This very useful addition to the Baader filter range is intended for the luminance channel in astro imaging. Using narrowband filters can often result in slightly dimmer images, which can be difficult to resolve to a focus. You can now use this filter to focus perfectly with a bright image and since they are also parfocal, switch to a different narrow band filter without loosing the focus. This filter has the same substrate thickness as the other Baader filters.
Baader balances each dielectric coating stack to avoid the ghosts and annoying halos that plague other manufacturers filters.