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I see there are two kinds of H-alpha filters: narrowband (~7nm) and broadbande (~13nm). I'm planning on buying one to use with my DSLR. Which one should I buy or do I really need both?

Which brand do you guys like?

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The filter to get is the Astronomik 6nm Clip Filter (with certified 90+% transmission at Ha, available only by direct purchase from Astronomik in Germany). The uncertified filters available from US retailers are probably fine, but I did not want to take the risk. The full answer can get very, very long. But these comments may lead you in the right direction:
Doppler line broadening is very minute, fast focal ratio broadening is also minute. Both these are generally under a nm. They do matter if you are talking of filter bandwidths of a few Angstroms (NB 1 nm = 10 Angstroms). Unsharp stars are due to misfocus, not really the Bayer Array, since the Ha leaks onto the G pixels in Canon DSLRs so loss of resolution is trivial. I have a few pages addressing filter issues on my website. 13nm filters are, frankly, too wide if you have a lot of light pollution. Been there, done that. Pay up and get the best up front, as indicated above. If you are willing to use 2" filters then the Bayer 7nm Ha CCD is also great. I do own the Bayer 7nm, Astronomik 13nm and Astronomik 6nm, but I have not shot the same target with a couple of these for comparison. Some things just take too long for our short lifespans.

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