Astronomy Specialist




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The Optolong LRGB 1.25" filters are specially designed for deep space photography with monochrome cameras. The strict manufacturing process and the quality control guarantees a tight tolerance of the filter thickness (± 0.05mm) which makes these filters parfocal. This allows the filter change without the need to refocus.

Each filter is fully multi-coated to minimise internal reflections, halos around stars and to assure a high transmission which is above 95%

Each filter cuts off the ultraviolet and infrared light so there is no need to use the L filter in combination with the colour filters.

The width of bandpass of the 3 RGB filters is balanced with the spectral response of the most common CCD and CMOS cameras. This expedient allows to balance the exposure of each channel with the ratio of 1:1:1. That means the exposure time in red is equal to the exposure time in green and in blue. During post-processing, it will be easier to obtain images with a natural colour balance thus without colour dominances. The fourth L filter assures a transmission above 95% across the visible spectrum (400nm to 700nm) cutting out ultraviolet and infrared light.

The design of these filters is optimised to cut out the typical yellow emission of the sodium and mercury lamps around 589nm.


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 L R G B
Glass thickness

2.0 mm

Optical flatness

1/4 λ (wavefront)

Band-pass transmission above 95%
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", 36mm



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