For extreme contrast enhancement, the OIII filter is especially useful for planetary nebulae In the OIII line, they show their most interesting structures. But [O III] regions are also embedded in other gas nebulae. So an OIII filter can show completely different details than a UHC filter or no filter.
The Baader OIII filter is very narrow banded with only 10 nm FWHM
We recommend it for apertures above 200 mm. Below, only low magnification is possible. It is a good addition to the broad band UHC-S filter from Baader.
Baader OIII filter transmission curve
The transmission curve shows a small 10 nm FWHM band - exactly on the OIII line at 501 nm. With the OIII filter, even under worse stray light conditions in big cities, successful observations are possible. The background remains black also at big exit pupils. The filter is dielectrically coated and aging-resistant, robust and scratch-proof. Highest image quality is guaranteed by plane-optically polished surfaces.