely high transmittance of over 85% at central wavelengths of 500.7 nm and 672 nm. Although it is not a special filter for very fast optics, the Duo narrowband filter has proven itself even when used with a fast telescope with an aperture ratio of f/1.9.
