The Optolong L-Quad Filter (2") (L-QEF) is a four-band, light pollution blocking filter specifically designed for astrophotography.
It effectively blocks the transmission of light pollution wavelengths, especially artificial lighting (including mercury and sodium lamps). This allows longer exposure times without "over-illuminating" the sky background, and also reduces the time needed to take images to a fraction of the time.
With the Optolong L-Quad filter, more contrast is achieved, even at Bortle 7 light pollution levels! For Bortle 8 and Bortle 9 light pollution (in urban, near city centre conditions), Optolong narrow band filters are recommended instead. Since you can work with a more contrasty, nicer raw material from the start, post-processing is also easier and faster when using a filter.
The Optolong L-Quad filter has more than 90% transmittance for H-Beta, OIII, H-Alpha, SII emission lines, therefore maximizing the color balance of nebulae, galaxies, clusters and other targets.
Optolong's L-Quad filters are made from German Schott glass and have a machining accuracy of at least lambda/4. The surfaces are finished with multiple layers of anti-reflective coating and, for durability, a hard, scratch-resistant (IBAD) coating.
Of course, the edges of the glass surfaces in Optolong filters are painted matte black, which prevents harmful internal reflections.