Optolong Filters

Optolong CLS Clip-On Filter Canon Full Frame

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Product Overview

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).


  • 95% transmission at major nebula emission lines
  • 0.1% transmission* at off-band at Na (589nm), Hg (435nm, 578nm).
  • Multi-layers anti-reflection coating
  • Precise and sharp roll-off at light pollutant lines
  • 2 mm filter thickness (1.25", 2" and 77mm) | 1mm (clip series)
  • Made on Schott substrate
  • Finely polished to 1/4 wave accuracy
  • Surface quality: 60/40 (MIL-O-13830 standard)

*equivalent to OD3

Recommended applications

  • CanonEOS Full Frame DSLR camera 5D, 5D Mark II , 5D Mark III, and 6D only
  • Deep sky colour imaging, particularly emission nebula or DSO emitting in the H-alpha, H-beta, O-III and S-II lines

Coating technologies

  • Multi layers anti-reflection coating
  • Non cementing optical substrate coating
  • Electron-beam gun evaporation with ion-assisted deposition coating technology to enhance coating durability and resistance to scratching as well as stability on central wavelength such that passband does not deviate with temperature change
  • Planetary rotation system offers high precision and homogeneity of coating surface ensuring high value on passband transmission and off-band blocking.

Filter cell technologies

  • CNC machined with aerospace grade aluminium
  • Black anoidised finish and extinction treatment to prevent refection
  • Laser engraved fade-free wordings


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