I've bought a Ha filter simply for imaging with the moon and my horrible light pollution problem...So if I can collect 2hrs worth of Ha data on 4 objects a month, then when I go to a dark site for new moon weekend, I'll collect 2hrs of color data, then use the Ha data as a Lum channel... (mind you, thats all in theory for me at the moment as I've still got to get normal imaging under control
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I would say that from what I've seen, a 13nm Ha filter is an essential item for deep sky imagers.