A H-alpha etalon filter designed for use on 3.5" Classic QUESTAR Maktsutov telescopes.  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".