Optolong CLS (City Light Suppression) is a dual-bandpass filter which selectively enhances emission from nebulas and suppresses the transmission of light pollution sources. CLS enhances the contrast of nebulas and reduce background brightness mostly dominated by light pollution. It has wider passband than UHC (Ultra High Contrast) and thus it enhances deep sky objects under mild to moderate light pollution yet offering a more balanced colour rendition.
CLS selectively cuts out light pollution emission. For example, mercury vapor lamps, high and low pressure sodium vapor lights and skyglow caused by emission from neutral oxygen in our atmosphere. Among the emission lines enhanced are OIII (496, 500nm), H-beta (486nm), NII (654, 658nm), H-alpha (656nm) and SII (672nm).
*equivalent to OD3