Hi Micheal,
The Passband of the rear filter appears like a modulation pattern with many maxima with the one near H-alpha at the maximum ampilitude. The blocking filter (at the front) just isolates the one near H-alpha. The (small) amount of other colours is useful for image processing both prominences and surface detail, an article in S&T recently explained this.
John
Ah, thanks John! I must dig out that article.
Regards,
  