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One last thing to consider with narrow band pass Ha filters (say 3nm or 4.5nm) is that they transmit less continuum radiation (starlight, skyglow, etc). From a Ha purist point of view, this is ideal as all they want to capture is nebulosity. However from an aesthetics perspective, this will also suppress many stars. You will find that many astrophotographers actually blend a narrow band Ha image with a luminance (unfiltered or IR/UV blocked) image. Thus, you can maintain strong nebulosity while keeping as many stars in the field.
The trick is the blending process is to ensure the background of luminance image is slightly brighter than the Ha image. In addition the highlights of the Ha image should be brighter than the luminance. When the two images are combined in PS, the brightest components of both images will be shown, thus maintaining nebulosity, subtle details and star formations.
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