Nebula filter. Only about 5000 angstroms of OIII (secondary ionized oxygen) is allowed to pass through. It's a little worse than UHC, but it also throws out other nebula filters for a few blocks. It has a good effect on planetary nebulae and supernova remnants. For NGC7000, NGC6804, M97 and M 17, the effect is better than UHC. But most nebulae are not as effective as UHC. But when the light damage is serious. OIII can surpass UHC in many nebulae, such as M42 and M8. In the country. OIII can also beat UHC on many nebulae, but it is not as obvious as UHC.