Filter Optolong OIII 6.5nm 50mm Optolong · OIII 6.5nm 50mm Key specifications Filter typeNarrowband PassbandOIII 500 nm Bandwidth6.5 nm Filter size50mm About Improved-version 6.5 nm OIII narrowband imaging filter centered on 500 nm; 50 mm diameter, 3.0 mm thick.