Optolong CLS Filters

The Optolong CLS filter is perfect for both visual and photographic applications in areas that suffer from artificial light pollution. Since the City Light Suppression Filter has a wider pass-band than the popular UHC, or Ultra High Contrast filter, many amateur astronomers favor its use when observing in mild to moderate light pollution. The CLS filter selectively reduces the transmission of light pollution wavelengths, especially artificial light pollution caused by high and low pressure sodium vapor and mercury vapor lights. Using a CLS filter will even help reduce skyglow caused by naturally occurring neutral oxygen emissions in our atmosphere.

The Optolong 1.25” CLS Filter is made Schott glass that has been multi-coated with ion-assisted deposition technology to ensure scratch resistance and stability on the central wavelength, or CWL. The filter cell is made of lightweight, strong metal and is CNC machined, sand-blasted, black anodized, and then laser engraved. The result is a super thin filter cell that will produce a clear aperture of 26mm.

Transmission Characteristics of CLS Filter

• 95% transmission H-Beta 486nm
• 95% transmission OIII 496nm and 500nm
• 95% transmission H-Alpha 656nm
• 95% transmission SII 672nm
• 0.1% transmission Hg at 435nm and 578nm
• 0.1% transmission Na at 589nm

Quality Filter Materials:

• Schott B260 substrate material from Germany, 2.0mm thick
• CNC machined and black anodized aero-metal filter cell that is ultra-thin to provide largest clear aperture; Laser-engraved to prevent fading
• Multiple layers of anti-reflection non-cementing coatings applied via electron-beam gun evaporation with Ion-assisted deposition

Optolong Filter Manufacturing Processes and Parameters

• Filter substrate is fine-optically polished to 1/4 wave and <30 parallelism over both surfaces
• Surface quality, according to MIL-O-13830, is 60/40
• Main nebula emission lines has a transmission of 95%
• Off-band (major emission lines of artificial light pollution) transmission is 0.1%
• High value transmission of pass-band and optical density of off-band is assured through Optolong’s planetary rotation system, a homogenous and precise coating procedure
• Thin metal cells have a clear aperture of 26mm for 1.25” filters
• Each filter comes with a plastic case with high pressure EVA case lining