Astronomy Specialist



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The Optolong L-eXtreme filter is a light pollution dual band filter designed to drastically reduce the effect of light pollution. Its two passing bands isolate the nebulae emissions in H-Alpha (red) and OIII (green-blue).

The advantage of the L-eXtreme filter is that there is no transmission between the H-Alpha and OIII lines as there are no nebula emission lines there. It blocks the light pollution, maximises nebulae signal and makes the sky background dark. The L-eXtreme filter works well under bright urban skies. Different from the Optolong L-eNhance filter which is a tri-band (transmits H-beta, H-Alpha, and OIII), the L-eXtreme filter is dual band to reduce the effect of light pollution even more.


NOTE: It is not recommended to use the L-eXtreme with telescopes faster than f/4 as the H-Alpha transmission decreases significantly. Instead, we recommend the L-eNhance filter.


This filter is designed for DSLR's and colour cameras used under urban skies. It works as a combined H-Alpha and OIII narrowband filter so photos will appear highly contrasted without the necessity to use a monochrome camera and two separate filters.

  • 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 at OIII (496nm and 500nm) and H-alpha (656nm).
  • Two transmission bands with a bandpass of 7nm.
  • Designed mainly for astrophotography.
  • Ideal for urban areas with very high light pollution.
  • It incorporates an IR/UV-Cut filter that is necessary in astrophotography, especially for astronomy cameras that are sensitive to the infrared radiation.
  • 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.

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 L-Extreme

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 on CWL (central wavelength) and no deviation affected by temperature changes. The planetary rotation system offers precision and homogeneity of coatings ensuring high transmission of 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 Dual band H-Alpha and OIII
Glass thickness

2.0 mm (for 1.25" and 2" filters)

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"


Optolong filter box

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