Mick,
Not scientifically of course as Kiwis are not overly edumicated but the 4Nm will allow less of the "other" light in, JUST the Ha, whereas the 13Nm will allow a wider selection of light, Ha as well as "normal". Stars will be less visible in the 4, and more in the 13.
I use the Baader 7Nm.
Gary
Gary said it all.
Unless there's some new artificial lighting emitting around the Ha wavelength - don't know of any, there's not much else in the sky which gives out light in or near Ha than nebulae or stars...
Its worth noting, the smaller bandwidth filters will require longer exposure times... Shots taken with 2~4nm filters usually have TINY little pin prick stars, where as at 13Nm, stars almost appear as normal..
I use an Astronomik 13nm bandpass Ha and it provides quite natural star diameters, (due to the fact that it picks up a portion of the red end of the spectrum as well), very reasonable exposure times at f/4 and almost complete elimination of light polution/moon light. Also, 13nm filters tend to be less expensive. 4nm will remove more wavelenghts and so further reduce the effects of light pollution and non-systemic noise but at the price of requiring longer exposures. I have included a couple of examples of my Ha stacks I think about 1hr. and 30min.total respectively with my 13nm filter taken with fairly average gear. Hopefully, someone else could post a 4 nm. example.
