Baader 31mm f/2 H-alpha High-Speed Filter 2459381
These Baader H-alpha High-speed f/2 narrowband filters are for use in fast optical systems such as Celestron's Rowe-Ackermann Schmidt Astrograph (RASA) and Starizona's Hyperstar systems. The use of conventional narrowband filters cause a heavy loss in transmission due to the strong centre wavelength (CWL) shift. In extreme (very fast) cases the CWL shifts out of the filter's full width half maximum (FWHM) transmission. Therefore these filters have a CWL-preshift which matches f/2 to f/3 optics perfectly.
In spite of the typical line broadening with such fast optical trains, these filters are able to deliver maximum contrast. These filters allow them to be used with fast optics really effectively for high contrast emission nebulae imaging. Using these filters between f/1.8 and f/3.5 shows a dramatic improvement compared to any regular narrowband filters.