This picture of the Sun in the light of ionised Hydrogen. Passing through H-alpha light (656,28 nm) only exposes granulation and several protuberanses. Due to the extreme narrow bandwidth (0,7 nm or less) which is required to observe the smallest details, an Etalon filter is used in combination with a blocking filter.
This is basically an interference filter composed of two parrallel glass plates. In order to get rid of , consisting of two in combination with a conventional absorbtion filter which gets rid of UV, IR and wavelengths not equal to 656 nm. The latter are only visible at a wavelength of (almost) exactly 656,28 nm a