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Ric,
Galaxies emit over a broad wavelength range and do not generally contain much information within the 656.3nm that is Ha. Therefore it is best to take exposures using a luminance filter (or unfiltered) to boost your S/N ratio for galactic targets. This is certainly important if you are imaging very faint galaxies.
Having stated this, there is an emergence in the astro imaging community (predominately in the US) around Ha imaging of galaxies. It is possible to achieve and does emphasise the HII regions, but best performed on much brighter subjects such as those in our Local Group.
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