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My guess is it blocks everything except a relatively broad band around Ha. The ERF knocks the brightness of everything down a bit, then the etalon effectively blocks everything except those wavelengths that resonate between the two reflective surfaces inside the etalon. The frequencies that resonate of course build up till they are so bright they can be seen through the etalon. As there are many frequencies which can resonate inside the etalon, the blocking filter blocks them all except the one you want to look at. The blocking filter just can't let through as narrow a band of frequencies as the etalon, so you need both.
Al.
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