Astronomy Specialist


$60.00 USD
SKU: OP-11801



The Optolong CLS (City Light Suppression) broadband filter is designed to improve the visibility of emission deep sky objects. It selectively reduces the transmission of wavelengths of artificial light, specifically those produced by mercury vapour lamps, both high and low pressure sodium vapour lights and the unwanted natural light caused by neutral oxygen emission in our atmosphere (i.e. skyglow).

The high transmission in the main nebula emission lines at OIII (496nm and 500nm), H-beta (486nm), NII (654nm and 658nm), H-alpha (656nm) and SII (672nm) enhances the contrast and details for both visual and photographic purposes in sub-rural areas. Compared to the UHC filter, the CLS has a wider bandpass so it is preferred in areas with mild to moderate light pollution. It rejects the typical wavelength associated with artificial light and it is highly transparent in the main emission nebula lines: OIII (496nm and 500nm), H-beta (486nm), NII (654nm and 658nm), H-alpha (656nm) as well as SII (672nm).

  • It blocks the artificial light from mercury vapour lamps, both high and low pressure sodium vapour lights and the unwanted natural light caused by neutral oxygen emission in our atmosphere (i.e. skyglow).
  • Maximum transmission of the main nebula emission lines: OIII (496nm and 500nm), H-beta (486nm), NII (654nm and 658nm), H-alpha (656nm) as well as SII (672nm),
  • It can also be used for visual observations as well as for astrophotography.
  • Ideal for sub-rural areas with moderate light pollution.
  • Ultra-thin filter cell to maximise the compatibility with most filter wheels.
  • Multi-layers anti-reflection coating.
  • Cell in Aerometal Material, CNC machined, sand blasted, black anodized and laser engraved.
  • Plastic PP Case, high pressure EVA Case Lining.

For astronomy cameras, it needs to be used in combination with an IR/UV cut filter. Alternatively the CLS-CCD filter can be used as it cuts off the infrared and ultraviolet wavelengths. In DSLR the IR/UV filter is embedded in the camera so the standard CLS filter is sufficient. 

Light pollution filters do not eliminate the effects of light pollution or increase the object’s brightness. It does increase the contrast between nebulae and night sky.

 Optolong CLS filter

The interferential coating is deposited on the substrate using electron-beam gun evaporation with ion-assisted deposition coating technology. This increases the filter durability and resistance to scratching, as well as stability of the CWL (central wavelength). The planetary rotation system offers precision and homogeneity of the coatings ensuring high transmission of the pass-band and high optical density off-band.

WARNING: Optolong filters are not designed for sun observations. DO NOT LOOK AT THE SUN WITH OPTOLONG FILTERS. You could become BLIND if you fail to observe the warning.

Passing band Broadband
Glass thickness

2.0 mm (for 1.25" and 2" filters) ; 1 mm (clip series)

Optical flatness

1/4 λ (wavefront)

Band-pass transmission 90% across major nebulae bandpass
Coating Fully multi-coated
Surface quality
60/40 (in accordance with US military watch MIL-O-13830)
Parallelism 30"
Glass type
Schott monolithic optical substrate (non glued)
Available sizes 1.25", 2", clip on EOS-C, clip on Eos Full Frame (FF)


Optolong filter box

Customer Reviews

Based on 1 review
Tian S.
Canon 700D modified with OPTOLONG CLS filtter

This Optolong CLS filter fits nicely and tide with my Astro modified Canon 700D, I'm living in a Bortle class 6 zone, got lots of LED street lights and neighborhood backyard light around, this CLS significantly improves the nebula contrast show as below.
This CLS filter does not work very well when I shoot with wide-angle milky way, which is expected.



From $60.00 USD


  • 1.25"
  • 2"
  • EOS C
  • EOS Full Frame

Optical design

Image circle

Focal ratio


Focal length

Optical thickness



Colour or Mono

Pixel size

Sensor dimensions


Sensor diagonal

Quantum efficiency

Read noise

Fullwell capacity

Bit depth


Camera backfocus

Filter size

1.25", 2", EOS C, EOS Full Frame

Filter size

1.25", 2", EOS C, EOS Full Frame

Bandpass (FWHM)

Bandpass (FWHM)

Glass thickness

2mm, 1mm

Mount type

Payload capacity

Latitude range

Dovetail saddle

Slew speed


Periodic error correction

Counterweight shaft

Azimuth adjustment range

Internal diameter

Illuminated diameter

Compatible telescope external diameter





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