The new Optolong L-Para filter (2") combines the effect of two narrow-band filters. It blocks light from light-polluting mercury, sodium and LED lamps, while transmitting OIII (500.7 nm) and H-Alpha (656.3 nm) wavelengths.
The Optolong L-Para filter is designed for use with colour astronomical cameras and DSLRs, especially for capturing faint diffuse nebulae. Even with extreme brightness (F2) optics, the Optolong L-Para filter has 85% transmittance over the emission lines of nebulae.
The transmittance curve above shows that the two narrow 10 nm transmittance ranges can be exploited by less luminous systems just as much as by luminous systems.
The manufacturer's efforts to achieve even higher contrast and to minimize the halo phenomenon around bright stars appear to be successful.
The filter has a narrow design, only 5 mm thick (without thread). It is made of high quality Schott glass, with multiple layers of anti-reflective coatings vapour-deposited on its surface.