Hi Jase,
The link I gave is an analysis of the spectrum passed by the 6nm Astronomic filter. It appears from this that while the spectrum includes the Ha line it is not centered around it. The centre of the passwidth is right shifted towards the blue. I cant believe Astronomic would release a product with such an obvious flaw, but what do I know
Definately I am going with the 6nm but am a bit unsure if i should trust Astronomic in which case I would get it or the sites published spectrum in which case I would probably get the astrodon.
Paul