Optolong City Light Suppression Telescope Filter

  • This CLS filter is suitable for color CCD cameras, unmodified DSLR cameras, and visual use
  • Select either 1.25" or 2" filter size
  • 95% transmission of main nebula emission lines: H-beta 486nm, OIII 496nm, OIII 500nm, H-alpha 656nm, and SII 672nm


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Optolong City Light Suppression Telescope Filter

The Optolong CLS, is a City Light Suppression Broadband Telescope Filter.

Do you have artificial light pollution where you set your telescope? The CLS (City Light Suppression) filter is designed to enhance the contrast and improve the visibility for both photographic and visual applications in artificial light polluted areas. This CLS filter has a wider pass-band than the Ultra High Contrast (UHC) filters and is favored by many when observing in mild to moderate light pollution.

The CLS filter reduces the transmission of light pollution wavelengths. It specifically targets the wavelengths of high and low-pressure sodium vapor and mercury vapor lights.

If skyglow is a problem for you, using a CLS filter will also help reduce the skyglow caused by naturally occurring neutral oxygen emissions in our atmosphere.


Transmission Levels of CLS Filters

  • 95% transmission of main nebula emission lines: H-beta 486nm, OIII 496nm, OIII 500nm, H-alpha 656nm, and SII 672nm.
  • 1% transmission off-band (the major emission lines of artificial light pollution): Hg 435nm, Hg 578nm, and Na 589nm.
  • Transmission of 0.1% is equivalent to OD3 (Optical Density), and a high optical density value indicates very low transmission. Low optical density indicates high transmission.
  • Increased Contrast - The effects of light pollution are not eliminated, but suppressed, and the brightness of objects are not increased, but let through.

 Optolong City Light Supression Telescope Filter-transmission


  • Multiple layers of anti-reflection non-cementing coatings were applied via electron-beam gun evaporation with Ion-assisted deposition.
  • 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


This 2-inch CLS filter is made with thick Schott glass which has been multi-coated with ion-assisted deposition technology. This ensures scratch resistance and stability on the central wavelength (CWL).  

  • The Schott B260 substrate material is from Germany.
  • Filters are either 2 mm thick on 1.25-inch, 2-inch, and 36 mm filters, or 1.0 mm thick on Clip series filters.


The filter mounting ring has been CNC machined from lightweight, strong metal, then sand-blasted, black anodized, and finally laser engraved to prevent fading. Resulting in a super thin 2-inch filter cell with a clear aperture of 45 mm.



BandpassMulti Bandpass
Filter Thickness2mm
Filter TypeCLS
Free ShippingYes



  • Each filter comes with a plastic case with high pressure EVA case lining


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Filter Reviews:
Larry W.
United States

Batting 500

Got the oorder for my filters . But I ordered apolar scope and only got the illuminator and not the scope. After several calls stll no scope. Got my doubts about these guys.