A H-alpha etalon filter integrated with a 4.3X telecentric Barlow, optimal for use on refractors with a focal ratio from F/4 to F/8.  From the [url=http://www.daystarfilters.com/downloads/QuarkFlyer.pdf]product flyer[/url]:  "Due to variation in the end user application, we cannot specify the precise FWHM of the filter. Instead, units of 0.8 - 0.6 Å are classified as PROMINENCE and units of 0.5 - 0.3 Å are classified as CHROMOSPHERE".