Astronomy Specialist



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The Optolong OIII narrow-band filter boosts the contrast of emission nebulae and supernova remnants for professional results, even under a bright sky affected by light pollution. This filter is used in combination with the H-Alpha and SII filter to combine a narrowband colour image.

The 6.5nm band pass is only 2% of the entire visible spectrum, increasing the signal to noise ratio and delivering high contrast images and extremely dark backgrounds.

The maximum transmission of the Oxygen III line (500.7 Nm) is above 80% and the transmission off band is only 0.1%. This is a value often undisclosed by other filter producers and it is critical to darken the sky background and minimise the halos around the stars. 

The filter cuts off the ultraviolet and infrared light so there is no need to use the IR-cut filter.

Every OIII filter is marked with its unique serial number and individual passing band certificate. This test is rarely provided by any producer and it is a standard for high quality filters.

Optolong SHO Filter Passing Band Certificate 


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 OIII 6.5m
Glass thickness

2.0 mm

Optical flatness

1/4 λ (wavefront)

Band-pass transmission above 80%
Coating Fully multi-coated
Surface quality
60/40 (in accordance with US military watch MIL-O-13830)
Parallelism 30"
Glass type
Monolithic optical substrate (non glued)
Available sizes 1.25", 2", 36mm


Optolong filter box

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