The f/2 Highspeed Narrowband filters from Baader are specially designed for the delicate requirements of extremely fast astrograph optics, such as Hyperstar, RASA, and ultra-fast instruments from TEC/AP/ASA/etc/. Conventional narrowband filters cause a significant loss in transmission due to the strong CWL (center wavelength) shift. In extreme cases, the CWL can even be shifted out of the FWHM (full width half maximum).
To minimize this, these highspeed filters have a CWL- pre-shift, which perfectly matches f/2 to f/3, also optimizing FWHM. Despite the typical line broadening with these fast optical trains, the filters are still able to deliver maximum contrast. These filters allow for the effective use of extremely fast optics for high contrast imaging of emission nebulae. You'll notice a dramatic improvement when using these filters between f/1.8 and f/3.5.